FEAS,B>UTY STUDY FOR THE CONTINUOUS · 4. Manganese(lI) and Zinc(ll) 36 V. Sampling System 40 A....

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SWEM-TR-72.il | „ 1

A FEAS,B>UTY STUDY FOR THE CONTINUOUS

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

>

Approved for public release, distribution unlimited.

DISPOSITION INSTRUCTIONS i

Destroy this report when it is no longer needed. Do not return it to the originator

DISCLAIMER:

The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

AD

RESEARCH DIRECTORATE

WEAPONS LABORATORY AT ROCK ISLAND

RESEARCH, DEVELOPMENT AND ENGINEERING DIRECTORATE

U. S. ARMY WEAPONS COMMAND

TECHNICAL REPORT

SWERR-TR-72-11

A FEASIBILITY STUDY FOR THE CONTINUOUS AUTOMATED ANALYSIS AND CONTROL OF

PHOSPHATIZING BATHS

February 1972

Pron A1-1-5005A-(Ol)-Ml-Ml AMSCode ^97.06.7037

Approved for public release, distribution unlimited

FOREWORD

This report was prepared by E. P. Perry and R. L. Myers

of the North American Rockwell Science Center, Thousand Oaks,

California, in compliance with Contract DAAFO1 -72-C-O130

under the direction of the Research Directorate, Weapons

Laboratory at Rock Island, U. S. Army Weapons Command, with

S. L. Eisler as Project Engineer.

This work was authorized as part of the Manufacturing

Methods and Technology Program of the U. S. Army Materiel

Command and was administered by the U. S. Army Production

Equipment Agency.

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TABLE OF CONTENTS

Page No.

Abstract 1

I. Introduction and Scope of Work 2

II. Introduction to the Phosphating Process 3

A. Mechanism of Coating Process 3

B. Composition of Phosphate Baths 3

C. Control of Bath Composition 4

III. Introduction to Automatic Process Control 7

A. Advantages of Automatic Process Control 7

B. Basic Elements of the Control System 7

C. Design of a Data Acquisition System 13

D. System Expansion 16

IV. Selection of Analytical Scheme 17

A. Current Methods of Phosphate Bath Analysis 17

B. Manganese and Zinc Bath Analyses 17

1. Free and Total Acid 17

2. Nitrate Ion 18

3. Ferrous Ion 2k

k. Manganese Ion 25

(a) Ion Selective Electrode 25

(b) Polarography 27

(c) Redox TItrations 30

(d) Complexometric Tit rat ion 30

(e) Colorimetric Methods 30

5. Zinc Ion 31

(a) Ion Selective Electrode 31

(b) Polarographic Methods 32

(c) Titration Techniques 32

(d) Colorimetric Techniques 35

C. Recommended Methods of Analysis (Summary) 35

I i i

Table of Contents (Continued) Page No,

1. Free and Total Acid 35

2. Ferrous Ion 36

3. Nitrate Ion 36

4. Manganese(lI) and Zinc(ll) 36

V. Sampling System 40

A. Obtaining a Representative Sample 40

B. Precision Sampling Valves 41

C. Solution Multiplexing 41

D. Routine Tasks of the Sampling System 42

VI. Control Strategy 44

A. Evaluation of Analytical Results 44

B. Determination of Optimum Reagent(s) 45

C. Quantity of Reagent(s) Added 47

VII. Reagent Addition 50

A. Stirring of the Bath 50

B. Addition of Reagents 50

VIM. Miscellaneous Considerations 52

A. Temperature Control 52

B. Solution Level Control 52

IX. Site Considerations 54

X. Calibration and Maintenance 56

References 57

Distribution 59

DD Form 1473 Document Control Data (R&D) 62

1 v

ABSTRACT

The feasibility of automating production phosphate

coating process baths has been studied under the guidance

of the Research Directorate of the Weapons Laboratory at

Rock Island. The study comprised a detailed examination

of the analysis and control requirements. Continuous

automated analysis and control of phosphate baths are fea'

slble. Analytical methods, sampling system procedures,

and control logic techniques are presented for incorpor-

ation into a prototype system.

I. Introduction and Scope of Work

It is the purpose of this study to investigate the feasibility of con-

tinuous automated analysis and control of phosphatizing baths. The scope of

this report is to define the major problems which would be encountered in con-

structing such a system, recommend approaches to the solution of these problems

and provide a general plan for the implementation of automated control.

The application of corrosion resistant coating is an important phase of

the metal finishing industry. Many techniques have been developed to achieve

this goal. In general, these techniques are based on a chemical or electro-

chemical modification of the metal surface. One specific process which pro-

vides excellent corrosion resistant coatings is the phosphate coating process.

This process was developed in the early 1900's (1,2,3). However, the coatings

produced by the early procedures were difficult to obtain and did not satis-

factorily withstand severe environments. Empirical refinements in the techni-

que were made and by World War II satisfactory coating was being routinely

applied. In the late 19^0's and 1950's, the empirical relationships were de-

fined and studied qualitatively (A,5,6,7)- As a result of these studies, the

optimum concentration for the chemicals in the phosphating baths were known.

The problem of maintaining optimum phosphate bath activity, however,

still remains. As yet, no control system has been built which automatically

controls all of the important bath variables. The lack of continuous control

of bath composition causes a severe problem, since coatings are then produced

which do not meet corrosion resistance tests, e.g., salt spray. The production

of poor coating causes a loss in material and in the manhours required to re-

work the failures.

II. Introduction to the Phosphatlng Process

As a prerequisite to automating any process, the process must first be

understood as it is performed manually. The phosphating process is no excep-

tion. This section of this report will briefly examine the mechanism of the

phosphattng process, the composition of the phosphate baths and the present

method of composition control.

A. Mechanism of Coating Process

A piece of ferrous material which is to be phosphate coated is first

sandblasted, degreased and/or pickled to prepare a fresh clean surface for the

protective coating. The material is then immersed in the phosphating bath.

In the first step of the process, the iron surface is attacked by the acid.

This generates hydrogen gas and ferrous (Fe (II)) ions. The bulk acid concen-

tration must be maintained so that local depletion of the hydrogen ion at the

metal surface will be sufficient to effect the precipitation of the metal

phosphates on the metal surface. The overall reaction which occurs is given

by

Fe + 3Zn(H„P0.) - Zn,(P0,), + FeHPO, + 3H,P0, + H. 2 * 3 * 2(5) *(5) 3 H 2

If manganese is present instead of zinc, both the secondary and ternary man-

ganous phosphates will be formed. The resulting protective coating is a mix-

ture of ferrous hydrogen phosphate and ternary zinc (or manganese) phosphate.

The ratio of the salts present on the surface is dependent upon the ratios of

their concentrations of the ions in the plating solution. Most often nitrate

will also be present in the baths. The nitrate in the bath acts as an accel-

erator to speed up the coating process primarily by oxidizing the hydrogen.

Further details of the mechanism are not important here, but can be found in

a study by GiIbert (7)•

B. Composition of Phosphate Baths

Research conducted on the composition of phosphating solution has shown

that there exist optimum concentrations which achieve the maximum corrosion

resistance of the coating while minimizing the time required and the material

consumed. As stated in Rock Island Arsenal Report 54-2906, the bath composi-

tions should be (in order of decreasing importance):

A. Zinc Base Phosphating Solution

Free Acidity 4.0 points maximum*

Total Acid 24-27 points*

Free to Total Acid Ratio Not lower than 1-6.0

Iron (Ferrous) .40 to .50%

Nitrate 2.0 to 2.5%

Zinc .15% to equal the percent iron

Total Metal Not lower than ,50%

Zinc to Iron Ratio 1-1 to 1-1.5

B. Manganese Phosphating Solution

Free Acid 1.3 to 4.0 points*

Total Acid 24-27 points*

Free to Total Acid Ratio 1-6.0 min. (1-6.5 preferrable)

Iron (Ferrous) .20 to .40% (.30% preferrable)

Manganese .20 to .50%

Total Metal .40 to 1.0%

Manganese to Iron Ratio 1-1 to 1-1.5

Nitrate .3 to 1.0%

*0ne point equals 1 ml of 0.1 NaOH per lOcc sample.

From the above specifications it is clear that the acid and iron concen-

trations are the most important. Although it is not explicitly stated in the

specifications, the difference between the total acid value and the free acid

value is a function of the amount of phosphate present in the solution.

C. Control of Bath Composition

Even though the optimum concentrations of the bath have been known for

sometime, a completely satisfactory procedure for controlling the composition

of the bath to meet the optimum values has not yet been developed. Presently,

a sample is withdrawn from the bath and analyzed using wet chemical techniques.

If the analyses show that one or more constituents do not fall within the

tolerance range, chemicals must then be added to correct the composition.

