The Recovery of Copper in Sulphide Ores by Roasting, Leaching, and Electrolysis Jack S

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Bachelors Theses and Reports, 1928 - 1970 Student Scholarship

6-3-1932 The Recovery of Copper in Sulphide Ores by Roasting, Leaching, and Electrolysis Jack S. Greenough

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Recommended Citation Greenough, Jack S., "The Recovery of Copper in Sulphide Ores by Roasting, Leaching, and Electrolysis" (1932). Bachelors Theses and Reports, 1928 - 1970. 21. http://digitalcommons.mtech.edu/bach_theses/21

This Bachelors Thesis is brought to you for free and open access by the Student Scholarship at Digital Commons @ Montana Tech. It has been accepted for inclusion in Bachelors Theses and Reports, 1928 - 1970 by an authorized administrator of Digital Commons @ Montana Tech. For more information, please contact [email protected]. TEE RECOVERY OF COPPER IN SULPHIDE ORmS

jjY ROASTING, LEACHING, AND ELECTROLYSIS.

:sy SACK S. GREENOUGH.

BUTTE. .MON1'ANA,. .J'U'.NE 3. I932.

~.'. ' .. ':" .... ;..~ .... , THE RECOVERY OF COPPER IN SULPHIDE ORES BY ROASTING, LEACHING, AND ELECTROLYSIS. Inaugural Thesis

submitted as a partial fullfillment of the requirements for the degree of

Bachelor of Science in

Metallurgical Engineering from the

Montana School of Mines.

Jack S. GTeenough

Bu-cte, Monta-na.. toPY 2-'

MONTANA SCij.OOL OF MiNES LiBRARY: BUTTE

42876

Butte, Montana, TABLE OF CONTENTS.

PAGE. Theoretical Considerations ••••••••••••••••••••••• I Roast ing •...... • :3

Results of Roasting, Table I, • •• •• •••• •• ••• •••• • 4 Curves: Fig. I, The Formation of Copper Sulphate with Time ••• 6 Fig. 2, Decomposition of Copper Sulphide with Time ••• 7 ~ig. 3, Formation of·Copper Oxide with Time •••••••••• 8 Fig. 4, Change in Total Copper with-Time •••••••••••• 9 Fig. 5, Formation of Soluble Iron with Time ••••••••• IO

Leaching Tests •• w •••••• , •••••• , •••• ~ •••••••••••••••• II Results of Leaching Tests, Table II, ••••••••••••••• 12 Curves: - Figures 6a to 9b, ••••••••••.•••••••••••••• 13 to 16 Electrolysis of the teach Solution ••~ •••••••••••••• 17 Diagram Showing Electrical Connections ••••••••••••• 18 Results-of Electrolysis, Table III, •••••••••••••••• 20 Conclusions •••• ~•••••••••••••••••••••••••••••••••.• 21 I.

THEORETICAL CONSIDERATIONS. Sulphide ores of copper are insoluble in dilute sulphuric acid leaching solutions, but a very high extraction ean be obtained if the copper ore is in the oxidized condition. The problem is to convert the sulphide~lntQ~the oxide form. This can be done by giving the sulphide ore an oxidiz~ng-sulphatizing roa.st; Copper sulphate is soluble in water, so acid will be saTed in the leaching process if copper sulphate is present. The iron in the copper sulphide ores is present as pyrite, or in combinations as bornite, or chalcopyrite. The object of the roast is to convert a,considerable portion of the copper to the oxide form and a lesser amount to

the form of sulphate; and all of the iron to ferric oxide,Fe203• which is insoluble in dilute acid. If iron existed in the electrolyte, the current efficiency would be lowered to a considerable degree, since the iron is oxidized to the ferric condition at the anode and is reduced ·to ferrous iron at the cathode, thus consuming current which· might otherwise be used to deposit copper. The decomposition temperature of copper sulphate i8

650 degrees Centigrade and that of ferrous sulphate is 590 degrees centigrade,(I)Bo that theoretically the correct temperature of roasting should exist between 600 and 650 degrees

Centigrade.

(I) "Hydrometallurgy Of Copper", by William E.Greenawalt. 2.

