52

Porous Plug Gas Injection in Anode Refining Furnaces

P. Goyal, S. V. Joshi, and J. Wang

SUMMARY

The technical feasibility of using bottom injection of gases for anode refin­ing has already been established. In the present work, data have been collected using industrial-size porous plugs in a laboratory induction furnace. The data have been compared to predictions made by a thermodynamic model and are scaled up to industrial operation.

INTRODUCTION

Gaseous refining of anode copper has been commercially practiced since the late 1950s by Phelps Dodge Corporation. I ,2 Although this was a revolu­tionary improvement over the prevalent practice of wood poling, the earli­est literature on using hydrocarbon contained in fuel oils dates back to 1934. Hulme et al,3 invented a process for injecting atomized oil by means of nitrogen gas under pressure through the bottom of the refining vesssel. Most major primary copper producers have adopted some kind of injection of raw or reformed hydrocarbon fuels or ammonia.

Several variations of anode refining techniques have been developed, patented, and practiced by the industry over time.4 ,9 The reagents used for the poling step during anode refining have generally been air and steam­reformed natural gas, propane, or ammonia. IO The efficiency of reagent utilization ranges from 20-40%. Figure 1 is a sketch of a typical anode refining furnace with side tuyeres. Some of the drawbacks of conventional anode refining practices are: high capital cost of steam reformers; expen­sive reagents, e.g., ammonia, poor mixing efficiency; over-oxidation during desulfurization; and emission of soot during the use of raw hydrocarbons for poling.

Kellogg and Hendersonll have discussed the energy costs of competitive copper manufacturing processes. The development of oxygen smelting technologies has reduced the smelting and converting energy consumption to almost half. Therefore, the relative energy use for anode refining has increased from 10% of the total energy required to produce copper to 20%. One reason for the high energy consumption during refining is the poor efficiency of reagent utilization. Goyal et al. 9 presented an alternative highly efficient anode refining process using bottom injection of gases in the cylindrical anode furnaces via a number of porous refractory plugs. The technical feasibility was demonstrated using laboratory developed po­rous plugs.

In this paper the authors have evaluated the feasibility of using alumina porous plugs encased in a steel shell such as those used by the steel industry for ladle stirring. An industrial size porous plug was installed at the bottom of a laboratory induction furnace and desufurization experi­ments were conducted. The data obtained were compared to predictions made by a thermodynamic model. Using thermodynamics and the concept of mixing-energy (energy resulting from injection of gas in liquid metal) extrapolations were made for the volume and flow rates of refining gases, primarily nitrogen and hydrogen, and the number of gas injection points. An economic analysis of the proposed practice is also presented.

APPARATUS

Figure 2 is a schematic of the experimental reactor used for the anode refining tests. An Inductotherm 15-kW induction generator was used to power a hand furnace containing a 30-lb clay-graphite crucible 9.25 in. deep and 5.25 in. in diameter (internal dimensions). An industrial-size porous plug was inserted through the bottom of the furnace and through a 3.25 in. diameter hole at the bottom of the crucible such that a 1.4 in. of the plug was above the bottom of the crucible. Heat loss through the top of the crucible was minimized by placing an insulating refractory brick over

JOURNAL OF METALS· December 1983

TAP HOl.E

the top of the crucible and sealing the sides with Fiberfrax. Three holes were drilled in the insulating refractory brick to accommodate a thermocouple, a quartz gas-sampling tube, and a copper sampling device. A typical commercial installation of the porous plug is shown in Figure 3.

Figure 4 is a schematic of the entire experimental apparatus. The re­agent gases-nitrogen, hydrogen, and air-were injected into the copper melt through the porous plug. Gaseous nitrogen and hydrogen were sup­plied from gas cylinders, while air came from the laboratory compressed air supply. The gases were fed into a control panel equipped with pressure regulators and flow meters at a pressure of 40 psi. The gases were then maintained at a pressure of 30 psi to be mixed and injected through the porous plug. The reagent gas composition was obtained by introducing the desired gases at a predetermined ratio.