../«../ . I 1-IX

The type and amount of chemical to be added is calculated from predetermined

tables. A typical table is shown here as Table I. This particular table is

for the zinc phosphate solution and was taken from Rock Island Arsenal Report

5^-2906. Immediately the complexity of the control is apparent. To correct

a given deficiency, the proper amount of one or more of five chemicals must

be added. For example, assume that the results of the analysis are:

Free acid 2.0 points

Total acid 23 points

Ferrous .k2%

Zinc .05*

Nitrate 2.2 %

Comparing these values to the optimum range quoted In the previous section,

one finds that: 1) the free acid is under its maximum values; 2) total acid

is less than the minimum permissible; 3) ferrous is within optimum range; k)

zinc content is low; and 5) nitrate is within optimum range. It is, therefore,

necessary to select a control chemical from Table 1 which causes total acid

and zinc to increase while not changing free acid, iron and nitrate. The

closest response which can be found is that for the modified replenisher. It

is noticed, however, that the addition of modified replensiher will also in-

crease the free acid concentration. The free acid is below its maximum allowed

value so a slight increase will be tolerable. Thus, modified replenisher

would be added to increase the total acid to the middle of the optimum range.

The addition of replenisher would hopefully correct the zinc value and not

change the free acid significantly. If, however, analysis after the addition

showed the same or other deficiencies, a similar decision and addition process

would be necessary.

From the above discussion, it can be concluded that the primary problems

of automatic control of phosphate baths are the analysis of solutions and the

complex problem of reagent addition. This study will concentrate on these two

areas and also include discussion of the lesser problems which are more easily

solved, but nonetheless still important to the successful operation of the

system.

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III. Introduction to Automatic Process Control

Encompassed in the area of most automatic process control systems are the

disciplines of chemistry, chemical engineering, mechanical engineering, elec-

trical engineering and computer sciences. A properly designed system will in-

corporate the knowledge and techniques of each of these fields in order to

achieve optimum results.

A. Advantages of Automatic Process Control

A principal advantage of automatic control is that of improved precision

of control. This results in a more uniform product, reducing the time lost due

to producing goods which fail to meet quality standards. A second feature is

that of constant supervision. The operator is then able to perform other use-

ful functions and the process is less subject to fluctuation caused by operator

mental lapses which so often occur when a human is subjected to a repetitive,

uninteresting task.

The principal disadvantage of automatic process control is the time and

expense required for implementation. As will be considered later in this

study, considerable work must be expended in order to consider every detail

of the operation.

B. Basic Elements of the Control System

The basic elements of any process control instrumentation are the sensor,

the comparator and the corrector (Figure 1). A simple example of this mech-

anism is the temperature control of a heated bath. The sensor can be a ther-

mistor or bimetallic spring. The set point can be set to a resistance or

spring position corresponding to a particular temperature. Should the temper-

ature fall below the set value, a contact could close signifying the results

of the comparison. The closed contacts could then be used to correct the

temperature by turning on an electric heater or actuating a steam valve.

The preceding example was a very simple one. Only one variable was con-

trolled and the control function was implemented with analog signals with the

resultant action being on or off. The extension of process control to more

complex systems requires considerably more elaborate equipment. This is

to

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especially true of situations where most or all of the variables are inter-

related. The advent of the small or mini-computer has made economically

feasible many process control applications which were previously too complex

for automatic schemes. The power of the mini-computer in these applications

is four-fold. First, the computer can perform computations very quickly.

Second, the ability to make decisions is an integral part of the computer.

Third, the computer can be interfaced to external devices so that they can be

controlled by the computer. Fourth, the computer is easily reprogrammed to

accommodate any modification which may be desired for the process. These

changes are very difficult and expensive to make in a hardwired controller.

Using the mini-computer, three different possible configurations are pre-

sented (Figures 2, 3 and k). In each case it is assumed that the computer has

been preprogrammed to analyze the data and to perform the decision making

process. In the first, the computer acts independently in its decision making

role (Figure 2). This is an open control loop, since operator interaction is

necessary. The operator must input to the computer the readings from the

sensors. The computer will then output the proper control actions to be taken

and it is left to the operator to actually execute these functions. The

second approach (Figure 3) is to have the computer control the analysis, read

the data directly and perform the necessary calculations. Since the operator

must perform the actual corrections necessary, the control loop is still open.

The final approach (Figure k) represents fully automatic control. The control

loop has been closed excluding the human operator. The computer controls the

analysis, reads the data, makes the decisions and controls the reagent addi-

tion. The human operator is needed only for routine maintenance or the remote

possibility of system failure.

Since the phosphate process is quite complex, the remainder of the dis-

cussion will tacitly assume that a mini-computer will be included in the final

configuration. The specification of continuous automated analysis and control

requires that the computer assume a role as shown in Figure 4. The remainder

of this study will develop in detail the specific interactions which will be

requi red.

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C. Design of a Data Acquisition System

The basic relationship of the computer to analytical instruments has

been described in the previous section. Communication between the devices is

bidirectional. First the computer sends control signals to the instrument

directing it and perhaps assisting in making the analysis. During or at the

conclusion of the analysis the analytical instrument sends data to the computer.

The necessity of communication between the computer and the rest of the system

imposes the stringent restriction that all information must be in the form of

electrical signals (digital or analog). In terms of this analysis, the re-

striction means that titration end points must be transduced to electrical

signals. For example, the computer and analog to digital converter alone

cannot detect the color change of an indicator. If a color change were to be

detected, a spectrophotometer would be necessary to provide an electrical

signal proportional to the intensity of light at a given wavelength. In terms

of sampling and analysis, the restriction means that all valves must be ultim-

ately controlled by electrical command whether directly (solenoid valves) or

indirectly (motor-controlled pneumatic valves).

A more detailed description of the interaction is shown in Figure 5- In

this example, the computer is selecting one of three process streams to be

analyzed. This is implemented by a series of solenoid actuated valves. The

selected stream is directed to the analytical instrumentation. This instru-

mentation may be passive in the sense the computer need only to sample the

data and not control the experiment. Examples would be a thermocouple or a pH

reading. However, for the majority of analyses, the computer must take a more

active role. A typical example of this type is an automatic titrator. This

device must be sequenced by the computer, a chore most normally performed by

the human operator. First, the buret must be filled and the sample must be

moved to the analysis cell. Next the start of the titration must be synchro-

nized with the start of the data acquisition so that the data points of the

titration curve accurately correspond to the volume of titrant addea. Finally,

the computer must stop the titration after the end point and purge the analysis

cell. All of these small steps in the analysis, which the human operator per-

forms as a matter of routine, rrust be designed into the computer interface in

13

STREAM 1 STREAM 2 STREAM 3

CONTRO SIGNALS I I ) I MULTIPLEXER

COMPUTER

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DATA A/D

CONVERTOR

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Figure 5 ~ Block Diagram of Automated Arulysis System

14

order to achieve a fully automated analysis.

In a manner similar to the selection of the process stream to be analyzed,

the computer must also decide which of the signals from the analytical instru-

ments are to be converted to digital data at what points in time. The device

which accomplishes this goal is known as a multiplexer. The multiplexer is

generally designed so that any one of the channels can be selected at random

or the channels will be sequentially selected upon receiving an advance com-

mand. The order and frequency in which the computer accesses each channel

will, of course, depend on the experiment being performed.

In order that the signals from the instruments be of any use to the com-

puter, they must be converted to digital format. This is the function of the

analog to digital converter. Analog to digital converters are available in a

multitude of configurations. The options include: conversion time, e.g.,

from less than lysec to 1 sec; accuracy and precision, e.g., from +25% to

0.01%; and full scale ranges, e.g., > 1 KV to < 1OmV. Selection of a partic-

ular converter will also be based on the needs of the system. A conversion

technique which is particularly useful in slower applications (conversion

time > 1 msec) is the use of an autoranging digital voltmeter. In order to

display digital numbers, an analog to digital conversion must be performed.

The coded outputs of the voltmeter are generally available for external devices

such as printers or computers. Autoranging digital voltmeters are particularly

convenient since they can typically convert analog signals over a range of 1

KV to lmV at 0.01% accuracy with a lower limit of lyV.

The data from the analog to digital converter is transferred to the com-

puter upon command of the computer. At this point, the data is available for

calculations. Further information can be obtained from the operator through

the teletype. Status and action messages are also sent via the teletype to

the operator.

The extension to automatic closed loop control is easily accomplished

from this point. It is only required that the computer control the corrective

addition procedure through the use of pumps and valves similar to those used

for the sampling system.

15

D. System Expansion

One of the advantages of developing an automatic process control system

for phosphating baths which involves a mini-computer is the capability of

extension of the control to more than one bath. The rate at which phosphate

baths need to be monitored is anticipated to be very slow compared to the

speed of the computer and analysis system. Once the system is developed, it

would be a straightforward project to extend the sampling system to sample

more than one solution. It would similarly be a straightforward, but more

complex procedure, to extend the computer programs to control more than one

bath. The point of emphasis is that only one computer system would be neces-

sary. Thus, the majority of both development and hardware costs are eliminated

in extending control to several -baths.

