CYANIDE RECOVERY

A diagram of the laboratory mineral processing equipment used in the precipitation of copper and the subsequent regeneration of cyanide. The general technique employed in these operations was as follows:

1) To pregnant solution in a 1-liter stainless steel precipitator (A) add the sulfide (Na2S; NaHS; CaS) required for precipitation.

2) Bolt the head of the precipitator in place over a Teflon gasket; add the required volume of sulfuric acid to the acid storage container (B).

3) Add acid through valve (C) into the pregnant-sulfide mixture in (A) with valves (H) and (D) closed and with magnetic stirrer (E) operating.

4) Keep system closed and stir for 5 min. to complete precipitation.

5) Open outlet valve (D) to allow the evolved cyanide to pass into traps (F) and (G) containing alkaline absorbing solution.

6) For gas stripping, connect inlet to a source of air or nitrogen, open inlet valve H, and pass gas into and upward through the contents of the precipitator (A) and then out through exit valve (D) into the traps (F) and (G).

7) For steam stripping, a water-cooled condenser and receiver (not shown) are inserted between (A) and (G), and (A) heated and stirred to remove a cyanide-water mixture which is collected (F) and cooled in an ice-water mixture. (F) is vented into trap (G) to prevent loss of cyanide.

8) The cyanide contents of (F) and (G) were determined by direct titration with silver nitrate. The copper bearing precipitate was filtered from the liquors in (A) after the stripping operation, was dried, and analyzed for copper content.

The types of receivers (F) and (G) were varied to facilitate cyanide recovery and manipulation (cooling, volume of distillate handled, quantity of cyanide recovered etc.).

CYANIDATION OF COPPER MINERALS

In a fast mineral processing reaction, copper minerals dissolve in cyanide and act as cyanides in the recovery of precious metals by cyanidation. However, the application of cyanide leaching to the recovery of copper from copper ores requires considerably different conditions than used in precious metal cyanidation (higher cyanide concentrations; shorter leaching times; oxidizing conditions unnecessary).

Cyanide Complexes of Copper: The predominant species formed when copper dissolves in cyanide is one containing 3 moles of cyanide to 1 mole of copper (Cu(CN)3). This indicates that at least 2.32 lb NaCN equivalent are required to dissolve one pound of copper, if no side reactions occur.

Cuprous Copper Minerals: The reaction of cyanide with typical cuprous sulfide and oxide copper minimum. Fig. 1:—Simplified precipitation-regeneration system. Not shown in the diagram are a condenser and receiver for steam stripping.

The reaction is generally written as follows:

CHALCOCITE— CuaS:

CU2S + 6 NaCN = 2 Na2Cu(CN)3 + Na2S [1]

CuaS + 3 Ca(CN)a = 2 CaCu(CN)a + CaS [21

CUPRITE—CuaO:

Cu20 + 6 NaCN + HaO = 2 Na2Cu(CN)3 + 2 NaOH [SI Cu20 + 3 Ca(CN)a + HaO = 2 CaCu(CN)3 + CalOH). [41

Cupric Copper Minerals: Cupric copper undergoes reduction in cyanide solution. Therefore, when cupric minerals are dissolved in cyanide, the following reactions occur:

2 CuC03 + 8 NaCN = 2 Na2Cu(CN)3 + 2 NaaCOa + (CN).

Cyanogen, (CN)2, in alkaline solution, undergoes a further reaction as follows:

(CN|! + 2 NaOH = NaCNO + NaCN ■ H-.0

The over-all reaction therefore is:
2 CuCOa + 7 NaCN + 2 NaOH = 2 Na2Cu(CN)3 + 2 NaaCOa + NaCNO + HaO
In this reaction, the reduction of the cupric copper and the subsequent oxidation of cyanogen to form cyanate results in the loss of 0.5 mole of NaCN for each mole of copper dissolved (0.39 lb NaCN equiv./lb Cu).

Thiocyanate Formation: A second important source of cyanide loss occurs through the formation of thiocyanates. The exact mechanism of this thiocyanate formation is not known. The reaction of cyanide with sulfides and thiosulfates does not readily produce thiocyanate. However, higher poly-thionates such as the tetrathionates and pentathio-nates react readily with cyanide to give thiocyanate as follows:

NaaSiOo + 3 NaCN + HaO =

NaaSOi + 2 HCN + Na2S2Oj + NaSCN [81 Na_S:,0.; + 4 NaCN + HaO =

NaaSOi + 2 HCN + NaaSaOa + 2 NaSCN [91

Ferrocyanide Formation: In general, ferric iron is not soluble in alkaline cyanide solutions. Ferrous iron, however, does dissolve to form ferrocyanide, Fe(CN),f‘. Since ferrocyanides do not decompose readily in cold, dilute sulfuric acid (used in subsequent copper recovery), such compounds may constitute a source of cyanide loss in the copper cyanidation process.

Leaching Tests on Copper Minerals: Five-gm samples (minus 100 mesh) of several copper min­erals (Ward’s National Science Establishment, Roch­ester, New York) were leached for six hours in 1000 ml. of NaCN solution at various cyanide to copper ratios. In the tests on bornite and chalco- pyrite, 10-gm samples of mineral were leached for four hours using Ca(CN)a solutions. Cyanide con­sumptions were calculated by analysis of the preg­nant solutions for non-regenerative cyanide loss (by distillation) and for ferrocyanide (by iron analysis).

Rate of solubility: The rapid rate of extraction of copper from several copper minerals was demon­strated by leaching 50-gm samples of synthetic ores (2% Cu, —200 +325 mesh) prepared from various copper minerals and quartz. These ores were leached in open beakers at 40% solids and a cyanide ratio of 3.0 gm NaCN equivalent per gm of con­tained copper. Results are shown in Fig. 2. As in­dicated, the rate of extraction of copper from the minerals tested increased in the order of bornite, covellite, chalcocite and malachite. Copper extrac­tions of 81.6% (chalcocite) and 94.2% (malachite) were obtained in 15 min., thus demonstrating that samples of a copper ore (minus 28 mesh and minus 10 mesh) and samples of flotation tailings and cleaner flotation tailings were leached with cyanide. The predominant copper mineral in these samples was chalcocite. Samples of the ore suspen­sion were withdrawn periodically, immediately fil­tered and washed and the filter cakes assayed for copper.

As indicated in Fig. 3, the rate of solubility of copper was rapid in the case of the flotation tailing (1.08% Cu), although the cyanide to copper ratio (2.34:1) was lower than that required to give op­timum copper extraction (i.e., usually 3.0-3.5:1). Leaching of the copper ore at coarser sizes also was rapid, since from the minus 10 mesh sample, 75.1% of the total copper dissolved in the first 30 min and 86.7% in 4 hr. From the minus 28 mesh sample of the same ore, 83.9% of the copper was extracted in 30 min and in 2 hr essentially all the extractable copper was in solution.

Leaching was conducted in a laboratory Fagergren flotation machine and in the first two min. 84% of the copper was dissolved. Thus the rapid solubiliz­ing action of cyanide on copper was demonstrated clearly and was subsequently utilized in flotation operations to eliminate the activation of pyrite by contaminating surface films of copper and to permit the production of final copper concentrates of high grade.

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