A chromel-alumel thermocouple assembly, in a quartz tube sheath, served as the sensing and as the controlling thermocouple. The thermocouple assembly was placed through one of the holes of the refractory brick and held at a point such that an inch of the tip of the thermocouple was submerged in the copper melt. The temperature was displayed on a digital readout and also on an analog Omega temperature controller. A control panel contained the switches to engage or disengage the temperature con­troller to the induction generator.

The product gas above the copper melt was sampled through a quartz tube placed through the second hole of the refractory brick such that the end of the tube was 2 in. below the roof of the crucible. The product gas was then withdrawn through a water adsorption train and then through a thermoconductivity gas analyzer. The gas analyzer used a thermistor and a wheatstone bridge circuit to compare the thermo conductivity of the sample gas to the reference gas. A continuous record of the gas analysis was obtained by connecting a strip chart recorder to the gas analyzer. The gas analyzer was standardized with nitrogen gas and calibrated by using a mixture of 2% S02 in nitrogen. Toggle valves on the sample gas lines permitted rapid switching between standardizing the gas analyzer and sampling either the reagent or product gas. An additional toggle valve allowed for simultaneous operation of a second analyzer such as an oxygen analyzer.

The third port on the refractory brick was used for taking copper samples. Copper wire samples were obtained by dipping a sealed evacuated glass tube into the melt by means of a 12-in.-Iong tube holder. At the end of the glass tube, there is a thin-walled bulb which melts very quickly when inserted into the copper melt. The vacuum in the glass tube draws up the molten copper, which upon quenching with water, forms a solid copper wire sample. This third port was sealed with Fiberfrax and opened only during sampling.

PROCEDURE

Twenty-five pounds of blister copper (supplied by the Noranda Quebec smelter) were placed into the crucible. The copper was melted and heated

JOURNAL OF METALS • December 1983

Figure. 1. Anode refining furnace.

... SETTING, I'l l!; F"iIIIoIICTO"'t

COIL.S

. ,. SE;TTIN(;o IIIHtltACTORY

POilllOU$ I "LUG ASS£MILY

PIIIODUCT GAS

lHEIIIMOCOUPL.E ASSEMllY

METAL SAMPL..I G TI,I'E AND ttOlot'"

INSUlAnNG "'EFIIIACTO"'Y ."'leK lCi POUND CUY.GIIIAPt+ITr CIIIUCIIlE

CEIIIAMIC AOHE$IYt;:

A511£$T05 PAPe ..

PACJ(ED PlUIII.-.CTOIllY

- POIlltOUS C!IIIIAMIC CONE

"" .-.SIESTO S SHEET

......... STEEL FililANIE

10.0SUINLI!!$S STEEL.. CASIN'G

IIItUGENT GAS

Figure 2. Experimental reactor.

MORTAR

Figure 3. Commercial installation of porous plug.

53

Figure 4. Schematic of experimental apparatus.

54

METAL SAMPLING TUBE HOLDER

TEMPERATURE I~I CONTROLLER/INDICATOR ~ - THER - SAMPLE TUBE

II COUPLE 1S KW : :

INDUCTION .........--. FURNACE "

INDUCTION GENERATOR

FLOW CONTROL VALVES

COMPRESSED AIR

REA

TOGGLE VALVES

SULPHUR DIOXIDE

BOTTLED GAS

CALIBRATION GAS

VACUUM PUMP

to 1150oe, while introducing 4.5 scth of nitrogen through the porous plug to prevent the pores of the plug from clogging. The temperature was maintained to ± 100 e by an Omega temperature controller operating on the chromel-alumel thermocouple assembly.

Hydrogen and air were used as the reagent gases to control the initial oxygen content of the copper melt. Hydrogen was injected with nitrogen serving as a diluent gas until a characteristic green flame appeared at the top of the crucible indicating a deoxidized melt. A calculated volume of air was then injected to raise the dissolved oxygen content to the desired level. It was found that the hydrogen reducing efficiency was almost 100%, while oxidation efficiency with air was about 70%.