16

IV. SELECTION OF ANALYTICAL SCHEME

This section describes the different methods of analysis which were con-

sidered to determine those most suitable for automation of the phosphate baths.

The methods were evaluated by comparing estimates of simplicity, reliability

and precision.

A. Current Methods of Phosphate Bath Analysis

For control of the phosphating baths, analyses for free and total acid,

nitrate ion, ferrous ion, manganese(I I) ion (for manganese bath) and zinc(II)

ion (for zinc bath) are desirable. The present analytical scheme, as described

in Rock Island Arsenal Report 5^-2906, involves a titration with base using

two different indicators to obtain free and total acid. Ferrous ion is deter-

mined by direct titration with potassium permanganate using the permanganate

color as the indicator. Nitrate ion is determined by a somewhat complex redox

method, while manganese is determined by oxidation to permanganate followed

by titration with ferrous ion. Zinc is determined by a precipitation titra-

tion with ferrocyanide. The most important of these analyses for bath control

appears to be the analysis for free and total acid and ferrous ions.

If modified, some of these currently used techniques serve as a basis

for an automated analytical system. For example, a titration for free and

total acid with potentiometric end point detection and a direct potentiometric

titration for ferrous ion with potassium dichromate as the titrant would pro-

vide quite satisfactory automated techniques. The procedures currently used

for nitrate ion, zinc ion and manganese ion, are somewhat complicated, invol-

ving a number of steps, requiring the addition of solid reagents and heating.

Thus, they do not appear to be suited for automation.

B. Manganese and Zinc Bath Analyses

1. Free and Total Acid

As indicated above, a titration with base using two indicators, methyl

orange - xylene cyanole mixed indicator and phenolphtha1ein, is now used to

determine the free and total acids. In the usual definition, free acid con-

stitutes the concentration of uncombined but hydrated hydrogen ions, while

17

total acid constitutes the sum of the uncombined and combined hydrogen ion

concentrations (as in weak acids). While the former could be measured directly

using a hydrogen ion (pH) electrode (activity is actually measured but concen-

tration can be derived if conditions are sufficiently defined), the only way

to obtain the latter is through titration with base. As long as titration is

required to obtain the total acid, it is no more difficult to obtain both free

and total acid by titration and generally much more accurate. Thus, potentio-

metric titration appears to be the superior technique to obtain these values.

For the manganese phosphate solution, the potentiometric breaks are not

very well defined. Figure 6 shows a titration for a 10 ml sample which was

obtained from Rock Island on the 12th of November. This curve is similar to

those shown by Gilbert (4). Since the samples were somewhat aged and not

heated, the quantitative results of titration are not too meaningful. The

shape of the titration curve may also have been affected, but probably not

significantly. Of particular interest is the pH range for the color changes

of the two indicators in Figure 6. These indicator changes, particularly the

second, do not coincide with the analytical (potentiometric) end point. It

would thus seem appropriate to titrate to a given pH value. For the manganese

bath, the free acid range is from 1.3 to 4.0 points (mis.) while the total

acid has an acceptable range from 2k to 27 points (mis.). Because of the

approximate 3 point range which defines the working concentration for both

free and total acid, an uncertainty of approximately 0.5 ml would seem to be

satisfactory, and uncertainties significantly less than this should be poss-

ible if a titration is done to a given pH value (see curve).

For the zinc phosphate solution, the titration curve is somewhat better

defined (Figure 7) but, as with the previous solution, the indicator does not

change color at the break point of the titration. Thus, titration to given

pH values is preferrable here also. The uncertainty of end point detection

would appear to allow adequate control capability, since the free and total

acid concentration ranges are about the same as for the manganese phosphate

solution. Potentiometric titrations have been shown to be quite readily

automated as described in the next section.

2. Nitrate Ion

18

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Highly selective nitrate ion electrodes are commercially available (8)

for the determination of nitrate ion in the presence of other cations and

anions. One of these electrodes was tested for the determination of nitrate

in the phosphating baths. The potential of this electrode responds to the

activity of the nitrate ion according to the relation

•n = TT° + K In aNQ [1]

where TT° is a reference potential and K is a constant. If the electrode

responds such that K is equal to RT/nF, then the electrode response is

Nernstian. If Nernstian behavior is assumed, differentiation of equation [1]

gives the expression

. _ 0.06 ,AC^ r ,

where the activity coefficients are ignored, and C represents the concentration.

Figure 8 shows a semi-log plot of nitrate ion concentration vs. electrode

potential. The slope of this line if 58mV. Since for nitrate ion n = 1 this

is close to the Nernstian value. From equation [2] we then calculate that

for a 5% change in concentration, the potential will change by 1.3 millivolts.

In actual fact, it is a 5% change in activity, not concentration, so if the

ionic strength of the medium changes, the potential change will be somewhat

different from this. Figure 9 shows a plot of ATT in millivolts vs. (AC/C)

for data obtained by dilution of a zinc phosphate solution. Equation [2] in-

dicates that the slope of this plot should be 26 millivolts. The actual slope

is 27 millivolts. The potential of the electrode is not affected by the pre-

sence of phosphate ion. For example, the electrode potential measured in a -2

10 M sodium nitrate solution was 51mV. Addition of .01 M sodium dihydroger

phosphate did not change that potential, while addition of 0.1 M sodium di-

hydrogen phosphate gave a potential value of 50mV. Because of this repeat-

ability, it is felt that the concentration of nitrate in the zinc bath can be

measured to + 5% relative. The concentration range which is acceptable for

the nitrate concentration in the zinc bath is from 2.0 to 2.5%, corresponding

21

-100

—J

a.

a o a: i— <_>

+50-

+100-

+150 -

+200-

[NaN03] moles/liter

Figure 8 - Response Curve for Orion Nitrate Ion Electrode

22

O >

Figure 9 - Plot of AC/C vs. Air for Nitrate Ion Electrode

23

to a relative range of 25%. It is felt that this precision is probably

sufficient for the measurement. However, some additional testing should be

done to back up this preliminary conclusion. The above concentration range

corresponds to a molarity range from 0.32 to 0.40 M. Because it is recommended

that this electrode not be exposed to high concentrations of nitrate, it may

be necessary to dilute the solution 50-fold in order to improve the measure-

ment. In addition, the slope of the potential vs. log concentration plot in

this range may be somewhat less because of ionic strength effects (see Figure

9).

In addition, since the activity of the ion is measured, the activity co-

efficient must remain constant from solution to solution in order to obtain

concentration from the calibration plot. It is advantageous that the ionic

strength of this bath should not change appreciably for these measurements.

This is realizable since the ionic strength is a function only of the total

number of ions and their valence present, not on any specific ion.

In addition, the electrode shows some response to the hydrogen ion con-

centration at these low pH values (9). If the hydrogen ion concentration

remains sufficiently constant in the bath so this interference can be corrected

for by the calibration curve, or if the response is small, no problem exists.

Data to date suggest this to be true; however, this must be evaluated experi-

mentally in more detail. One of the problems noted with this electrode is

the instability over longer periods of time. A drift in potential occurs

which requires that a calibration technique be added to the electrode operation.

The potential of the nitrate electrode in the manganese plating solution,

although in the right potential "ballpark'1, was found to drift steadily up-

ward with time. The reason for this drift was not determined. Additional

studies will be required to explain this drift and to determine if the elec-

trode can be used to measure nitrate in the manganese bath.

3. Ferrous Ion

The presently used technique for ferrous ion involves the direct titra-

tion with potassium permanganate using the permanganate color as an indicator

of the end point. A modification of this approach which would be suitable

for automation involves the direct titration of ferrous ion using a potentio-

2k

metric end point. No pretreatment of the sample is required. Potassium di-

chromate or eerie sulfate would be preferred for the titratlon since standard

solutions of these reagents are highly stable. The titration curve obtained

with these reagents has a well defined break and a derivative of the titration

curve as obtained by the computer can give a very accurate indication of the

end point. The relative precision of this titration for the ferrous ion con-

centration is expected to be better than 1%.

The concentration range specified for ferrous ion in the zinc bath is

from O.k to 0.5%- For the manganese bath, it is from 0.20 to 0.30%. These

represent relative ranges of approximately 20 and 10%, respectively. Thus,

the precision achieveable by this titration is more than adequate.

A polarographic technique can be used for the determination of ferrous

ion. It has a significant advantage in that either manganese or zinc and

ferrous ion can be determined simultaneously (see below). Unfortunately, the

precision obtainable with this technique is not as good as the titration.

Previous studies have demonstrated that a precision from 3 to 5% could be

readily achieved using automated polarographic procedures. This might be im-

proved slightly. Even the poorer precision could still be used, provided a

proper analysis frequency and control scheme were used. This will be discus-

sed further in the next subsection. It is obvious that if such an approach

were used, the direct titration of iron would not be necessary.

