A study of the preparation andproperties of the ferrites of copper

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Authors Thompson, Alvin Jerome, 1903-

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A STUDY OF THE PREPARATION AND PROPERTIES OF THE FERRITES OF COPPER

W

, Alvin Jerome Thompson

. . v

Submitted in partial fulfillment of the . requirements. for the degree of

Master of Seienee in Metallurgy

in the College of Mines and Engineering of the

1933

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Chapter BageI. Introduction........ 1II. Apparatus, Analytical Methods, and

Materials used....................... 2III. Experimental Work.... ............... .... 8

Preparation of Copper Ferrites..... 8IV. Formation of Cupric Ferrite Under Differ­

ent Conditions of Time and Temperature 16V. Experimental X-ray Methods............... El

Determination of the Crystal Struc­ture of the Ferrites....... .. 28

VI. Magnetic Properties of Ferrites.......... 28VII. Density and Solubility Determinations.... 21VIII. Summary and Discussion.............. 25

90847

ACKIIOV/LEDGIJEIITS

The writer wishes to acknowledge the help and supervision of hr. F. S. Wartman, Assistant Metallurgist, U. S. Bureau of Mines, in all phases of the work and to express his appreciation for the cooperation he has shown during the year.

To Dr. T. G. Chapman, head of the Department ef Metallurgy, and Dr. miter Seller, Professor of Physics, whose time and assistance have been so freely given; and to members of the U. S. Burma of Mines staff, all of whom assisted in making it a pleasant year, the writer also extends his thanks.

A STUDY OF THE PROPERTIES OF COPPER FERRITES

CHAPTER I-- -

Ferrites are certain combinations ef ferric oxide with other metal oxides formed in either the wet or dry way. Some are definitely known to be chemical compounds while the nature of others has not yet been established. Among the ferrites known to exist may be mentioned those of calcium, eepper, sad- alum, Iren, lead, magnesium, manganese, nickel, potassium, silver, strontium, and zinc. Calcium ferrite and zinc ferrite, the former bemuse of its importance in the manufacture of ce­ments and the other because of its influence on the roasting and leaching of zinc ores and concentrates, have been the eeb- jeots of considerable experimental study.

Copper ferrite, like zinc ferrite, has also from time to time attracted some attention in certain fields of copper metallurgy. The study of copper ferrite, unlike that of zinc ferrite, has not, however, resulted in definite determinations of its properties and therefore the properties of this iron- copper combination are relatively unknown.

Copper ferrite is described by t!Orley and I M r 1 as a brown­ish black magnetic solid having the formula: CuO.FegOg; they prepared the compound by precipitating the oxides of copper and

^Watt's Dictionary of Chemistry,vol.II, p. 547:Longmans, Green and Co., Hew York, 18##.

iron in a potassium lyrdroxide solution and heating in vacuo.' 2According to Forestier» copper ferrite is a stable and strongly

magnetic substance. The lattice dimensions and Curie constant of copper ferrite have been determined by Holgersson3 and by Holgersson and Serres.4 The properties of copper ferrite as described comprised in essence the published data available to the writer before starting the experimental work described in this paper.

J. D. Sullivan, formerly of the United States Bureau of Hines staff, studied the formation temperatures of copper fer­rite.5 In this study Sullivan assumed the existence of a chem­ical .soaps®#*. Sullivan's study of copper ferrite under differ­ent conditions was not completed, but his data and suggestions formed a basis for further experimental work reported in this

%orestier, H., Magnetic Transformation of the Sesquoxides of Iron, of its Solid Solutions and of its Ferromagnetic Com­binations: Annales Chemie, vol. 9, pp. 316-401, 1928.

3Holgersson, Sven, Rontgenographic Examinations of the Min­erals of the Spinel Type, and of Synthesized Substances of the Spinel Type: Lunds. Univ. Arsskriff, U.F., Avd. 2; 23, 9, 1929.

4Holgersson, Sven, and Serres, A., Magnetic Properties and Crystalline network of the Ferrites: Comptes, rendus hebdomed- aires des seances de 1'academic its sciences, vol. 191, pp. 35-7, 1930.

5Sullivan, J. D., Preparation and Properties of Copper Fer­rites: Unpublished report of U. S. Bureau of Mines, 1930.

CHAPTER II.— APPARATUS, ANALYTICAL METHODS, AND MATERIALS USED

The results reported in this paper were obtained chiefly -by X-ray, chemical, and magnetic methods. The copper ferrites used were prepared by heating mixtures of chemically pure cop­per (cupric or cuprous) and iron (ferric) oxides in one of three Hoskins electric resistance furnaces. Two of these fur­naces were muffle type FA and FD, capable of withstanding tem­peratures up to 1,000°C. The third was a type FH combustion tube furnace which could be operated to a temperature of 1,S00°C.

Temperature measurements were made with platinum and plat­inum-rhodium thermocouples, using a Leeds & Horthrup potenti­ometer iadleator, or an optical pyrometer of Leeds & Horthrup manufacture. Both instruments were accurately calibrated and were considered to give results correct to within 5°.

The produetiea of the copper ferrites required extended experimental work and their preparation is described in a later chapter.

X-ray diffraction patterns were obtained from an X-ray diffraction apparatus of the UWC type, manufactured by the General Electric Co.

The photo-densitometer used in conjunction with the X-ray machine is of an original design by Dr. Walter Seller of the

Physios Department of the University of Arizona, and operates by means of a photo-electric cell and mirror reflection gal-

vanometer to make a record on bromide paper. It is essenti­ally the same as that sold by the General Electric Go. for smeh work.

Copper determinations were made by a modification of the short iodide method.6 With but few exceptions the samples for analysis dissolved readily in hydrochloric acid. It was found that after insuring complete oxidation of the iron and copper to the higher states by adding a little potassium chlorate, it was only necessary to make alkaline with ammonia and reacidify with hydrochloric acid, adding about 10 drops excess, before proceeding with the regular titration using potassium iodide and sodium thiosulphate. If iron were present, 3 grams of sodium fluoride were added before adding the potassium iodide. The method was rapid, and, on this particular type of sample at least, checked very well with the standard methods.