The initial sulfur content was adjusted by adding copper sulfide to the molten copper bath. Fourteen grams of copper sulfide powder was sealed in a short copper tube and dropped into the 25-lb melt. The dissolved sulfur content was expected to be approximately 200 ppm. Thus by adjusting the initial oxygen and sulfur contents, blister copper was simulated.

Desulfurization experiments were conducted by varying the initial sulfur and oxygen content of the copper melt. Nitrogen was used as the reagent gas to de sulfurize by the reaction between dissolved sulfur and oxygen. The flow rate of nitrogen (20.5 scth) was determined and adjusted to obtain suitable mixing by observing the amount of splashing of the melt.

The thermoconductivity gas analyzer connected with the strip chart re­corder provided continuous sulfur dioxide analysis in furnace offgases dur­ing desulfurization experiments. The gas analyzer was also used to com­pare hydrogen content of the offgases to that of the reagent gas stream during deoxidation experiment. This served as a double check for determin­ing the end of deoxidation.

In addition to the offgas analysis for sulfur dioxide, copper wire samples were also taken every 2 min throughout the experiments until there was no appreciable sulfur dioxide in the offgases. The samples were then ana­lyzed for sulfur and oxygen content.

The gas flow rate through the porous plug was as low as 4.5 scth at 30 psi during melting of the copper. Even at this low flow rate there was no appreciable seepage of the molten copper into the plug. At the end of a set of experiments, the porous plug was removed and sectioned into 114 in. slabs. Some copper was found in the top 114 in. slab directly in contact with the melt. This copper gets blown into the crucible when the charged copper is molten and the gas flow is started.

ANALYSIS OF THE DATA

The overall chemical reactions taking place during anode refining may be represented as:

JOURNAL OF METALS· December 1983

Oxidation:

Reduction:

or

02(g) ~ rolcu

[Slcu + rolcu ~ S02 (g)

[Olcu + H2 (g) ~ H20 (g)

[Olcu + 112 CH4 (g) ~ 112 C + H20 (g)

(1)

(2)

(3)

(4)

Based on the available data,12,13 the following equations may be written to represent equilibrium between the various gases and the molten copper bath during desulfurization:

log P~02 = 3~5 _ 0,529 _ 1,1~7.8 (%0) _ 76;.8 (%S)

+ 2 log (%0) + log (%S)

0.58 _ 7,~88 _ 62i·6 (%0) 48~.2 (%S)

+ log (%S)

(5)

(6)

Let the partial pressure of S02 in the gases coming off the bath be related to the equilibrium partial pressure by the following equation:

PS02 = P*S02 • Feq (7)

where Feq represents the fractional equilibrium attained between the exiting gas and the bath. If Feq is equal to 1, the two phases are in complete equilibrium and in all the other cases (Os Feq <1), the depar­ture of Feq from the value of 1 may be taken as an indication of the severity of mass transfer limitation. Assuming ideal gas law and assuming that nitrogen and sulfur dioxide are the only gases coming off the molten copper bath, as nitrogen is neither absorbed into or evolved from the metal, it can be easily shown by mass balance considerations that:

_ PS02 Vso2 - VN2 --- (8)

I-PS02

The above equation assumes that the total system pressure is essentially 1 atm. By mass balance of sulfur

W d(%S) = Vso2 x 32 x 100 dt 379

Combining equations 8 and 9,

d(%S) = 84 V PS02 dt . N2

I-Pso2 1

W

(9)

(10)

Where, VN2 is the volumetric flow rate of nitrogen in scfm and W is the weight of the molten copper. The value of PS02 can be calculated by using Equations 5-7. Equation 10 is valid for desulfurization by a pure nitrogen blow, and similar equations for air or air/nitrogen mixtures can easily be written.

Equation 10 is a first order non-linear differential equation, and with the proper initial conditions was numerically simulated by means of a computer.