Other techniques could be employed (i.e., colorimetric procedure using

ortho phenanthrophthalein) but would not be as precise and would be equally or

more difficult to automate because of possible additional sample handling.

h. Manganese Ion

A number of techniques have been used for the determination of manganese

(Table 2), but only those which appear to be suitable for automation will be

discussed in detail. The various approaches to manganese determination will

be discussed individually.

(a) Ion Selective Electrode. A technique which has several advant-

ages (if it is feasible for this determination) is the use of an ion selective

electrode. A divalent cation-ion elective electrode is commercially avail-

able. With this electrode, therefore, it should be possible to determine

25

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26 ^3

the sum of ferrous and manganses(I I) cations. No experimental work has been

done with this electrode as a part of this study. Such work Is required to

determine Its ultimate suitability for this analysis. As with the nitrate

electrode, the potential of the electrode Is determined by the activity of the

cations involved. Thus, the same factors as evaluated for the nitrate elec-

trode are important for this experimental program. Such things as ionic

strength effects, uncertainty of potential measurement with subsequent un-

certainty in concentration and nature of electrode response with concentration

will need to be determined. In addition, the selectivity of the electrode to

manganese(I I) ion compared to ferrous ion will have to be determined. With

suitable electrode calibration, the concentration of manganese could probably

be determined directly to +5%. This might be adequate for control purposes

but greater accuracy would be better.

However, the real potential of the divalent cation electrode is not real-

ized in a direct potential determination. A significant advantage is gained

by using the electrode to detect the end points of complexometric titrations.

The divalent cation electrode is not sensitive to EDTA complexes since they

do not have a charge of +2. Thus, although the potential error of direct

measurement may be several millivolts (corresponding to 5-10% change in con-

centration), this error is insignificant compared to the normal potential

change of several hundred millivolts at the end point of the titration

(corresponding to a typical change of concentration equal to 5 or 6 orders of

magnitude). For this reason, the best results will probably be obtained by

using the divalent action electrode for end point detection.

(b) Polarography. While manganese is not the ideal ion for polaro-

graphic determination, it does have some favorable characteristics for deter-

mination. One of the main advantages of the polarographic technique is the

possibility that both manganese ion and ferrous ion can be determined simul-

taneously in the same medium, thus simplifying the analysis. Both normal and

classical polarography, as well as pulse polarography, should be considered.

If significant amounts of ferric ion exist in the bath along with the ferrous

ion, a supporting electrolyte for application of pulse polarography needs to

be selected in which the ferrous-ferric ion couple is somewhat irreversible.

27

For normal or classical polarography, a reversible wave can be used, as long

as the manganese(I I) ion wave is sufficiently separated from the ferrous ion

wave. The concentration of ferric ion which can exist in phosphating baths

has been determined (]Q) and has been found to be highly dependent on the nit-

rate ion concentration and temperature. The greater the nitrate ion concen-

tration, the more soluble is ferric phosphate. In a phosphating bath contain-

ing ]% nitrate, the solubility of ferric phosphate is 0.03 g/1 at 200°F.

This is approximately 1/100 that of the concentration of ferrous ion. At room

temperature, the solubility of ferric phosphate is 1.48 gms. per liter in the

presence of 1% nitrate. However, if a sample is taken at 200°F and it is

separated from all sludge and particulates before cooling, the interference

of ferric ion would still be expected to be negligible. The withdrawal of

adequate samples from the bath without obtaining sludge in the sampling lines

and valves is discussed later. Moreover, it would be expected that the con-

centration of ferric ion would be maintained reasonably constant under all

acceptable bath conditions, because it is determined by the solubility of

ferric phosphate. Therefore, the use of an irreversible wave for iron in the

application of pulse polarography to the determination of ferrous ion does

not seem to be necessary. This needs to be experimentally demonstrated.

Table 3 lists possible candidates for the electrolyte choice for simultaneous

determination of ferrous and manganese ions in the bath. Inspection of the

table indicates that there are several electrolytes which look quite promising

for the simultaneous determination of manganese(I I) and ferrous ions. The

specified range for the concentration of manganese in the phosphating bath is

from approximately .04 M to .09 M. By a fifty-fold dilution of this original

solution, the concentration would be essentially in the optimum range for

polarographic analysis. Because ferrous iron has about the same concentration

range, the determination of both ions would appear possible using the same

dilution.

Previous studies have indicated that it indeed is possible to use a

dropping mercury electrode as an indicator electrode in automated procedure.

However, it is probably the most serious drawback of this technique, and

techniques are under investigation for replacing this electrode with a solid

28

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29

stationary electrode.

(c) Redox Ti trations. Perhaps the most common titrimetric method for

manganese is the oxidation to permanganate with an oxidizing agent, then

titration with either ferrous ion or arsenite. There are, however, problems

associated with the employment of the usual oxidizing agents. Thus, common

oxidizing agents such as potassium periodate, ammonium persulfate, sodium

bismuthate, etc. require a filter step, heating of the solution or some other

complication to the sample handling procedure. It has been stated (11) that

manganese(I I) can be oxidized to manganese(VI I) by bubbling with a stream of

oxygen containing 5% ozone. The excess ozone is removed by bubbling with a

stream of C0« or N„. This technique would appear simpler to implement from

a sample handling viewpoint.

Another titration technique which appears reasonably direct is the

titration of manganese(I I) with permanganate in a near neutral pyrophophate

solution (12). In this titration Fe(III) does not interfere. However, Fe(ll)

would be expected to interfere. If Fe(ll) is stoichiometrically titrated with

the permanganate under this condition, the sum of Mn and Fe(ll) could be

determined. If not, a preliminary oxidation would be required to eliminate

this interference.

(d) Complexometric Titration. Although several direct complexometric

titration methods have been suggested (13), only one appears to be sufficiently

simple to be considered for automation. This involves the titration of Mn(II)

by EDTA and masking the interference of iron by reducing to Fe(II) and com-

plexing with cyanide as the ferrocyanide. However, a heating of the solution

is required (lA,15) for complexing the iron, as well as the desirability of

back titration which makes this procedure rather unattractive.

(e) Colorimetric Methods. In general, colorimetric methods have the

disadvantage that if any other colored specie; are present, a spectrophoto-

meter is necessary to discriminate the different wavelengths. If species

which absorb at the same wavelength or particjlates are present, interference

will be found. The most common colorimetric method involves oxidation of the

manganese(I I) ion to permanganate with a suitable oxidizing agent (i.e., per-

sulfate, periodate, bismuthate, lead dioxide, etc.) and measuring the color

30

of the permanganate ion. The same problems apply here as in the oxidiation

step of the titrimetric method. The solution must be heated when most of the

oxidizing agents are used (exception; bismuthate) or else there is a problem

of separation of the excess of the (insoluble) oxidizing agent. This type

of procedure, therefore, would tend to complicate the automated analysis. The

most common oxidizing agent is periodate (16). However, ferric ions are also

said to slow down the oxidation (17), and ferrous ions will be oxidized before

the oxidation with periodate.

Another colorimetric procedure which has been studied by several workers

involves the reaction with formaldoxime. However, iron interferes and the

masking of the iron is not straightforward (18). Other colorimetric procedures

involve the Mn (Ml) ion in sulfuric acid, the Mn(lll) diethyl dichrocarbonate

complex and the green Mn(til) complex in basic triethanolamine (12), but the

methods are considered of less practical importance, and have not been studied

thoroughly. In addition, interference of iron is a general problem.

5. Zinc Ion

(a) Ion Selective Electrode. As described in the previous sub-

section on manganese, the possibility of using a selective ion electrode is

attractive. There is not an electrode which is specific to zinc, but there

is a commercially available electrode which is highly selective to divalent

cations. These two are the only divalent cations of any consequence in the

bath. The selectivity coefficient for ferrous and zinc ions is reported to be

identical (3-5 to 1) (9). The electrode response is, therefore, the same for

both ions, and zinc ion can be used for calibration. The same variables as

discussed previously need to be considered; namely, possible ionic strength

changes in the bath, electrode reproducibi1ity and stability, both short and

long term (which dictates uncertainty in the measured metal ion concentration),

and the effect of hydrogen ion and phosphate ion. If the sum of zinc and

ferrous ions can thus be determined, (ferrous ion concentration can be deter-

mined separately by titration as described earlier) zinc could then be obtained

by difference. In this manner good precision would still be obtained for

ferrous ion, the more important species for bath control, while the precision

of the zinc analysis would be about that of the total divalent cations.

31

As discussed previously, another use of the divalent cation electrode is

for the end point detection of a complexometric titration. In a later section

the specific details of the analysis procedure will be examined.

(b) Polarographic Methods. As described in the manganese subsection,

if the concentration of ferric ion in the bath is small enough to be ignored,

either pulse or normal polarography could be used to monitor simultaneously

both ferrous and zinc ions even if the wave for the iron couple is reversible.