Comparative results as obtained on the original test runs are given below2

SampleHo. •Character of sample !Method of analysis 22

Copper, per cent

1#:Mixed oxide of Cu : and Fe

:1$

Electrolytic222

23.70

1 : Do. 2 Modified method 2 23.652 %

:Ferrite plus oxide ; mixture

12$

Standard iodide12 2

21.602 2 Do.

22$

•e. 2:

21.50

Determinations on duplicate samples ordinarily checked to

one-tenth of one per cent.

bSoott, W. V/., Standard Methods of Chemical Analyses, vol. I, 3rd edition, p. 194i D. Tan Hostrand Co., Hew York, 1922.

Determinations of the amount of ferrite present in a sam­ple were made by dissolving out the free copper oxide in a potassium or sodium cyanide solution and, assuming the undis­solved copper to be in the form of a ferrite, calculating the ferrite percentage. Sullivan, in his preliminary work on cop­per ferrite, tested the reproducibility and rapidity of the solvent action of sodium cyanide on CuO in the pressnoe of CuQ.FegOg and FegOg. His conclusion that 15 minutes was ample time for treatment of a 250-mg. sample with sodium cyanide solution in order to dissolve the free CuO from the mixture was verified as being true for all practical purposes.

A sample of cupric oxide, prepared by precipitating cup­ric hydroxide from solution and Igniting at 820°C., completely dissolved in a boiling solution of sodium cyanide in 5 minutes.

Samples of a mixture of copper and iron oxides heated for 1 hour, which were treated with sodium cyanide solution for 15, 30, and 60 minutes, gave an analysis 3.4, 3.5, and 3.6 per cent soluble copper, respectively, the total copper content was 26.25 per cent.

Another sample of the precipitated oxide mixture after heating to 800°C., was treated with sodium cyanide solution for 1 hour, and analyzed. Then it was given another cyanide treatment for about a half hour and reanalyzed. Both analyses gave the copper content of the residue as 25.4 per sent.

7A recent article in the Engineering and Klning Journal

^Avetislan, C. K., Composition of Copper IMte: Eng. and Min. Jour., Deo., 1932, p. 627.

has diocussei the solubility of copper minerals in cyanide solution. Among the compounds considered, copper ferrite and ohaleopyrite are listed as the only compounds of copper in­soluble in cyanide.

The procedure adopted for making cyanide soluble analyses m o as follows:

50 c.c. of a solution containing 80 grams of sodium cyan­ide per liter was poured over the sample. The beaker contain­ing the solution was covered with a watch glass and the solu­tion allowed to come to a gentle boil. After 50 minutes' boiling, the cyanide solution, whlsh had dissolved that part of the copper originally present as free copper oxide,was sep­arated from the residue by filtration. Both the filtrate and residue were, in most cases, analyzed for copper.

Sodium and potassium cyanide have been used throughout the experimentation as both analytical and purifying reagents. Cyanide-treated residues as referred to in the text have been treated with a sodium cyanide solution and are assumed to be free from uncombined copper oxide. Ferric oxide, of course, remains unaffected.

In the analyses of mixtures of cuprous and cupric fer­rite, no thoroughly reliable method for differentiating the two compounds was discovered. An indication of the amount of cuprous ferrite present was obtained by dissolution of the sample in strong E d in an inert atmosphere, with subsequent dilution of the solution with 300 c.c. of cold water and

?

titration with standard diohromate, using diphenylamine as an internal indicator. Results of analyses so made have been reported as bivalent iron, because it was at first believed that the reducing substance titrated was formed as a result of reduction of part of the iron content to magnetite. Inter work showed the redmelng substance to be cuprous oxide or cuprous ferrite.

CHAPITER III. --EXPERIMENTAL WORK

Preparation of Copper Ferrites

With regard to the production of cupric ferrite for ex­perimental work the writer believed that the screen sizes of the oxides used and the temperatures employed in treating the oxides determined, at least to some extent, the properties of the resulting ferrite. Since Sullivan used grain mixtures of oxides in forming cupric ferrite, the writer dedlced to form the ferrite under the most favorable conditions with respect to grain size and intimacy of mixing, and for these reasons used precipitated oxides.

A quantity of Baker1o analyzed ferric chloride (FeClg.®SgO) was dissolved in water and the solution analyzed for iron. Similarly, Baker's analyzed copper chloride (CuClg.EHgO), in an amount which would approximately yield one equivalent of copper to two equivalents of iron, was put into solution and the solution analysed for copper. The two liquids were mixed in sueh proportions as to yield a mixture of the correct pro­portion of copper to iron for the formation, of cupric ferrite (CuO.FegOg). The method of using weighed amounts of the chlor­ides was not relied upon, because of the variable analyses found in such hydrated compounds.

9

Hjriroxidee of eopper and iron were precipitated from the eolutien mixture by a strong potassium hydroxide solution. The total alkaline solution used was added at one time, and in such quantity as v/ould precipitate all of the copper and iron as hy­droxides and only allow for a very slight excess.

The precipitate was washed by decantation until the wash water was free of chloride, as indicated by the silver nitrate test.

The precipitate was then dried in an electric oven at 120°C. and contained 23.70 per cent of copper and 40.95 per cent of iron. A certain amount of water of hydration was apparently retained at the temperature used, since pure oxides would have given copper and iron contents corresponding to that expected from a mixture of one mol of CuO and one mol of FegOg or 26.87 per cent of copper and 46.68 per cent of iron. Portions of this oxide mixture (later referred to as mixture 4} were heated to various temperatures and for varying time periods, as shown in Table 1, in order to study the effects of these variables in the formation of ferrite. A discussion and interpretation of the data in this table will be found in a later chapter.

Table 1.— Copper Ferrite Data for Mixture Ilo. 4

Samplelie.

Tempera­ture of heating, °C.