By roasting at this temperature, a maximum amount of copp~r oxide and of copper sulphate and a minimum amount of soluble iron should be obtained. After the ore has been r0asted and leached, the result- ing solution is transferred to the electrolytic tanks, where it is electrolyzed using insoluble anodes. The anode used was lead while the cathode was copper. For every pound of cop~er deposited at the cathode, 1.54 pounds of sulphuric acid will be regener- ated. The ideal working plan is to regenerate enough acid at the anode SO that it will not be necessary to buy any for leaching purposes. If there are preoious metals present, they will occur in the residue. According to Greenawalt, they can be recovered by cyanidation, chloridizing, or by direct smelting. He states(2) that the cyaniding of material which has alreadY been finely ground and leached to remove cyanicides i8 a cheap and simple _\ process and the preoious me~alsare produced in bullion form, acoeptable to the U.S. Mint. Experimental work indicates that with properly roasted concentrate there will be no difficulty in cyaniding the copper leached residue at a cost of $0.50 to

$1.00 per ton.

(2) American Electro-Chemical Society, Pre-print 61-29, "Electrometallurgy Applied to COPJ?er and to Complex Ores". By, W.E.Greenawalt. ROASTING.

The material used in the present investiga~ion was Anaconda copper flotation concentrate, containing 26.42 % eu and 22.95 % Fe, the size varying from 100 to 2240 mesh. Two hundred grams were weighed out, placed in a large roasting dish and roasted in an electric muf~le, with a plentiful supply of air and with frequent rabbling; the temperature was 600 degrees Centigrade. A sample was taken from the main roast at half hour intervals and analyzed for water soluble copper, acid soluble copper (1:9 RCI), insoluble copper, and soluble iron. In analyzing the samples, the copper existing as sulphate was found by boiling in 100 cc., of water for three minutes, filtering, and titrating with potassium cyanide until the blue color formed by the addition of ammonium hydroxide dissappeared. The oxide copper was found by leaching the above residue with 100 cc., of boiling 1:9 RCI for three minutes, filtering and titrating as indicated in the preceding determination. The copper as sulphide was determined by dissolving the residue from the last step in hydrochloric acid and stannous chloride and titrating with potassium cyanide. The iron was determined by precipitating it with ammonium hydroxide, and analyzing it by the permanganate method. Total coppers were also run and these results cnecked very closely with the values obtained by adding the oxide, sulphate, and sulphide forms of copper.

MONTANA SCHOOL OF MIN ss LI.... ARY BUTTE 4.

TABLE I.

Roasting Copper as . Copper as Copper as Soluble Time,hrs. Sulphate,%. Oxide,%. Sulphide,%. Iron,%. t 0.00 41.6 58.4 6.60 I 0.00 51.5 48.5 23.'70 It 10.00 78.1 11.9 22.50 ... 2 20.10 79.0 0.9 4.90 2t 31.00 69.0 0.0 I.65 3 25.80 74.20 0.0 I.50 3t 30.00 70.00 0.0 1.50 4 32.70 67.30 0.0 1.60 5.

The results-are shown in Table I. These results were also plotted and will be seen by observing Figures I,2,3,4,and5. Figure I illustrates the formation of copper sulphate with the time of roasting. It is seen that no copper sulphate was formed· until the material had been roasted for one hour. The percentage of the total copper in the form of sulphate steadily increased until after two and one-half hours of roasting, when it reached 30 per-ccerrt, - Figure 2 correlates the decomposition of copper sulphide with the time of roasting. When the first sample was taken for analysis at the end of a.roasting period of half an hour it is observed that 58 per-cent of the total copper existed in the form of sulphide,but as the roasting progressed this value steadily decreased until at the end of two and one- half hours it became zero. The formation of copper oxide with roasting time is illustrated in Figure 3. At the end of two and one-half hours of roasting, 70 per-cent of the total copper·was in the oxide form. As roasting progresses the volatile constit- uents were expelled so that the percentage of total copper increased. This is shown by Figure 4. The behavior of acid-soluble iron in this type of

roast is shown in Figure 5. No water soluble iron was formed. The percentage of a~id-'Boluble iron increased from 0 to 23.90 per-cent in a roasting period of one hour and then decreased until it became I.6 .per-cent at the end of two and 6. Figure I.

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o I I~ Z 77~,. (fl~ /i'ol.r/,/7) II. one-half hours of roasting. In the preparation of the material for the electrolytic part of this work this value was fUrther decreased to 0.39 per-cent. This iron is believed to be in the form of FeO, since FeS04 is soluble in water. Rca.at Lng at high- er temperatures produced a material in which the soluble iron was less, but likewise in which the soluble copper was less.

LEACHING TESTS.