Pulse polarography has the advantage that it is less subject to interference

(e.g., oxygen), to small changes in the drop time of the electrode, and to

bath conditions (e.g., stirring); in addition, neither the solution at the

electrode nor the electrode is changed as a result of the measurement. Thus,

there is more chance of using a solid electrode with this technique.

Table h gives some possibilities for a suitable supporting electrolyte for

the simultaneous determination of zinc and ferrous ions. All those listed

which show possibility involve only the addition of a single reagent mixture,

and requires no heating, or other sample handling to obtain concentration of

both ions. This obviously has advantages for automation provided the concen-

tration measurements can be made with sufficient precision.

(c) Titration Techniques. The classical and most commonly used

technique for the determination of zinc involves the precipitation titration

with ferrocyanide. This technique is currently being used for this analysis.

The ferrous ion present in the original solution must be removed. This is

currently done by careful titration to the stoichiometric end point with

potassium permanganate. The interference of ferric ion can be eliminated by

use of pyrophosphate ion, and manganous ion does not interfere.

If this technique were used in an automated scheme, either ferrous would

have to be oxidized by stoichiometric titration, or through the use of an

oxidizing agent. The use of ozone as suggested earlier, would seem to have

many advantages. Modification of the presently used procedures to eliminate

addition of solid reagents and heating of the solution would be highly desir-

able and would not appear too difficult. Either a potentiometric (19), amper-

ometric (20), or dead stop end point (21) is preferable and would seem to be

adaptable (Table 5)- This titration cannot be recommended for an automated

32

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ro 4-J 4-J

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x; £ ro in TJ QJ QJ 4-1 i_ .— ro C O in in — O O QJ O i- 3 r> a 2

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scheme with much enthusiasm. The method is empirical and the composition of

the precipitate depends on temperature, concentration of reagent and reactant,

addition rate, etc. (22). A false end point is also apparently common when

both indicators and potentiometry are used. While the former problems might

be overcome by suitably standardizing the procedure, the false end point would

be difficult in an automated analysis.

Complexometric titration with ethylenediaminetetraacetic acid (EDTA) is

also rather common for zinc. Ferric and ferrous ions however interfere. Ferric

ion has been masked by glycerol (23). Ferrous ion can be masked with cyanide

(2^) but heat is required. However, cyanide is not recommended for use in an

automated field analysis, and thui, one is left with an oxidation, then masking.

Moreover, end point detection schemes other than indicators have not been

exploited to any extent, and most work has involved a back titration of excess

EDTA.

(d) Colorimetric Techniques. The classical colorimetric technique

for zinc involves the use of dithizone (25). Many interferences exist.

Zincon has also been used for colorimetric determination of zinc (26), but

iron interferes. Other lesser known reagents (27) have been used, but inter-

ferences are also a problem. In addition, with colorimetric techniques, rather

high dilutions of the bath would be required to use colorimetric techniques

and the attainment of the required precision in the analysis may be a problem.

C. Recommended Method of Analysis (Summary)

There are many factors which must be considered in designing an analysis

system. Among these are accuracy, reproducibi1ity, reliability, simplicity,

maintenance, calibration, and cost of development. This section will describe

an analysis scheme felt to be most suited to this application, selected from

the previously discussed techniques. It hopefully will also indicate how and

why certain techniques were selected when others seem just as satisfactory.

Table 6 lists the methods of analysis in order of preference. Obviously, if

any of the principal techniques prove unsatisfactory when tested in the lab-

oratory, the alternative methods would be developed.

1 . Free and Total Acid

Since these variables are probably the most important, this problem was

35

approached first. The free acid could be determined to within several per-

cent by a direct pH measurement. However, there is only one method to deter-

mine total acid and that is by titratlon with base. Therefore, this method

was selected also for free acid due to the added precision of titration with

potentiometric end point detection compared to a direct potentiometric measure-

ment.

2. Ferrous Ion

Here also the most precise results can be obtained by titration with

potentiometric end point detection. Dichromate or ceric(lV) are more suitable

as a titrant than permanganate due to greater stability. The choice between

dichromate and ceric(lV) cannot be made on paper, but will depend on which

gives the better results in the laboratory.

3. Nitrate Ion

Subject to the results of additional laboratory tests (described earlier),

the nitrate ion selective electrode seems to be the best method for nitrate.

It is simple, fast, and requires only the electrode for equipment, since one

of the other titration vessels can be used as the sample container. Should

this method fail, the colorimetric method for nitrate appears to be the next

best approach.

k. ManganeseQ I) and Zinc(II)

Considerable development and equipment costs would be saved if the same

technique were used for both manganese and zinc. Because of increased simpli-

city in sample handling and measurement, the use of direct potential measure-

ment using a divalent cation electrode to determine the sum of Mn and Fe or

Zn and Fe should be investigated. A calibration procedure may have to be

employed. However, if this measurement cannot be made with the required

precision or if the electrode is too unstable, then a complexometric titration

procedure seems very attractive. Costs are further minimized since the pro-

cedure is titrimetric using identically the same equipment as the analysis for

acid and ferrous ion.

An aliquot of phosphate solution is titrated with a basic EDTA solution

in a vessel containing a reference electrode (SCE) and two indicator electrodes,

(a Pt wire electrode and a divalent cation electrode). Initially, as titrant

36

CM

0)

ro c l_ <D

-C -S= -C Q- Q. CL 03 <D 13 l_ >_ l_ Oi Ol en o o O u 1_ i_

o> TO 0)

O o o O- O- O-

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-o o

-C 4-i <D z:

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E O u

(J

o>

o

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i

o c 1

o i O

o •— O •— — •— 4-1 — 4-J 4-1

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m E c o 4-1 o o Q. O O O- Q- Q_ — *— — H u 4-> 4-< 4-J U ^—* 4-1 O 4-1 CJ

c c <o u .— X > o .— L) — — - <D u o — <D u 4-1 i_ 4-1 X) *>—* i_ 4-J l_ 4-1

0- o — — OJ 01 <D • — 0) — 0) a. \- Q E a£ o Q E Q E

o <

Ol -J

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0) T3 in c <1) in t) 03 4-1 3 c

0> o OJ <u 1- L. o O) 0) 4-1 1_ c c 1_ — 0) • — 03

37

is added, the ferric-EDTA complex will be formed (pKF /.,.\ = 25 - t) - When

all the ferric ions are complexed, the potential of the platinum electrode

will change significantly. The Pt electrode is only required if a significant

amount of ferric ion is present in the bath. As noted before, it is not

anticipated that this will be true. As the titration continues the solution

becomes more basic forming the complexation of ferrous and zinc or manganese

(whichever is present). The complex formation constants are sufficiently

close (pKFe(M) = 14.33, PKMn(M) • '3.79, pkZn(n) = 16.50) so that no break

point will be detected as the individual species are complexed in turn.

However, when the end point for the sum of both species is reached, the pot-

ential of the divalent cation electrode will change significantly. Calculated

titration curves for equal amounts of Fe (III), Fe (II) and Mn (II) are shown in

Figure 10. The zinc or manganese concentration can then be calculated from

the amount of titrant used from the first break point (Pt) to the end of the

titration, minus the contribution due to ferrous ion when it was independently

determined.

Serious interference could be caused by other divalent cations. These

species would have to be at concentrations greater than ]% of the zinc or

manganese. However, from available information concerning bath composition,

the problem seems remote. If such a titration procedures does not prove to

be satisfactory, then a polarographic procedure should be investigated.

38

POTENTIAL

DIVALENT CATION ELECTRODE

PLATINUM ELECTRODE

Figure 10 - Theoretical EDTA Titration Curve for Fe(

Fe(lI), and Mn(I I)

39

III),

V . Sampli ng System

The function of the sampling system can be very simply stated. The

sampling system must deliver a representative sample of accurately known

volume to the analytical instruments. The key words in the previous sentence

are "representative" and "known volume". The emainder of this section will

discuss the problems associated with meeting those criteria.

A. Obtaining a Representative Sample

The first prerequisite for obtaining a representative sample is that the

bath composition is homogeneous. Recognizing that the sludge is not properly

part of the specified analysis, homogeneous is meant to imply that the solution

phase is for the most part in equilibrium. In fact, since particulates are

especially detrimental to the reliable and accurate operation of precision

sampling valves, it is quite probable that a filter will have to be installed

at the intake to the sampling line to remove any particulate matter (sludge)

suspended in solution. Studies by Gilbert have shown the processing of normal

work introduces a local concentration gradient at or near the surface of the

work (k). Thus, sampling near the work would produce biased results and

should be avoided. A further aid to achieving homogeneity is stirring, a

topic to be discussed In a later section.