If

ll

ll solublecopper

Fe as femmus -iron

Per centef f eiUteformed

Per cent Degrees throw on spring (magnetic balance)

23 465 1 day 24.4, 42.2 16.1 0 31.2 1.73 2923 465 2 days 25.1 4 M 13.7 0 42.9 1.73 3724 465 5 days 25.8 44.6 12.0 0 51.9 1.73 4625 465 13 days 25,9 44.8 10.3 0 53.1 1*78 3828 655 1 hour 25.95 44.8 8.1 0 07.3 1.73 8429 655 2 hours 26.0 48*0 7.7 0 68.8 1.75 8633 055 4 hours 26.0 45,0 7.6 0 69.2 1,73 8350 655 8 hours 26,0 48.0 6,0 0 fS.8 1.73 84SS 655 12 hours . 36.0 45.0 7.0 0 71.4 &«** 8126 655 1 day 25.95 45.2 6.5 0 73.2 1.74 95m 655 4 days 26.1 45.4 6.0 0 73.8 1.74 8634 655 14 days 36.2 45.7 7.2 0 * w 7511 820 1 hour 20.4 45.5 4.0 0 84.4 1*72 11412 820 4 hours 26.35 48.8 3.7 0 85.2 1*73 13920 830 10 hours 26.35 45.5 3.5 0 85.8 1.73 ' . 15110 820 26.35 45*5 3.5 0 85.8 1*73 1489 820 67 hours 26.3 45.5 s*? .1 88.8 1.73 23519 930 1 hour 26.25 45.4 3.5 .1 85.6 1*72 225If 930 2 hours 26.3 45.4 3.4 .1 86.2 1*72 22215 930 9 hours 28.4 45.5 3.2 .1 87.2 1.72 23316 930 26 hours 26.5 45.7 2.8 .2 89.2 1.72 24718 930 48 hours 26.4 45.7 2.8 .4 88.8 1.73 24837 1,040 1|2 hour 26.4 48.8 .# •*0 97.0 1.72 22435 1,040 8 days 25.4 46.4 1.0 5.5 92.5 1.82 236SS 1,040 11 days 25.3 46.6 1.0 5.2 92.1 1.84 2368 1,130 1 hour 28.6 46.2 5.2 8.0 81.8 1.73 2397 1,180 8 hours 26,5 46.0 6.0 8.0 77.8 1.73 227

11

For the purpose of making density, magnetic susceptibility, melting point, and solubility determinations, cupric ferrite was prepared in a somewhat different manner from that already deserihel. The fact that the above mixture at no time yielded a product in vhioh the copper and iron were quite as high as they should have been had only pure oxides been present was in­terpreted as being due to the presence of small amounts of chlorides, fhe greater readiness with which nitrates are washed free from precipitated oxides or hydroxides led to the follow­ing method of preparation.

Yielding rods of the highest grade of pure iron were dis­solved in nitric acid. Such carbon as the iron contained was evolved as a gas, and the iron went into solution as ferric nitrate. Electrolytic copper foil of a similar standard of purity and in suitable amount was then dissolved in the same nitric acid solution.

Three separate mixtures, numbered 1 to 5, inclusive, were prepared, each containing approximately 100 grams of iron. In the first mixture the copper used was such an amount as would exactly form the euprio ferrite, if there were complete reac­tion between the oxides. The mixed hydroxides of the two metals were precipitated with a dilute solution (7 per cent) of sol­ium hydroxide, added slowly and with constant and vigorous stirring. An excess of sodium hydroxide was avoided, as cop­per hydroxide is slightly soluble in alkaline solution.

12-

V/ashlng by decantation v/as carried seven or eight washes past the point where a sulphuric acid solution of diphenyla- mene sulphate gave a test for nitrates. Drying the washed precipitate at 120°C. for two days in the electric oven re­sulted in a black slag-like product which was still partly hydrated.

It was found that complete reaction did not take place when mixture Ho. 1 was roasted. A second batch containing a large excess, corresponding to 1.8 mols of CuO to one of FegOg, of copper oxide was therefore prepared. It was hoped that such an excess might force all of the iron into ferrite combination. After roasting, the excess CuO could be removed since cuprle oxide dissolves in a number of reagents which do not affeot either the ferrite or ferric oxide. On the other hand, no solvent for the ferric oxide has been found which will not dissolve the ferrite also. Differential solvent ac­tion must, therefore, be applied to the cupric oxide, and any uncoabined ferric oxide will remain as an impurity In the fer rite.

The third preparation of this series contained only a slight excess of cupric oxide. The results of roasting the three mixtures are presented in Table 2.

Table 2,— Results cf Ferrite Formation txoa Different Oxide Mixtures Roasted to Vasioto TgaTOraturca

MixtureTtee ef beating, turs of

heating,Ce

Total copper, per cent

Totalixoa,

per cent

KCSHsol— uble copper, per cent

It of KOI residues ;e + PogOgJ

Copper Iron For cent ferrite

1 1 700 26.2 46.9 5.0 22.4 50.1 84.21 1 800 26.3 4¥.0 1.0 25.4 * 7.7 95.51 72 800 26.3 47.0 1.0 25.4 * 7.7 95.51 1 900 26.3 * 7.0 0.0 25.3 47.2 97.01 1 1,000 26.3 * 7.0 0.7 25.6 47.5 96.32 1 700 S3.® 36.4 23.0 49.6 86.52 1 800 38.1 36.5 24.0 48.7 S0.42 1 950 33.1 36.5 SS.S 48.4 92.33 1 700 23.0 44.3 17.7 54.1 66.63 1 900 28.2 44.9 25.3 47. S 95.2

-14-

5?he writer believes that the decrease in the amount of ferrite formed with increasing cupric oxide content is to a large extent due to a tendency of the oxides to segregate em precipitation when the cupric oxide is in excess. One small batch made with a large excess of cupric oxide gave a distinct layer of this oxide after several washes of the precipitated mixture. This same mixture, after redissolving and repre­cipitating, gave a homogeneous product which yielded a cyanide treated residue after heating to 900°C. which ran 25.4 per cent of copper. The second precipitation was made by adding potass­ium hydroxide very slowly to a large volume of dilute solution and stirring constantly and vigorously. Under these conditions the precipitation of the hydroxides seemed to be delayed until nearly sufficient alkali had beta added to precipitate all iron and copper; then the precipitation occurred almost Instantaneous!; This feature of the preoipitatiea was more noticeable and more delayed in the first case, where the CuOiFegOg ratio was 1:1, than in any of the others.