Predictions of bath composition with time for several starting composi­tions of blister copper were made using the model. A range of values for Feq was tested from 0 to 1. The experimental data were then compared statistically with the model predictions at various values of Feq. The best agreement between data and prediction was found to be at Feq = 0.55, indicating that the gases achieved only 55% equilibrium in the metal bath. Figure 5 shows results of the experiments with different starting oxygen compositions. The solid lines are predictions made by the computer assum­ing 55% equilibrium and the symbols represent experimental data.

DISCUSSION AND EXTRAPOLATION TO COMMERCIAL SCALE

In the experiments reported here, the gas bubbles traveled a distance of approximately 14 cm through the melt and 55% of the equilibrium be­tween the exiting gases and the molten bath was achieved. The percent approach to equilibrium for the industrial baths that are much deeper (3-6 ft) may be estimated based in the analysis of Themelis and Schmidt,14 as follows:

1 (1 PS02) - K Ab Z n --- - tv P*S02 b

JOURNAL OF METALS • December 1983

(11)

200

150

! 100 !!:.

50

--- - MODEL PREDICTIONS AT Feq ~ 0.55

•••• 0 - EXPERIMENTAL DATA

10 12 14 16

DESULFURIZATION TIME (MINUTES)

Figure. 5. Sulfur content versus desulfuriza­tion time, thermodynamic model versus ex­perimental data.

55

FLOWAATE.. 0 (NM'/min/l)

D.Ol 002 D.05 .. 0.2 • • 5 ., . :>.. 5 .•

.00

50

I •• I I I I I I I I I

: LAB CONDITIONS

.00 200 500 .000 2000 5000 .•. 2 "- '0' S 10' 10'

MIXINC ENERGV, i:. ,Wit'

Figure 6. Bottom gas flow rate (Q) at 14200K and metal depth = 150 cm.

Figure 7. Porous plug-Injected anode furnace.

56

Where K t is the mass transfer coefficient, Ab and Ub are the area and the terminal velocity of the gas bubble, respectively, and Z is the depth of the molten bath. The extrapoltation of the experimental results to the industri­al scale operations by means of Equation 11 indicates that near complete equilibrium between the exiting gases and the bath can be achieved on the industrial scale.

The work done by a gas bubble injected into a melt consists of two components: one relates to the displacement of a liquid "column" as the bubble rises, and the second to the expansion of the bubble as the hydro­static pressure around it diminishes . Nakanishi and Szekely15 have defined the concept of mixing energy per unit time or the power dissipated in mixing the bath as the product of work done by a rising bubble times the number of bubbles injected per unit time.

The relationship between mixing power input and the gas flow rate, as calculated by Themelis and Goya}l6 is as follows:

Pm = 2 (i1A) RT In (1 + Pm • g • Z/Pa) (12)

A useful operating factor is the ratio of mixing power input per ton of molten bath.

E = Pm _ .JL RT Wm - 2 (22.4) Wm In (1 + Pm • g • Z/Pa) (13)

Where: Pm Mixing power input (J/s)

Q Gas flow rate (lis)

R Universal gas constant = 8.314 (J/mole OK)

T Temperature (OK)

pm Density of molten metal = 8.96 (g/cm3)

g Acceleration due to gravity = 980.6 (cm/s2)

Z Metal depth (cm) Pa Atmospheric pressure = 1.016 x 106 (dynes/cm2)

Wm Weight of metal (tons)

"Mixing time," T, is defined as the time required to attain complete mixing in a stirred bath. Nakaniski and Szekely15 also correlated the concept of mixing time to the mixing power input per unit weight for several steel systems. Their correlation is expressed as follows:

T = 800 E - 0.4 (14)

Where, T Mixing time (s) E Mixing power input (W/ton)

JOURNAL OF METALS· December 1983

The authors have used this correlation to predict gas flow rates required for a commercial bottom-blown anode refining furnace. The gas flow rate of the laboratory experimental results was used as the basis for extrapolation. As shown in Figure 6, the mixing energy in the laboratory experiment was about 1,650 W/ton. This translates to an equivalent gas flow rate of 0.113 Nm3/min/ton for a commercial anode refining operation with a metal depth of 150 cm and mixing time of 42 s. Compared to the reaction time for anode refining operation, the mixing time is insignificant and hence is of academic interest in this application.