Another difficulty in obtaining a representative sample is that of time

fidelity. If an hour were required to obtain the sample, the results of the

analysis would only be of marginal validity in terms of making decisions con-

cerning corrective additions. To overcome this problem, a recirculating

sampling line should be used. This would require a pump to circulate the

solution through a pipe from the bath to the analytical instruments back to

the bath. If a 1 cm (i.d.) pipe were used, the linear velocity in the pipe

would be approximately 13 meters/min. per liter/min. of volume flow. Thus,

if the instruments were 40 meters from the bath, the sample would arrive in

3 minutes, flowing at a rate of one liter/min. This would seem quite reason-

able in comparison with the time response of the other bath parameters. The

actual sampling valve would be located very close to the analytical instru-

40

merits to minimize the number of precision components necessary.

B. Precision Sampling Valves

An important criterion which should be used to select sampling valves is

that of reproducibi1ity. This satisfies the goal of delivering a known amount.

It is not necessary that the samplinj valve deliver 10.000 ml, but it is

necessary that the valve deliver the same amount every time to better than ]%.

The computer can easily correct for a sample size of 9-37 ml or 11.04 ml, as

long as the amount is known and reproducible.

Sampling valves are available as standard items from several firms.

Specific valves for this project should be chosen on the basis of: (a) deli-

very volume as required by the analysis scheme; (b) resistance to corrosion;

and (c) reliability and wear resistance. For example, one manufacturer (Carle

Instruments) markets a sampling valve which is particularly well suited to

this application. The valve is constructed of tantalum, Teflon, and gold-

plated stainless steel. The delivery volume, which can be from microliter

to milliliter size, is reproducible to at least 0.1%. It could be argued

that such a corrosion resistant valve would not be necessary for this system,

but that stainless steel would be adequate. However, under certain conditions

phosphoric acid will mildly attack stainless steel. This is not considered

to be a severe problem for the switching valves, but for the sampling valves

no corrosion is permissible.

C. Solution Multiplexing

The sampling system should be designed so that any one of several differ-

ent sample lines can be selected. The important advantage of this approach

is that one or more of the sample lines can be a calibration solution. The

computer can then automatically check the calibration of the analytical instru-

ments at regular time intervals. Another advantage of this approach is that

the system is thus easily expandable to more than one bath. The main hazard

of this design is the possibility of cross contamination of the solutions.

This is not severe for the bath solutions, since the amounts involved are

virtually negligible compared to the volume of the bath. However, calibration

solution reservoirs would be much smaller and thus more susceptible to con-

41

tami nation.

A simplified schematic diagram for a sampling system is shown in Figure

11. The stream selector valve (commercially available) connects one stream

to the sampling line and returns the other streams to their respective sources.

The sampling line passes through a series of precision sampling valves which

deliver an aliquot of solution to the analysis vessels. Due to the possibility

of solution contamination caused by the mixing of solutions in the sampling

line, the exit of the sampling line is treated as waste.

D. Routine Tasks of the Sampling System

The sampling system must be capable of more than just obtaining the

sample. An automated analysis requires also that the normal "housekeeping"

functions of analysis must be performed. Included in this category are such

tasks as: draining the analysis vessel, rinsing the analysis vessel, refill-

ing the motor driven buret for the next titration, and other similar functions.

As an example, an automated analysis of the free and total acid would involve

the following sequence, which would be done by the computer through the sample

handling system.

(1) Add appropriate sample aliquot to cell.

(2) Add water to dilute, if necessary.

(3) Begin titration, adding base from motor driven buret and

monitor pH.

(4) Record volume of solution to predetermined pH for first end

point.

(5) Titrate to second end point and record volume to predetermined

pH for second end point.

(6) Dump contents of cell.

(7) Rinse cell with water (or acid, if necesarry) to remove

precipitate and residual sample.

(8) If acid is required to remove precipitate, rinse with water.

(9) Refill buret from reagent storage.

(10) Start again at (1).

kl

ANALYSIS 1

ANALYSIS 2

ANALYSIS 3

WASTE

Figure II - Schematic Diagram of Liquid Sampling System

43

V I. Control Strategy

The analysis and sampling systems have provided the computer with the

concentrations of each of the important species. The next phase is to decide

what action, if any, should be taken. This step is not as straightforward as

it might appear and can be broken down into three separate problems. First,

it must be determined if any species are outside acceptable limits. Second,

if one or more species are not within the proper range, what reagent(s) should

be added? Third, what is the optimum amount of the reagent (s) that should be

added?

A. Evaluation of Analytical Results

As the first step in the control process, the analytical results must be

compared to the optimum values (Section II.B). This comparison must also in-

clude the error of analysis. Thus, the questions which actually must be asked

are: "Is the measured value minus the analysis error less than the minimum

and/or is the measured value plus the analysis error greater than the maximum

limit?"

It is important to note that for all species there will be a concentra-

tion range including the analysis error which will be within the predefined

limits. This establishes a deadband, i.e., there is no corrective addition

necessary for this species. In this application, the deadband is quite valu-

able due to the problems associated with stirring after an addition (Section

VI I.A).

An important optimization will have to be made when defining the maximum

and minimum concentration limits for each species. It is important to narrow

the range since it is quite desirable that the bath never exceed the limits

as delineated earlier (Section II.B). On the other hand, it is also import-

ant that the range be as wide as possible to minimize the number of additions

to the bath. One of the important criteria which will be used as a basis for

this optimization is a comparison between rate at which the bath composition

changes and the rate at which composition errors can be detected and corrected.

The relative time lags are also important since the stability of the control

loop is dependent upon their interaction.

B. Determination of Optimum Reagent(s)

Having determined that a particular species is out of range (preceding

section), the optimum corrective reagent (s) must be selected. The most

obvious approach would be to define a set of simultaneous equations which

quantitatively describe the effect of addition of every reagent on the total

bath composition. These equations could then be solved to arrive at an exact

solution for any and all bath deficiencies. However, it is apparent from this

study that the phosphating bath is sufficiently complex that all these rela-

tionships are not quantitatively understood and moreover are difficult to

obtain. Therefore, another approach must be found.

The simpler and recommended approach is an iterative method. In this

approach, the computer would be programmed to react in virtually the same

manner as a human operator. For example, if the iron content is low, the

operator would go to the manual (or more likely recall from previous exper-

ience) and determine that either Iron syrup should be added or that the solu-

tion should be artificially loaded, depending on the acid values being low or

high. Thus, the computer could be programmed to go through the same type of

decision making process. An example of a flowchart sequence for the decision

making process is shown in Figure 12. This example is quite simplified and

shows the decision process for only two cases. In the first case, all species

(free acid, total acid, and Fe(ll)) are within range and no control action is

necessary. This is seen from descending vertically from START, through the

OK response for each decision stop and finishing at NO ACTION. In the second

step, free acid is low and total acid and Fe(II) are satisfactory. The L

branch from the first decision is taken, followed by the OK responses of the

next two decisions, thus ending at ADD PHOSPHORIC ACID. Initially these

responses would be based on previous results and he experience o' current

operators of the baths. However, during development work the computer program

could be written so that poor decisions could be recognized and modified as

control continues.

^

<_>

1°

ADD

SP

HO

R

AC

ID

) o n: a.

c o

-a

< c a) 01 <t> (U

en

u o

o o 1-

Q.

c O

o 1)

1-

o

rsi

<0 1-

^6

C. Quantity of Reagent(s) Added

Sections V.A and V.B have discussed two aspects of the control strategy.

The final aspect is that of determining the amount of reagent which is to be

added. This element is equally as important as the first two, since the de-

gree of control established over the bath is dependent on the amount added.

If, for example, the proper strategy is selected, but the quantity is wrong,

the concentrations will oscillate above and below the desired value if too

much correction is made or will only slowly approach the proper concentration

if the quantity added is too small.

Much of control theory has been devoted to the topic of loop gain and

stability (28,29,30). Indeed if there were only one variable in the system,

the problem of control would be very similar to that of the servo system of a

recorder. In that case the pen is moved to a position which minimizes the

difference between an internal potentiometer and the applied signal. If the

gain of the servo system is too high, the pen oscillates. If the gain is too

low the pen response is sluggish and does not quite follow the signal. Ideal

response occurs when the gain is at an intermediate values ("critically damped")

This pattern is quite identical to that described for the phosphating bath.

It is the purpose of this section to describe the basic elements of control

with reference to how they will be used in this application. In no way will

an exact procedure be specified. An approach will be given which specifies

the starting point and indicates modifications which are available to over-

come problems which may arise.

The error signal needed to activate the control strategy will, of course,

be based on the results of the analysis. For the purpose of the following

discussion, the error signal will be represented by AC . AC is derived from

the difference between the measured concentration of a particular species (C )

and the optimum values for that species. For example, Fe (I I) has an accept-

able range of from 0.40 to 0.50% in the zinc phosphate solution, from this

range the optimum value could be established as 0.45%. Based on these values,

AC for Fe (II) could be defined as

C - 0.45% for |C - 0.i»5| >0.04%

and 0 for |c - 0.45| < 0.03% .