The product of mixture 3, which was roasted to 900°C. for 1 hour, was ground to pass a minus 350-mesh screen and roasted again for 1 hour at the same temperature. It was thought that if the failure of some of the oxides to combine was due to

lack of contact, a reorientation of the grains by finer grind­

ing and further mixing might effeet further combination. *e increase in ferrite formation resulted.

All of the heated products of mixtures 1 and 2 were re­heated with copper oxide added as minus 550-mesh powder in quantities sufficient to make the total copper to iron ratio 5:1. The cyanide-soluble residue obtained from the reheated mixture contained the same excess of ferric oxide, indicating again that contact of ferric oxide with copper oxide was not in itself sufficient to cause complete reaction.

Samples of cuprous ferrite mere prepared by heating pow­dered cuprous oxide and powdered ferric oxide together at 1120°C. An excess of cuprous oxide corresponding to one for­mulae weight mas used. Baker's analyzed cuprous oxide, most of which would pass a 200-mesh screen, was the source of cuprous oxide. The ferric oxide was a minus 200 mesh product prepared by J. D. Sullivan when he began his work on ferrites. Be precipitated Fe(0H)g from Fe3 (304)3 with HB4OH and ignited the precipitate after drying. It analyzed 69.75 per cent iron as against 69.94 per cent in pure FegOg.

The heating was continued for 15 minutes after the furnace reached 1120°C.; then the product was taken from the furnace and immediately quenched in water.

The excess cuprous oxide readily dissolved in potassium cyanide solution and the residue gave an analysis of iron and copper which checked within two-tenths of one per een that theoretically required to form CugO.FegOg.

t of

CHAPTER IV.— FOPJ.IATIOH OF CUPRIC FERRITE UHDER DIFFERENT CONDITIONS OF TIME AND TEMPERATURE

• Table 1 gives the results of the analyses made on temper­ature and time heatings of mixture No. 4.

NON-soluble copper represents the copper in the sample which did not unite with the iron oxide present* to form cupric ferrite. The residue from the KCN treatment contained in each case the ferrite formed in the roast plus the excess oxide of iron. The percentage of ferrite formed was calculated by sub­tracting the KCN-soluble copper from the total copper and cal­culating this difference into its equivalent amount of ferrite.

Iron in the bivalent form was estimated by dissolving the sample in hydroohlorio acid in a nitrogen atmosphere and titrat­ing with dichromate, using diphenalamine indicator. As ex­plained previously, the amounts of bivalent iron reported do not represent bivalent iron in the sample but equivalent amounts of univalent copper. X-ray patterns of samples of cupric fer­rite which have been heated for long periods of time above 1,000°C. have shown, besides the cupric ferrite lines, the lines of the cuprous ferrite, and indicate a pronounced ten­dency for the cuprio ferrite to change to the cuprous ferrite at high temperatures. Below l tOOO°C. the change is but slight, but nevertheless persists to some extent down to 900°C.

tim aetml quantities of cuprous ferrite originally pres­ent are not represented in the table, since all of the samples therein reported were air cooled. Water quenching of samples resulted in a slightly higher ferrous iron content end furnace cooling almost entirely eliminated it. Table 3 lists a few typical analyses.

Table 3.— Effect of Rate of Pooling on Ferrous Iron in the Final Sample (Samples taken from Mixture Ho. 4)

: Temperature ! : Method : Fe asSample: of heating of i of : ferrousHo. : °C. s heating * treatment : iron

: :1/4 hour

$ :40 $ 1,110 i • quenched 2 8.2

t i1/4 hour

I 141 t 1,110 i 2 air cooled 1 6.1

i s $ 235 $ 1,040 i 8 days : air cooled : 5.5

: 2 s 238 $ 1,040 I 8 days s furnace cooled 2 .1

$ 1. I %43 2 1,043 $ 8 minutes 2 air cooled 2 3.5

2 2 1 244 $ 1,043 i 8 minutes ; furnace cooled .2

: i % :

Samples of cupric ferrite heated above 1,OGO°C. and then stirred or remixed before any annealing treatment have shown much less reversion of cuprous ferrite back to the cupric fer­rite. It seems probable that the change either way takes place most rapidly at the surface, and if this surface layer which has become the most altered is scattered throughout the charge, as by remixing, the reversion to the original form is much hindered. The small difference which exists between the bivalent iron content of samples 35 and 43 above, one of which

18

has been heated for 8 minutes and the other for 8 days. Is Interpreted In the same manner.

As mieht be expected, heating cupric ferrite to high tem­peratures in an oxygen atmosphere under pressure decreases the amount decomposed. Heating at 1,200°U. in a sealed quartz tube containing oxygen at 350 mm. pressure at room tempera­ture, with subsequent quenching in water gave a product con­taining univalent copper equivalent to 5.3 per cent bivalent iron. At this temperature the dissociation pressure of thesystem 4CuO(S) — — SCngO(3) * Og (g) is given by Roberts and

fiSmyth as 19.3 atmospheres.In column 8 of Table 2 is shown the variations in the

amounts of ferrite formed with variations in the temperatureand time of heating. It will be seen that the percentage offerrite formed increases with the temperature of heating up toabout 1,0Q0°C», or the temperature at which cupric oxide begins

oto decompose. At 1,040 the decomposition pressure of cupricoxide in the system GuO(S) --- Cug0(3) ♦ Og (g) as given byRoberts and Smyth is 196 millimeters of mercury,9which is well above the partial pressure of oxygen in air at atmospheric pressure. It is, therefore, to be expected that at and above this temperature cuprous oxide will to a large extent be formed.

--------- " ■ ■ ' -----Roberts, II. S., and Smyth, F. H., Jour. An. Che*. See.,

1920, vol. 42, p. 8888.9Lo o. cit.

