The cyanide added at the start of the leaching operation goes into the pregnant solution chiefly as free or unreacted cyanide, soluble copper and iron cyanide complexes, and by-products, such as thiocyanate, cyanate and possibly small amounts of ammonia. On acidification of the pregnant solutions in the precipitation step, the free cyanide and the cyanide in the copper complex are converted to hydrocyanic acid which is recoverable in stripping operations at ambient temperature or higher temperatures. The cyanide consumed in the formation of by-products such as thiocyanate, cyanate, etc., is non-recoverable by stripping either at ambient or higher temperatures.

Cyanide recovery from the acidified liquors from the precipitation step was conducted by air and inert gas stripping at varying temperatures and also by steam stripping.

Laboratory Stripping With Air: The copper con¬tent of a pregnant solution (11.7 gm Cu per liter; 40.0 gm NaCN equivalent per liter; prepared from cyanide and copper oxide) was precipitated with sodium hydrosulfide and sulfuric acid. In separate tests the acidic slurry was aerated in the laboratory precipitating apparatus (Fig. 1) or by means of laboratory Fagergren or Agitair flotation machines at 33 °C.

No difference in cyanide removal was noted in the two flotation machines at equal aeration rates. The more rapid stripping obtained by aeration in the laboratory precipitating apparatus was probably due to the increased flow of air, as was demonstrated in further stripping studies at varying air flow rates. The shape of the cyanide recovery curve indicates that at room temperature the stripping of cyanide becomes more difficult as the solution becomes more dilute. For example, in the tests in the flotation ma¬chines, 80% of the cyanide was removed in 1 hr, but 2.2 hr was required to remove 98%.

The effect of temperature on cyanide stripping was studied. The stripping operation, conducted at 33° and at 59°C was, as expected, more rapid at the higher temperature.

Cyanide vs. common leaching agents: The superiority of cyanide as a solvent for copper sulfide ores, as compared to ammonia, acid, and acid plus oxidant, was demonstrated by leaching a flotation tailing (1.077% Cu; 500-gm charges; 47% solids; 1-hr leach). Copper extraction was determined by preg­nant and residue assays. (See Table III). The Table shows that copper extraction (1-hr) of 75-80% was obtained with cyanide as compared to only 54% with H2SO, plus oxidant, 10% with H2SO,, and no extraction with ammonia.

The mechanism for the dissolution of copper from sulfide minerals in cyanide solutions is given in Equations I and II above. Addition of acid to a copper-bearing pregnant solution reverses these equations and precipitates copper as the sulfide and releases the cyanide in the copper complex.

The Effect of pH on Copper Precipitation: The effect of pH on copper precipitation at a reduced added sulfide level was studied, using a pregnant solution (14.9 gm Cu per liter) prepared by leaching a flota­tion cleaner tailing (4% Cu) with cyanide. Suffi­cient sulfide was added (0.22 gm NaHS per gm Cu) to give approximately 50% of the total sulfide needed to precipitate all of the copper as Cu2S (the remainder of the required sulfide being formed in the cyanidation step). Aliquots of the solution were then acidified (in open beakers) to various pH levels with 50% H.SO4. After removal of the precipitates, the barren solutions were analyzed for copper. It was found that copper precipitation was complete (>99.9%) at and below a pH of 2.4.

Copper Content of Precipitates: The copper content of the precipitate depends upon the type of cyanide and alkali used in leaching, and the type of acid and sulfide used in precipitation. The use of NaCN, Na2S (or NaHS) and HaSCh should produce nearly pure Cu2S (79% Cu).

What a metallurgist in a Laboratory Steam Stripping Tests can do: The increase in cyanide removal rates by air stripping at elevated temperatures suggests the possibility of using steam stripping as a method of cyanide recovery. Steam stripping is also suggested by the fact that the cy­anide distillation procedure is a standard method for the quantitative determination of cyanide. Cy­anide recovery tests on slurries resulting from the copper precipitation from pregnant solutions con­taining 17-40 gm Cu per liter indicated that about 99% of the total recoverable cyanide was removed
in 1-hr heating at 100°C, a procedure which simu­lated steam stripping.

Steam stripping has the advantage of removing water from the cyanidation circuit, thus favorably influencing water balance in the process. Also, after steam stripping, the settling-filtration characteristics of the copper bearing precipitate are improved, probably because of crystal growth and agglomera­tion during the process.

The use of cyanides in the flotation of copper ores was described: The presence of films of sulfide copper on pyrite was suggested by Gaudin. Such films have been photomicrographed by Cadwell. Both these authors describe the influence of such contaminations on the pyrite surfaces. The applica­tions of a cyanide leach to rougher copper con­centrates, as produced, dissolved relatively large quantities of copper. However, it has been indicated that, if certain rougher copper concentrates are split into a sand and slime fraction, the addition of cyauide in refloating the sands yields a high grade copper concentrate. The copper in the slime portion may also be floated, usually without cyanide addi­tion, to give concentrates of high copper content. The combined sand and slime concentrates thus produced are higher in grade than those obtained by the usual procedure of regrinding-refloating the entire rougher concentrates and frequently repre­sent higher over-all copper recovery. T

Formation of the cuprocyanide complex is ex­tremely rapid, which indicates that about 85% of the copper in an ore fraction containing 4% Cu was dissolved by leaching for only 2 min in a laboratory Fagergren machine. In this work, the cyanide to copper ratio was 3.5:1. Lower concentrations of cy­anide, such as are used in flotation operations, are also consumed rapidly.

SUMMARY

This describes laboratory work on the de­velopment of a process for recovering copper by cy­anidation. The extraction of copper from sulfide and oxide copper-bearing minerals, various ore products, and converter slags is rapid and the cy­anide requirements are low. A method of copper re­covery has been developed involving the following steps.

 1.    Leaching with alkaline cyanide solutions at atmospheric temperatures and pressures.

 2.    Recovery of the dissolved copper by precipita­tion by addition of acid and sulfide; additions of sulfide are less than the stoichiometric equivalent of the dissolved copper, since sul­fide, thiocyanate and ferrocyanide by-products are formed in the leaching operation and are utilized as precipitants.

 3.    Regeneration-recovery of cyanide for recycle by air, inert gas, or steam stripping after pre­cipitation.

The rapid rate of dissolution of common copper minerals in cyanide solutions has found application for removal of copper films from gangue minerals such as the pyrite contained in copper flotation concentrates, thus facilitating the separation of such gangue components in subsequent flotation operations.