47

This definition incorporates both the error of analysis (5% relative is

equivalent to 0.09% absolute of 0.45%) and an anticipation factor such that

an error signal is generated before the concentration exceeds the specified

limits. The region where AC equals zero is the deadband for this species.

With no a priori reason to suspect that the corrections to the bath will

have to be rapid or that control stability will be critical, proportional con-

trol seems to be the best method with which to start. In this method the

amount of correction is directly proportional to the error signal. Thus,

ARt = aACt [3]

where AR is amount to be added and AC is the concentration error. a is a

combination of several other constants and will include: conversion factors

for units change as appropriate, volume of working solution, effectiveness

coefficient (e.g., 1 pound changes concentration by 0.1%) based on existing

knowledge, and an empirical weighting factor. The empirical weighting factor

is an important parameter. It should first be set to unity and then modified

to reflect the relative success or failure of previously applied corrections.

Should the proportional method of control be too slow to respond, a

derivative component can also be added to increase the response. Derivative

control could be used alone, but suffers from the fact that long term drifts

are not adequately corrected, since the correction is based on the difference

between the last error signals. Using a combination of proportional and

derivative control, the correction takes the form

AR = aACt + B(AC - AC ,) [4]

t Before proceeding further, two points should be made concerning this analysis and description. First, for simplicity the discussion will reference only one variable. The extension to more than one variable is possible, but is not necessary for illustration here. Second, due to the nature of chemical control, ARt will be assumed to be positive. This is quite permissible, since in reality the sign of a will change if the reagent raises or lowers the concentration of the particular species under consideration.

48

where AC . refers to the error existing at the time of the previous analysis

and 3 is the weighting coefficient for the derivative portion. 3 contains the

same factors as a, but its final value is found by increasing the value of the

empirical factor from 0 to a value which optimizes response. Two points are

worth mentioning. First, if the value of 3 is too large, wild fluctuation

will occur in the bath. Second, the previously determined value of a will

probably have to be decreased to maintain stability.

The most general form of control is proportional, plus derivative, plus

integral. This is expressed as

k AR = aAC + 3 (AC -AC ) + Y £ AC [5]

k=0

where y is defined as 3 and k is the "reset" frequency and is determined

empirically. The integral component corrects for changes in loading which

might otherwise cause long term drifts of concentration which are uncorrect-

able by the other components.

The computer programs developed for the project should be constructed

with form three; proportional, plus derivative, plus integral. At the begin-

ning of the test period a best guess is made for a, 3, and y. As the experi-

mental results become available, the values of the coefficients would be

appropriately modified. This approach would be much more efficient than try-

ing to add, say, an integral component at a later time.

Another problem which has not been considered here, but must be a part

of the development work, is that of initial start of a bath. This may re-

quire a separate program, since the normal program for control will probably

require that the bath concentrations are close to their working range.

^9

VII. Reagent Addition

The problem of reagent addition is a crucial one to the ultimate goal of

continuous analysis and control of phosphating baths. Several factors compli-

cate this section of the system immensely. One problem is stirring to achieve

homogeneity of the baths and another is the physical form of the currently

used correcting reagents.

A. St i rring of the Bath

Regardless of the final form of reagent addition used, the problem of

bath stirring will have to be solved. From available information (4,6), it

appears that the poor phosphate coating produced by a stirred bath is not

caused by the agitation. Indeed this is what is expected after observing the

agitation caused by hydrogen evolution during the course of applying a good

protective finish. The problem seems to be caused by the dispersion of sludge

through the solution which becomes entrapped in the coating mechanism.

Irrespective of whether corrective reagents can be added while material

is being processed or not, a baffle will have to be designed and added to the

bath. Even if the reagent is added between processing, the sludge should not

be dispersed because of the time required for resettling. The baffle must

have the properties that: a) allow the sludge to settle to the bottom of the

tank, and b) permit sufficient interaction between the solution and the sludge

so that equilibrium or quasi-equi1ibriurn is maintained. One possible solution

to the problem is simply to mount a series of slats just above the bottom of

the tank. The sludge could settle to the bottom and if the stirring propellor(s)

were directed at approximately right angles to the slats, the dispersion of

the sludge would be minimized.

The method of stirring (propellors, turbines, etc.) will have to be de-

signed in conjunction with the design of the baffle. This is particularly import-

ant since the effectiveness of the baffle is dependent upon the direction and

velocity of the solution which is moving around it.

B. Addition of Reagents

Just as it was necessary to develop new methods of analysis which are

50

compatible with continuous automated analysis, it may also be necessary to

modify the reagent addition scheme currently in use. For the majority of

cases, the corrective reagents are compatible with automated control (6).

However, there are several notable exceptions as described below.

The reagents which are easy to dispense are homogeneous liquids such as

phosphoric acid, replenisher and manganese and zinc nitrate solutions. If the

mechanical equipment (pump, pipes, and valves) is constructed of the proper

material (e.g., stainless steel, Monel , etc.), it is a straightforward process

to deliver a known amount of compound to the phosphating bath. The delivery

can easily be controlled by solenoid valves actuated by the computer.

The main problems of reagent addition occur in the cases of slurries

(e.g., manganese carbonate) and irregular solids (e.g., steel wool, sheet shot

or scrap iron). In these cases the most cost effective approach is to develop

suitable liquid substitutes rather than design additional equipment to handle

the present reagent form. The development of suitable reagents will be

greatly simplified throujh the use of the continuous automated analysis equip-

ment developed as part of the program. The analysis will be available almost

immediately after equilibrium has been established or before equilibrium, if

that is appropriate.

According to current procedures, it is anticipated that the reagents will

be at room temperature when added to the baths. The main advantages are that

no cost is necessary to preheat the reagents and that many of the reagents

(those with phosphates) have a higher solubility at room temperature. It also

seems reasonable to assume that the relative volume of reagent added versus

the total bath is small enough so that no substantial temperature gradient

wi 11 occur.

A further advantage of an all liquid reagent addition scheme is that it

is easily expanded. After the initial shakedown period when one bath is being

controlled, the delivery system could be expanded to several more baths by

adding suitable feeder pipes and solenoid valves. Additional major invest-

ments would thus be avoided.

51

VIM. Miscellaneous Considerations

In addition to the major problems of automatic analysis and control of

phosphating baths discussed in Sections III to VI, there are several other

variables which also must be controlled.

A. Temperature Control

Temperature control of the bath is very important. The best coatings are

obtained if the bath is operated at temperatures over 200°F. However, if the

temperature exceeds the boiling point of the solution, the composition balance

of the solution is destroyed. Apparently, the changes which occur upon boiling

are so great as to render the solution virtually useless. This necessitates

removing much or all of the solution and adding fresh chemicals to restore the

bath to satisfactory composition. This is expensive in terms of both materials

and downtime.

It is, therefore, necessary to install or implement automatic temperature

controllers on the bath. Since thermostats and Fenwal type temperature con-

trollers are readily obtainable and have histories of proven performance; they

would be the most economical and reliable method of temperature control. It

is important that the data acquisition system include a thermocouple to moni-

tor the temperature of the bath and provide a record of the bath temperature.

An alarm could then be sounded if the temperature of the bath were outside the

acceptable limits. The main point is that temperature control is more suitably

achieved if the computer is not included in the loop, but merely monitors the

performance of the controlling device.

B. Solution Level Control

Current phosphating baths have not been provided with any form of solution

level control. The operators have been relied on to add water to supplement

rinse water in maintaining the solution level against constant evaporation

losses. Furthermore, the phosphate baths do not have drain or overflow pro-

visions. This presents an additional consideration for automatic control.

A level sensor will have to be included in the final configuration of the

bath. Unlike temperature control, level control should involve the computer.

52

Several factors must be considered. A deadband should be included to allow

for the volume change when work is immersed in the bath. A decision should

be made to determine whether corrective reagent additions made will be suffi-

cient to raise the level back to within limits or whether water should also

be added.

It would be most feasible in the automation project to just add a level

sensor to the bath. The sensor should provide a direct measure of level of

the bath. A possible choice for the sensor would be a float connected to a

slide wire potentiometer to give a voltage signal proportional to the liquid

level. If the preliminary tests indicate that an overflow provision Is

necessary, it could be designed and implemented at that time.

53

Site Considerations

The environment of a typical phosphating installation as

set up at Rock Island Arsenal, not unlike other process en-

vironments, is unsuitable from the point of view of process

control instrumentation. The temperature range would be in

excess of 100°F during the summer months and could probably

drop to 50°F - 60°F during the cold winter months. Also, the

area immediately surrounding the bath has a high humidity due

to evaporation loss from the baths operating at 200°F. The

ambient air could also contain corrosive vapors from phos-

phate baths, chromic acid baths, and pickling operations not

to mention sandblasting and degreasing operations occurring

nea rby.