The foot that small amounts of ferrous iron have been de­tested in analyses of samples heated to as low a temperature as 930° (samples 18 and 16 in Table 1) is, however, not in accord with dissociation pressure data on either ferric or cupric ox­ide. The dissociation pressure2-0 of cupric oxide at 900° is given as 12.B millimeters of mercury and the dissociation press­ure of ferric oxide at 1,100° is only 5.3 millimeters of mer­cury.1* * It does not necessarily follow, though, that the dis­sociation pressure of cupric ferrite is no higher than these of the compounds from which it is formed.

The length of heating at any given temperature has only a limited effect on the ferrite formed. At any given temperature the percentage of ferrite in the sample appears to approach a limiting value as if attaining equilibrium. To test for this possibility a number of the products formed were reheated at lower temperatures in order to observe if there would result a decrease in the total ferrite. A slight decrease in some eases was noted, samples 70 and 17, but as further heating did not result in further reduction, its significance has been given no interpretation. Table 4 lists a few of the results obtained.

10Roberts, H. S., and Smyth, P. II., Loc. eit.*1Ralston, 0. C., Iron Oxide Reduction Equilibria: Bull.

296, U. S. Bureau of Mines, 1928, p. 50.

fable 4.--•Changes Produced In the Per cent of Ferrite by Reheating at Lower than Formation Temperature

Sample •Original heating :Copper T : insol.in; Reh<iating : Copper

:insol. inHo. : Temp., : Time, .; HaCH, ; leap.. ; Time, : HaCH,

i °C. s hours :per cent: °C. ; hours : per cent00 s 800 : 1

:: 25.4 *: 600 % 16 % 25.4

?0 : tOO : 1i:li

24.9 l 370%: 72

12 2

24.770 : »00 : 1 24.9 :* 270

:% its 24.7

17 : 930 : 2i; 22.9 !• 370

#: 332

e

22

22.616 : 920 : 26

:s 22.7 ; 370

%46 23.7

: : : 2 2

The ratio of iron to copper in the samples as shown in column 9 of Table 1 was calculated to study the loss of copper due to volatilisation. Except for long roast at high tempera­tures it may be judged from the ratio values that the loss was inappreciable.

-21

CHAPTER TT.— EXPERIIIEIiTAL X-RAY HETHOBS

At the beginnins of the work it was anticipated that X-ray diffraction methods would be of value from three stand­points:

1. By study of the variations in line intensities with different: amounts of impurities it was hoped to obtain a check on chemical methods of analyses.

2. Samples showing magnetic variations were to be subjected to X-ray analyses to observe the changes in crystal structure with changes in magnetic susceptibility.

8. The crystal structure and atomic arrangement of the eubstances formed were to be determined.

For the purpose of making a systematic study of variations in line intensities with variations in impurities, physical mixtures of cupric oxide and ferric oxide, in percentages rang­ing from zero to 100 per cent each, by weight, in 10 per eent intervals, were subjected to X-ray analysis. The photo-densi­tometer oharts of each of these were compared. The intensities of the stronger lines as measured by the height of the peak changed in general with the changing percentages, as may be observed by a critical analyses of the accompanying charts.The comparison must be made on those lines which are due to the impurity alone. In the charts in Figure 1, peaks Eos. 4, 11, and 12 represent reflection planes due entirely to copper

oxide, and in like manner peaks lies. 1 , 5, and 10 represent reflection planes due to ferric oxide. The remaining peaks are ones in which intensity peaks of both oxides oeenr close together. The effect of reflection angles occurring within l/E degree of each other is to completely obscure the weaker line when subjected to photo-densitometer measurements. It has, therefore, appeared that the recognition of the presence of impurity in a sample by X-ray analyses and an estimate of the amount of the impurity is dependent upon the location of strong lines of the impurity. The optimum condition for the detection of an impurity would, therefore, be a strong first order reflection angle of the impurity occurring with the near­est reflection angle of the specimen several degrees away.Actual estimations of the amount of an impurity are complicated by variations from a true intensity record due to slight changes in line voltage, thickness of sample, length of exposure, a- mount of binding material used, and the current from the stor­age batteries in the photo-densitometer circuit. In addition it is to be remembered that the darkening of the X-ray film is not a straight-line function of the X-rays that fall upon it.For comparative measurement, however, these variations need not give serious trouble if care is taken to treat all samples as nearly as possible alike.

Photo-densitometer charts of the diffraction patterns of those products described in Table 1 which analysed the highest in ferrite were made and examined for the presence of impurities. The peak which corresponds to the one marked lid. 4 in Figure 1

FIGURE I.

appeared ©a all those samples which were treated below 1,000°. The chemical analyses of the free CuO in most of these samples were around 3.5 per cent.

On the charts of the samples heated above 1,000° a series of slight humps appeared at the points corresponding to the cuprous ferrite. This was indicated chemically by the amount of ferrous iron obtained.

These X-ray intensity measurements have not resulted in any definite conclusions as to the sensitivity of the X-ray diffraction method or its limits of accuracy as a quantitative method. Yet by comparing the size of the hump resulting from the impurity in the ferrite samples with the size of the hump in the FegOg. - CuO standards, a rough estimate of the percent­age in the sample has been obtained. The evidence indicates that the chemical results obtained for the percentages of fer­rite formed are substantially correct. For detecting the pres­ence of impurities amounting to less than 1 per cent or for estimating the amount of the impurities to closer than 2 or 3 per cent, the method is not reliable.

Determination of the Crystal Structure of the FerritesX-ray diffraction patterns of the various samples of the

ferrites, prepared as described in a previous chapter, have disclosed three types of crystal structure. One is associated with the cuprous, and the other two with the cupric ferrite.

Bromide prints of films showing the diffraction patterns of these three substances, together with some others used for reference, are shown in Figure 2. The distance between the

cubeCu0fs*03itrtraqonaiCuOfidOj

CuiOFetOj

FaC/. r r i m■ u i u n n i m n

FIGURE 2.