Based on the experience of steel industry, with bottom-blowing technology, thermodynamic equilibrium data and less time constraint on a typical anode furnace practice, we expect that a gas flow rate of 0.068 Nm3/min/ton, corresponding to mixing energy of 1,000 W/ton and mixing time of 50 s will be adequate to perform an efficient refining practice. For a typical 300-short-ton melt this gas flow rate would be about 40,000 ft3. An aver­age 30-ft-Iong anode refining furnace equipped with 6-8 porous plugs along the bottom (as shown in Figure 7) would require a gas flow rate of 100 ft3/min/plug.

ECONOMICS

By using the equilibrium model discussed previously, anode refining costs for using nitrogen and hydrogen are calculated. Figure 8 shows the computation of refining costs for various combinations of starting oxygen and sulfur impurity concentrations. The X and Y axis represent starting sulfur and oxygen concentrations. The Z axis represents the total cost of using nitrogen and hydrogen to bring the final sulfur and oxygen levels to 20 ppm and 800 ppm respectively. The cost surface shows a mInImUm refining cost for starting oxygen contents between 2,000-3,000 ppm at a range of sulfur values.

Figure 9 is a cross section of the cost surface at a fixed sulfur concentra­tion of 400 ppm (typical industry value). As seen in this figure, the minimum refining cost occurs between 0.2-0.3% 02. At lower oxygen impu­rity levels the partial pressure of sulfur dioxide is very low. Large volumes of nitrogen are therefore required to oxidize the desired sulfur impurity resulting in a high nitrogen cost. When the oxygen impurity is high, although the partial pressure of sulfur dioxide is high and the nitrogen cost for sulfur removal is low, the cost of using pure hydrogen for deoxidation becomes excessive.

Similar cost curves can be generated for any combination of refining gases, e.g., hydrogen-nitrogen mixtures and the optimum cost process can be selected.

CONCLUSIONS

It was established through laboratory experiments that commercially available porous plugs are suitable for use in the anode refining furnace to substitute for the low efficiency side tuyeres.

Desulfurization of simulated blister copper containing varying amount of dissolved sulfur and oxygen using pure nitrogen instead of air was found to be a technically feasible process. The test data were compared to the predicted values from a thermodynamic model. At the flow rate employed, only 55% of equilibrium was achieved in the melt depth of 14 cm. Extrapolation of laboratory data using Themelis and Schmidt's analysis showed that in a commercial anode refining process, bottom-blown reagent gases will achieve near thermodynamic equilibrium with the copper melt.

Based on the concept of mixing energy, it was found that adequate stirring could be provided in an anode furnace by using approximately 40,000 scth reagent gases through 6-8 porous plugs located at the bottom of the furnace.

Computation of the economics of using nitrogen and hydrogen for anode refining showed that the new process is an economically viable alternative for blister copper with certain specific range of impurity concentration.

References 1. L. Klein, "Gaseous Reductionof Oxygen-Containing Copper," J. Metals, 13, 181 (19611 pp. 545-547. 2. C. R. Kuzell, M. G. Fowler, D. L. Klein and J. H. Davis, "Gaseous Reduction of Oxygen-Containing

Copper," U.S. Patent No. 2,989,397, June 20, 1961. 3. P. M. Hulme, et a!., "Process for Refining Copper," U.s. Patent No. 1,965,190, July 3, 1934. 4. H. L. Jandras, "Method of Producing High Purity Copper Castings," U.S. Patent No. 3,470,936, October 7,

1969. 5. M. C. E. Bell, and J. K. Pargeter, "Refining Copper Pyrometallurgically to Two Stage Subatmospheric

Treatment," U.S. Patent No. 3,767,383, October 23, 1973. 6. J. B. Brinkman, et a!., "Oxygen Control in Continuous Metal Casting System," U.S. Patent No. 3,987,224,

October 19, 1976. 7. J. A. Dantzig, et a!. "Method and Apparatus for the Degassing of Molten Metal," U.S. Patent No. 4,235,627,

November 25, 1980.