Needless to say, the analytical and process instrumenta-

tion should be located in a more controlled environment if

optimum results are to be obtained. As instrumentation is

currently envisioned, it would occupy very nearly the same

amount of floor space as that currently provided for the

analysis of the bath solutions. Some additional space would

also be required for carboys containing the analytical reagents

and waste materials. Several limitations do arise from this

choice of location being approximately 75 feet from the baths.

However, neither seems to present a great difficulty.

The first problem caused by the distance between the

baths and analysis instruments is that of transporting the

sample. Several solutions to the problem exist. The first

solution is to run the sample lines to the control room in a

stainless steel tube covered with an insulating material such

as asbestos. This introduces a time delay equal to the length

of tubing divided by the flow velocity. However, this would

5k

not seem to be a problem since the composition of the bath

is estimated to change over a longer period of time. If

cooling of the sample during transportation presents a

problem, the transport lines could be steam or electrically

heated to maintain the temperature. The steam could be ob-

tained from tapping into existing facilities. If necessary,

return lines could also be heated in a similar fashion to

avoid any gradients in the bath. However, in view of the

relative quantities involved, this appears to be a very re-

mote pos s i b i1 i ty.

The second problem caused by distance is that of the

transmission of electrical signals. Typically, minicomputer

manuals indicate that peripheral devices should be located

within 25 or 50 feet of the computer. This restriction is

not applicable here since it applies only to pulse type

signals in which the rise time must be less than 100 nsec.

The signals which would be required to go from the computer

to the baths are d.c. levels or level changes which can have

rise times on the order of msec or more. This is permissible

since these transition times are small compared with the

time required to actuate solenoid valves.

55

X . Calibration and Maintenance

One of the main criteria for judging the performance of an automated

\ control system is reliability. It is extremely important that the system be

capable of operating for a reasonable period of time (week, month, etc.) with-

out operator intervention. It is difficult to predict a priori the mean time

between failure of various components. The best approach is to select the

most reliable components when constructing the system.

The routine calibration of the analytical instruments should be done auto-

matically and at specific intervals. Suitable calibration solutions could be

made which would be stable and yet simulate, as close as possible, the actual

phosphate solution conditions. These solutions would then be circulated

through the sample line and analyzed identically to the unknown phosphate

solutions. The results of the analysis would be used to update the normal data

analysis program. In addition, if the standardization data showed drastic

changes from previous results, the operator would be notified and/or an alarm

would be activated.

Routine maintenance for the automated system would not require signifi-

cant amounts of time or material. Typical maintenance tasks would, for

example, include: a) restocking the solutions used for the chemical analysis

and calibration; b) inspection of valves, pipes, and pumps for leakage and

corrosion; and c) lubrication of pumps, fans, etc. as necessary. Also, if

filters were required in the sampling and analysis system, they would also

have to be inspected and changed at regular intervals. The computer and

electronics will most likely be the most reliable components of the system.

Maintenance, other than periodic recalibrat ion, would not be required for

these components except in the case of failure.

56

References

1. Coslett, T. W., British Patent 8,667 (1906) and U. S. Patent 870,937

(1907).

2. Allen, W. H., (to Parker Rust Proof Company), U.S. Patents 1,167,966

(1916) and 1,291,352 (1919).

3. Tanner, R. R. and Darsey, V. M., (to Parker Rust Proof Company), U. S.

Patent 1,888,189 (1933).

4. Gilbert, L. 0., Rock Island Arsenal Report 56-2995 (1956).

5. Gilbert, L. 0., Rock Island Arsenal Report 57"2536 (1956).

6. Gilbert, L. 0., Rock Island Arsenal Report 54-2906 (1954).

7. ASM Handbook Committee, Metals Handbook, Vol . 2_, American Society for

Metals (Menlo Park, Ohio) 1964, pp. 531-547-

8. Orion Research, Inc. Catalog.

9- Personal communication, Orion Research, Inc.

10. Chamberlain, P. G. and Eisler, S., Rock Island Arsenal Report 53-638

(1953).

11. WiI lard, H. H. and Merritt, J., Ind. & Eng. Chem. Anal. Ed. J_4, 482

(1942). . .

12. Copper, M. D. and Winter, P. K., in 'Treatise on Analytical Chemistry",

edited by I. M. Kolthoff and P. J. Elving, Vol. 7, Pt. II, Interscience,

New York, 1961, p. 464.

13- Schwarzenbach, G., Complexometric Titrations, Interscience, New York,

1957, PP. 614-615-

14. Welcher, F. J., "The Analytcal Uses of Ethylenediaminetetraacetic Acid",

A. Van Nostrand, Princeton, New Jersey, No. 5, 1958.

15- Flaschka, H. and Puschel , R., Z. Anal. Chem. 143, 330 (1954).

16. Sandell, E. B. , Colorimetric Analysis, 3rd Ed., Interscience, New York,

1959, P- 605.

17- Ibid., p. 608.

18. Ibid., p. 612.

19. Kanzelmeyer, J. H., in "Treatise on Analytical Chemistry", edited by

57

I. M. Kolthoff and P. J. Elving, Interscience, New York, 1961, Vol. 3,

Pt. II, p. 156.

20. Woodson, A. L., et. al., Anal. Chem. 2k_, 1 198 (1952).

21. Swinehart, D. F., Anal. Chem. 23_, 380 (1951).

22. Kanzelmeyer, J. H., loc. cit., p. 125.

23. Krleza, F., Chem. Abstracts 6_7, 39860p.

2k. Flaschka, H. and Puschel , R., Z. Anal. Chem. ]k3_, 185 (1956).

25- Sandell, E. B., loc. cit., p. 9^5 and p. 9^7-

26. Adachi, T. , et. al., C.A. 6]_, 17582x (1967) •

27. Horiuchi, Y., et. al., C.A. 6^, 7367** (1968).

28. Lee, T. H., Adams, G. E., and Gaines, W. M., Computer Process Control,

J. Wiley 6 Son, New York, I968.

29. Elgerd, 0. I., Control Systems Theory, McGraw-Hill, New York, 1967•

30. Perry, R. H., Chilton, C. H., and Kirkpatrick, S. D., ed., Chemical

Engineer's Handbook, McGraw-Hill, New York, 1963-

58

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61

unclass ITiea Secu rit£ Classification

DOCUMENT CONTROL DATA -R&D (Security claaalllcatlon ol till: body ot abatract and indaxlng annotation mutt b* antarad whan tha overall rmport It claaallladj

I. ORIGINATING ACTIVITY (Corporatm author)

North American Rockwell Science Center 1049 Camlno Dos Rlos Thousand Oaks, California 91360

2*. REPORT SECURITY CLASSIFICATION

UnclassIfi ed lb. CROUP

a. REPORT TITLE

A FEASIBILITY STUDY FOR THE CONTINUOUS AUTOMATED ANALYSIS AND CONTROL OF PHOSPHATIZING BATHS (U)

4. DESCRIPTIVE NOTES (Typa ol raport and Inelualwa daft)

o AUTHORISI (Flrat nama, middla Initial, laat nama)

Edward P. Parry R. Lee Myers

B REPORT DATE 7a. TOTAL NO. OF PACES

62 7b. NO. OF REFS

30 Sa. CONTRACT OR GRANT NO.

DAAF01-72-C-0130 b. PROJEC T NO.

Pron Al-1-5005^-(01)-Ml-Ml e.

AMS Code 4497.06.7037 d.

Sa. ORIGINATOR'S REPORT NUMTBER(S)

SC523.1ER

Bb. OTHER REPORT NO(S) (Any othar numbara that may ba atalgnad thla raport)

SWERR-TR-72-11 10. DISTRIBUTION STATEMENT

Approved for public release, distribution unlimited.

II. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

U. S. Army Weapons Command Res., Dev. and Eng. Directorate Rock Island, I 11 inois 61201

13. ABSTRACT

The feasibility of automating production phosphate coating process baths has been studied under the guidance of the Research Directorate of the Weapons Laboratory at Rock Island. The study comprised a detailed examination of the analysis and control requirements. Con- tinuous automated analysis and control of phosphate baths are feasible. Analytical methods, sampling system procedures, and control logic techniques are presented for incorporation into a prototype system. (U) (Parry, E.P. and Myers, R. L.)

1

DD rout I MOV •• 1473 Unclassified

Security Classification

Unclass1 fled Security Classification

14. KEY WORDS

LINK A LINK C

1 . Phosphat ing

2. Automated Analysis

3. Automated Control

k. Phosphatizing Baths

Unclassified

Security Classification

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FEAS,B>UTY STUDY FOR THE CONTINUOUS · 4. Manganese(lI) and Zinc(ll) 36 V. Sampling System 40 A....

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Transcript of FEAS,B>UTY STUDY FOR THE CONTINUOUS · 4. Manganese(lI) and Zinc(ll) 36 V. Sampling System 40 A....