-24-

vertioal reference lines is equivalent to 5 millimeters, and the line of zero refraction is at the extreme left edge of the ■trips. A molybdenum target, producing X-rays of a wave length #710 angstromsi was used in the X-ray tube#

The unit cell of the cuprous ferrite is a rhaabohedron containing 4 molecules with the chemical formula GuFe02. Ex­pressed in terms of rhembehedral axes the parameters of this cell are a0 = 6.69; e - 53°68w. In terms of hexagonal axes a0 = 6*06; e = 2.82 where o is the axial ratio.

A possible atomic arrangement is given in Figure 5. Rhembehedral axes are used in the drawing to denote the atomic positions and parameters* This arrangement has cheeked very well with estimated intensities, and atomic radii. The oxy­gen atom fits into one corner of the small rhombehedren (one- eighth of the unit cell) one-third of the distance up on the altitude of the tetrahedron formed by the one copper and the three iron atoms. It touches all four metal atoms and bulges out on the faces of the tetrahedron. A slight over lapping (.07 angstroms) of the oxygen atoms exists when the small rhombohedrons are fitted together to form the large unit cell.

The lattice dimensions of the cubic lattice of Cu0.Fe20g were given by Holgereeon and Serres12 as 8.445 2 .002 angstroms. The structure is of the spinel type containing 8 molecules per unit cell. This particular structure was shown in the X-ray patterns of all the cupric ferrites formed by heating the pre­cipitated mixtures above 900° and cooling in air or by quenching.

ISSee footnote 4.

(Dlstancca refer to the height of the atoms as measured along the 0 axis and above the

ab plane)The atoms occupy the following special positions (ihoabohedral axle):

Cu = 0, 0, Oj 1|2, 0, 1|2; 0, 1|2, 1)2; 1|2, 1|2, 0.Fo - 0, 0, 1|2; 112, 0, 0; 0, 1|2, 0; 1|2, 1|2, 1|8.0 « a, a, u; 1(2 ♦ u, 1(2 + u, u; 1|2 — u, 1 - u, lj2 — u;

1 - u, l|2 - u, 1|2 - u; u, u, 1|2 * u; %1|2 + u, 1|2 * u, 1(2 ♦ u; 1}2 ♦ U, u, 1|2 ♦ u. u » 2|1B.

in which the aro the face diagonals mit cell are:

Cu » 0, 0, 0.Fe = 1|2, 1|2, 112. • = U, u, u; TT, n, 1

this it is

(l|8 unit)

: Fub. 318, Carnegie Inst, of Wash., 1922, Wash., B.6e

Measurements made on the lattice dimensions of this struc­ture have indicated a somewhat lower result than that reported by Holgersson and Serres. The method used, however, does not lend itself to great accuracy, and the results are only re­ported because of their consistency and better agreement with density determinations.

All dimensions of distance between reflection planes were obtained by direct measurement of distance on diffraction pat­terns or on photo-densitometer charts made from these patterns. These distances were measured both with respect to zero re­flection lines and diffraction lines given by substances whose distances between atomic planes have been accurately measured. Sodium chloride has been in most cases taken as the standard and samples of this mineral have been so used that its diffrac­tion pattern occurs on one side of the film containing the diffraction pattern of the ferrite sample. The zero lines of the samples are essentially the same, and comparisons have been made by setting the lines of the sodium chloride on these de­termined values and then measuring the ferrite dimensions.By means of a logarithmic scale expressing distances in ang­strom units the distances between atomic planes have been de­termined directly.

The dimensions of the cubic lattice of cupric ferrite, secured in this manner, gave the side of the unit cube as 8.40.

The density calculated from this value is 5.228 against 5.244, calculated from the lattice dimension given by Holgersaon and Serres.

nil i3The densities are calculated from the formulae p = ~ ,

where m is the number of chemical molecules in the unit and II the mass of one molecule, is the molecular weight of the com­pound multiplied by the mass of a hypothetical atom of unit molecular weight (1.65 x 10**24 grams); V is the volume of the ooll and p the crystal density.

Densities calculated in this manner are lower than those14given for calculated densities in Tables Annuelles. The value

there given for the density of CuFegO^ as calculated from the lattice dimension 8.445 is 5.28. Density measurements made on a sample of this ferrite gave the value 5.309.

Ferrite samples made by heating above 900° and cooling rapidly have disclosed an altered crystal structure when sub­sequently annealed for a long time at a lower temperature.While sample 70 in Table 4 gave the cubic structure as discussed above, sample 70a, which was the product formed by annealing sample 70 for 3 days at 360°C., showed definite displacement of the diffraction lines. The new structure was determined to belong to the tetragonal system with the dimensions of the unit cell: a0 = 8.28; c0 = 8.68. There is no change in the number of molecules associated with this unit. The calculated density is 5.307. The measured density was determined to be 5.295.

This same structure was found on products in which the temperature of heating in the formation of the product was

^Wyckoff, R.W.G., The Structure of Crystals, 2 ed., p. 182: The Chemical Catalog Co. Inc., Hew York, 1931.

Tables Annuelles Internationales de Constantes ct Donneia Hueerlquea, vol. VIII, 2me, Parte Anndes 1927-28, p. 2187.

800° or less. Low magnetic susceptibility and a reddish brown color are other properties which distinguish this form from the highly magnetic, grayish black cubic variety.

CHAPTER VI.— KAGRETIC PROPERTIES OF FERRITES

The magnetle pull of the series of heated products, in terms of degrees throw of spring on the magnetic balance, is shown in the last column of Table 1. Although all samples did not have the same percentage of ferrite, conversion to a 100 per cent ferrite composition does not alter the trend of rapid increase in magnetic susceptibility with increased tem­perature of heating. The time of heating has less effect and it is probable that the temperature reached and the ferrite formed at that temperature determine the magnetic properties of the material.

In order to maintain the magnetic properties instantan­eous cooling would be necessary. The difference between in­stantaneous cooling and water quenching is assumed, however, to be inappreciable, since the difference between water and air cooling is not great. Furnace cooling, though, greatly reduces the magnetic pull, as also does annealing at tempera­tures above a certain minimum point. Since no changes in magnetic properties result from heat treatment at 150°C. and marked changes occur at 300°, the minimum temperature is as­sumed to be in the range between these two values.