JOURNAL OF METALS· December 1983

COST 25 (S/TON)

U

SULFUR 1%)

0.6

Figure 8. Anode refining cost using nitro­gen and hydrogen.

2.7

2.'

2,5 •.. Z- .. , ~ 2.2

..: ' .1 '" 0

" 2 .•

1.'

I.' 1.7

1.1 r

0.2 0.3 •.. 0.' D.' OXYGeN (%)

Figure 9. Cross section of anode refining cost using nitrogen and hydrogen at fixed sulfur concentration of 400 ppm.

57

Pradeep Goyal,De­velopment Engineer, Air Products & Chemi­cals, Inc. , Allentown , Pennsylvania 18105.

Mr. Goyal received his bachelor ' s in metallurgical engin­eering from the Indian

8. K. Hidaka, and Ken-Ichi Yoshimura, "Operat ing of Refining Furnace for Copper Smelting," Japanese Patent Office Application No. 1971-39,321, October 18, 1980. 9. P. Goyal, N. J. Themelis and W. A. Zanchuk, "Gaseous Refining of Anode Copper," J . of Meta ls, 35 112)

11982), pp. 22-28. 10. J . E. Foard and R. R. Beck. "Copper Smelting-Current Practices and Future Development." AIME Annual Meeting, New York, 1971. 1l. H. H. Kellogg, and J. M. Henderson, "Energy Use in Su lfide Smelting of Copper," in Extractive Metallurgy of Copper, edited by J. C. Yannapoulos and J. C. Agrawal , The Metallurgical Society of AIME. New York. (1976), pp. 373-415. 12. A. Yazawa and T. Azakami. "Thermodynamics of Removing Impurities During Copper Smelting," Can. Met. Quarterly, 8 (3) 11969), p. 257. 13. K. Sana and H. Sakao, "Thermodynamic Considerations for the Experimental Resu lts of the Equilibrium Between Liquid Cu and CO-C02 -S02 Gas Mixtures," J . Japan Inst. of Metals, 19, (1955). p. 655. 14. N. J. Themelis, and P. R. Schmidt, "Deoxidation of Liquid Copper by a Submerged Gas Jet," Trans. Me t, Soc. of AIME, 239 (1967). p. 1313. 15. K. Nakanishi . T. Fugii, and J . Szekely. "Possible Relationships Between Energy Dissipation and Agitation in Steel Processing Operations." Ironmaking and Steelmaking, 3 (1975). pp. 193-1 97 . 16. N. J . Themelis and P. Goyal, "Gas Injection in Steelmaking: Mechanism and Effects." paper presented at the 20th Annual Conference of Metallurgists, Hamilton . Ontario. August 1981.

ABOUT THE AUTHORS

Shriram V. Joshi,Air Products & Chemicals, Inc., Allentown, Penn­sylvania 18105.

Dr. Joshi received his bachelor's in chemi­cal engineering from the Indian Institute of Technology, Bombay,

Jimmy C. Wang, Graduate Research Assistant , Henry Krumb School of Mines, Columbia Uni­versity, New York , New York 10027.

Mr. Wang received

Institute of Technology, Kanpur, India, and his master's in metallurgy from the Massa­chusetts Institute of Technology. As devel­opment engineer in the Applied Research and Development Department of Air Products, he is responsible for new indus­trial gas applications for the metallurgy industry. Mr. Goyal is a member of The Metallurgical Society of AIME.

India, and his doctorate from the University of Delaware. He is currently responsible for new industrial gas applications in high-tem­perature processes. He is a member of The Metallurgical Society of AIME.

his BA in biochemistry from Columbia College in 1981 . He is com­pleting his graduate studies at the Henry Krumb School of Mines at Columbia Uni­versity and will receive his MS in mineral engineering in 1984. He is a member of The Metallurgical Society of AIME.

58

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JOURNAL OF METALS· December 1983