The leaching of the roasted samples was done in 4000c.; beakers. Five grams of the roasted material was placed in~·the beakers and was leached for one hour with sUlphuric acid of different amounts and strengths. The contents of the beakers were stirred every ten minutes. The results obtained are tab- ulated in Table II, and shown graphically in Ftgures 6a to 9b. The theoretical amount of sulphuric acid necessary to convert the copper in five grams of roasted concentrate into copper sulpha.te was "found to be 2.29 grams. This amount 'of acid was made up into strengths of 1%, 3%, 5%, 7%, 10%, 15%, and 20%; Figures 6a and 6b- show the extraction of iron and copper. ~The extraction of iron varied between 1.3 and 5.8 per-cent, while the maximum extraction of copper was 80 per-cent, using 20 per-cent strength acid. Other leaching solutions were made up in strengths of 5%, 10%, 15%, and 20%, and using It, If, and 2 times the theoretical amount of acid. As indicated by Figure ?b, there

LEACHING TESTS

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t <1 s 6 '1 8 9 10 11 12 13 14 Ii 16 17 18 was not much change in using 11-times the theoretical amount of acid, an 82 per-cent extraction of copper being obtain~d wi th an acid of 20 per-cent strength. There is a--marked in- crease in the extraction of copper bY'using It times the I theoretical amount of acid, as Figure 8b illustrates. with " an acid strength of 15 per-cent the extraction of copper increased to 97.5 per-cent, th~ soluble iron being 0.78 per- cent. From Figure 9b it is seen that by using twice the theoretical amount of acid in a strength of 15 per-cent an extraction of copper of 98.5 per-cent is obtained. The best test was the 15 per-cent strength acid, It times the theoretical amount, in which the copper extraction was 97.5 per-cent. This is illustrated by Figure Sb.

ELECTROLYSIS of the LEACH SOLUTION.

According to the results obtained in the roasting and leaching tests on the Anaconda concentrate, a batch of two hundred grams was treated in order to· secure a suitable solution for t~e electrolysis. The concentrate was roasted for four hours at 600 degrees Centigrade, and then leached -1 for two hours with a cold solution of sulphuric acid, having a strength of 15 per-cent and using It times the theoretical amount. From this leach, 150000;, of solution ~as obtained, containing 40 grams per liter 'of copper and 3.9 grams per

liter of iron. IS.

Figure 10.

STORAGE BATTERY SLIDING CONTACT RHEOSTAT. +'

AMMETER To Cathode L----4 A }------t

VOLTMETER v

SINGLE POLE SWITCH

GLASS TANK, 5"XI2"X5".

DIAGRAM SHOWING ELECTRICAL CONNECTIONS •

.. 19.

This solution was placed in a glass cell and con- nected to a lead storage battery with the arrangement as shown in Figure IO. The copper cathode and lead anode were two and one-half inches by two and five-eighths inches, and were placed two and one-half inches apart. The temperature was maintained at 21 degrees Centigrade. A current density of 20 amperes per square foot was used, although it was found that lower current densities also gave high efficiencies. The potential required was 2.2 volts and the current used was 0.92 amperes. The electrolyte was agitated by means of an electrical stirrer. The copper cathode was weighed at intervals of fifteen minutes and thirty minutes. The results will be found in Table III. Since it is known that 1.186 grams of copper are deposited theoretically at the cathode per ampere-hour, the current efficiency can be calculated if the weiglit of copper deposited, the time, and the current are known. It was found that the efficiencies were high, 92 per-cent being the average. There was not enough iron in the electrolyte to materially affect the current efficiency. 20.

TABLE III. ~ECTRQLYSIS. , "

Time, (Hour-s};' Current· Efficiency,%. t 100.00 I 97.00 It 92.75 It 90.00 2 91.00 2t 91.00 2t 90.10 93.50 90.90 92.75 3t 89.40 3t 87.00 (Ferrous Iron ••• 0.4 grams per liter)~ (Ferric Iron ••• 3.5 " 11 " ). 41- 92~60 41- 92.60

5t 89.60 5t 92.50 21.

CONCLUSIO~

It was found that in order to make all of the copper soluble and most of the iron insoluble,. the roasting temperature should be 600 degrees Centigrade, and the time of roasting should be two and one-half hours. The leaching solution which gave the highest extraction in the shortest time contained one and one-half times the theoretical ~ount of sulphuric acid in a strength of fifteen per~cent. The extraction of copper given by this solution in a time of one hour was 97.5 per-cent. An average current efficiency of 92 per-cent was obtained in electrolysis, using a current density of·twenty amperes per square foot. There was only 0.39 per-cent iron in the electrolyte, so the current efficiency was not materially affected by this amount of iron. This process offers possibilities for the production of pure electrolytic copper by using hydro-metallurgioal methods on copper sulphide ores. This work was done under the supervision of . Doctor C. L. Wilson and Professor J.U. Mao Ewan, of the Metallurgy Department at the Montana School of Mines •

. 42876