Table 5 summarises the effect of rate of cooling and annealing on the magnetic pull of ouprio ferrite.

Table 5.— Effect of Bate of Cooling and Anneal lag on maaetioPoll of Cupric Ferrite

:40 : 1,110 ; 1/4 hot

%ir :quenched (water) 2604

41 : 1,110 ; 1/4 hoty? :air cooled 232

38 : 1,040 : 8 dayss:air cooled 236

$38 : 1,040

%: 8 days •furnace cooled 199

16 s 930 1 26 horn%

m :air cooled 24716a Product

♦

of t reatmerit of sample Ho. 16 67ti

TO I

annealed for 2 days at 300°C. i :

900 : 1 hour :quenched1 :annealed for 3 days at 360*0.

260470a z. 36

: : :

Degree throw of spring (Magnetic balance off of spring balance)

IJagnetic tests mate on the cuprous ferrite have shorn it to have a surprisingly small magnetic pull (4° throw on the magnetic spring balance as compared to 280° throw with some samples of the cupric ferrite). The decrease in the magnetism of some samples when heated for long periods of time at 1,000°C was probably due to the formation of the cuprous ferrite. It was the observation of the cuprous ferrite lines in the X-ray patterns of some of these samples that first led to the suppo­sition that there was a change in the structure of the cupric

ferrite with changes in magnetism.

90847

-80-

Ihe spring magnetic balance usea for this work was cali­brated against mixtures of I,lineville magnetite of 98 per ©tat purity and ground to pass 3E0 mesh, with pure ignited ferric oxide of the same grain size. The sensitivity of the balance was not great enough to detect any magnetic effect in the fer­ric oxide, which therefore served as a neutral material for making diluted magnetite mixtures. Table 6 shows the calibra­tion data for the balance.

Table 6.— Calibration Data for Magnetic Balance

f c l o T - \ c ^ % ^ e : Degrees aefleotion-----------------------------------------------------------------------; .... -..-.... -................... " ........

10 : 21mo ; U80 : 7840 : 102#0 : 13260 : 160TO : 18280 i 202to : 22098 : 233

:

Absolute measurements by the ballistic method on this magnetite showed it to lave a permeability of 3.24 in a field of 20 gauss and a packing density of 2.0 grams Pe^O^ per c.o.

-31-

CHAH'ER VII.— DENSITY AIID SOLUBILITY DETEELHUATIONS

ISioh time was consumed In an attempt to prepare analytically pure cupric ferrite, as is indicated in Chapter III. The final samples chosen for density and solubility determinations were the purest that could be obtained and contained from 1 to 1 l/2 per cent of free trivalent iron. As ferric oxide was the only impurity present in significant amount, densities were corrected accordingly and the solubilities were assumed to be unaffected by the ferric oxide present.

A 10-c.c. capacity Gay-Lussac specific-gravity bottle with perforated glass stopper was used in making the density meas­urements. Six to eight grams of minus 100 plus 300 mesh mate­rial were used in each determination. The weighed sample was boiled in water to expel occluded and dissolved gases and the weighing made at a constant temperature of 20°. The corrected densities cheeked within .003 grams. The corrections made for the iron present in the sample were made on the basis of a den­sity value for Pe20s of 5.24. The results are given in Table 7.

Table 7.— The treasured Densities of Different Types of ConnorJj.0 y p 1 te S

Sample :Formulae of compound: Crystal system * Density:

56 i:

CUgO.FegOg $ Hexagonals$ 5.478

70 : CuO.FegOg $ Cubic : 5.31047 : Cu0.Pe20s $ Tetragonal : 5.296

Sample Bo. 56 m s prepared as described under Preparation of Cuprous Ferrite on page 15. Sample 70 was taken from a cyanide-treated residue of a portion of mixture 3 which had been heated for 1 hour at 900°C.

Sample 47 was taken from a cyanide-treated residue of a portion of mixture 1 which had been heated for 1 hour at 800®C.

Determinations of the solubility of each ferrite were made from the same head sample. The minus 350 mesh cupric ferrite used was screened from the product on which the solubility de­terminations on the plus 200-mesh material were made. The results, therefore, bear a comparison value which might not have been significant had different head samples been used for the determinations. It has been noticed throughout the work that some cupric ferrite samples are much more readily soluble than others. Those formed at the high temperatures are the most insoluble. Time did not permit the securing of quanti­tative results on this phase. The data on solubility tests are shown in Tables 8 and 9.

(

Table 8.— Solubilities of Cupric Ferrite in Some of the m Solvents

: Solvent % lima of :Temperature :Per cent of % Per cent ofSample i (50 o.o. of # treatment, lof treat- # Fe g Cu

i solution) :a*a.t. °c. ... : die solved : dissolved: : 1 : 1 ' ' '

—100 meah OuO.Fe2Oa i1

50 H3SO4100 HB30*

:s

2424

$g

2525

:s

9*116.6

s1

9.3M e#

% 200 H3SO* g 24 g 25 1 37.6 1 37.9* % Ham* g 1 # 95 i 81.7 1 81.9$ 100 HsS0* : 1 g 05 1 1 0 0 .0 1 1 0 0 .0* 200 n2so4 1 1 g 95 l 1 0 0 .0 : 1 0 0 .0t 1000 ffilOo • 24 g 25 t 19.8 : 16.6i 1000 BI0o t 1 # 95 1 — - 1 01.71 100 HOI t 24 g 25 1 24.4 : 24,1i 100 HOI 1' 30 g 95 l 100.0 1 1 0 0 .0i 1000 HOI $ 30 : 25 1 100.0 : 1 0 0a 0I# 1000 HOI 1# 5 g 95 l 100.0 1 100,0#

-350 moah CuO,FoqOo i 50 HsSO*#$ 24

#g 25

11 mmmtm 1

$ 23,0100 HaS0 4 : 24 g 25 t —— 1 40,7

# 200 HsSO* : 24 g 25 1 1 76.7$ 50 HnSO* # 1 g 95 l 1 100.0# 100 BaSO* # 1 : 95 1 1 100.0* 200 H3SO* g 1 # 95 1 — : 100.0: 1000 HIJOo % 24 # 25 1 — 1 22.1% 1000 HIJOo : 1 g 95 t — 1 1 0 0 .0% 100 HOI g 1 g 05 1 — l 100-0# 1000 HC1 g 25 min* g 25 t — i 1 0 0 .0: 1000 HOI g 3 min. g 95 i — i 1 0 0 .0t 2 : z :

Table 9.— Solubl 11 ties of Cuprous Perilte In Seise of the Comon Solvents

Samplet#:

Solvent (80 c.e, ofsolution)

2$

Time of treatment

: Temperature :Per cent of :Per cent of ls : of treat- : Fe t Oa

: nent. °G. : diooolved : dissolved.; 2 : 2 1

-350 mesh CugO.Fe^s $ 5jS H2SO4 $ 24 i 25 $ 32.8 2 32.8: # t 2 2: 103 H2SO4 * 24 t 25 2 43.0 2 42.9: * i 2 2# 20/5 IIsS04 : 24 i 25 2 72.2 2 72.3* # l 2 2# &;5 H3SO4 # 1 > 95 2 100.0 2 100.0$ # 1 2 2# lOyJ HOS04 # l 1 95 2 100.0 2 100.0# # 1 2 2% 20^ HC504 $ 1 1 95 2 100.0 2 100.0: : 1 2 2* 10̂ HHOo : 24 1 25 2 39.2 % 39.2; # l 2 2$ 10JS HOI 2 24 1 25 2 53.5 2 53#5: 2 i 2 2

CHAEEER VIII. AHD DISCUSSION

X-ray work M s revealed three forms of copper ferrite, each belonging to a different crystal system, and each possess­ing certain rather definite characteristics. The cubic form, assumed by the cupric ferrite when heated to high temperatures and rapidly cooled, is ferro-magnetic and grayish black in color. The tetragonal form of the same compound is associated with lower formation temperatures and is only slightly magnetic with a color possessing a definite shade of dark red or brown. The cuprous ferrite belongs in the hexagonal crystal system and its color while black has a distinctive velvety luster in the powdered state. Of the three types this is the least magnetic. Each of the three forms, by changing the temperature of heating, can be converted into one of the other forms possessing the special characteristics of that form.

A comparison of magnetic pulls as measured m the magnetic spring balance gives values in the order mentioned above as 2604, 30, and 4. The value given the tetragonal form may be high. There is no evidence to show that the sample of the tetragonal form used for measurements was entirely free from the cubic form. The variation in magnetic properties with heat treatment may be explained on the supposition that at any par­ticular temperature, there is an equilibrium between the two forms. As the temperature increases, the proportion of the strongly magnetic cubic form increases. The rate of inversion

is so slow at room temperature that the equilibrium tempera­ture may be "fixed” by rapid cooling.

The actual range in which some formation of cupric fer­rite, from precipitated mixtures of cupric and ferric oxides, occurs extends from drying temperatures (120°C.) to decompo­sition temperatures (around 1,000°C.). The formation of some ferrite (2#) at 120°C.f which was the lowest temperature on which tests were made, indicates the likelihood of some slight formation even with precipitation, v/ithin this range the per­centage of ferrite formed in the sample increased in a regular manner with increasing temperature of heating. This behavior has the aspect of an equilibrium reaction. The fact that re­grinding of a heated sample to a finer mesh, remixing and re­heating produces no change in the percentage of ferrite formed is a point in favor of this view. A number of discrepancies, however, exist. 1. Contrary to the law of mass action, large excesses of cupric oxide do not increase the percentage of ferric oxide which goes into combination at any given temper­ature. 2. The action has shown no appreciable tendency to be reversible. Such slight reversions as were detested were not sufficiently definite or pronounced to serve as bases for conclusions.

A comparison of the results obtained in this work has shown them to be in accord with the results obtained by Sullivan in the experiments on grain mixtures. His data

\show a marked increase in the percentage of ferrite formed

in the mixture. It was to he eacpeetea from his work that intimate mixing hy preeipitation would prcanoe a greater fer­rite formation than he attained. At 800°C., his grain mix­tures of minus 200 plus 350 mesh material in molecular ratio of FegOg to CuO of 1:1 yielded a ferrite content after 1 hour of heating of 8.9 per cent. Hinas 050 mesh material with the same proportion of oxides and the same heating temperature gave a ferrite content of 17.7 per cent. Under similar con­ditions, the precipitated mixtures used in the present work show after roasting a ferrite content of 90 per cent.

A few test roasts were made on chalcopyrite in which the copper and iron are moleoularly mixed in equimolar propor­tions. Fifty per cent of the copper is all that could com­bine as cupric ferrite. Boasting of minus 100-mesh material containing 97 per cent chalcopyrite for a period of 4 hours at 820°C. caused 48 per cent of the copper to go into com­bination as ferrite. Calculation shows 96 per cent of the iron in the sample is in the form of ferrite. Roasting of chaleopyrite at lower temperatures was not tried, but it is probable that 820°C. does not represent the lowest tempera­ture of ferrite formation in chalcopyrite roasting.

As 820°C. is not above the maximum temperature attained in the roasting of copper concentrates, and as considerably higher temperatures are attained by some of the particles in the flash roast between hearths, copper ferrite in roasted copper concentrates should not be a rare occurrence.

Leaching of the roasted products of chalcopyrite ores is complicated by the presence of copper ferrite. The possi­bility of leaching ouch a compound if formed may be estimated from the table of solubilities (Tables 8 and 9). Although not readily soluble in a normal leaching solution (2^ solution) at moderate temperatures, its solubility is greatly increased with temperature and strength of solution. A magnetic separa­tion of cupric ferrite from a calcine containing large amounts of it and a special treatment in a strong H2S04 solution might be practical.

It would seem that there is little hope of roasting a concentrate containing chalcopyrite or bornite to produce a calcine entirely free from copper ferrite.

1^33 -75 C5

6 3 900;