Aluminium Precipitation

The use of aluminium as a precipitant for gold out of cyanide solutions was patented by Moldenhauer in 1893 but he does not appear to have made any practical use of the process. H. F. Julian experimented with it, in the same year, but soon abandoned the idea. In 1910 S. F. Kirk patrick introduced it at the Deloro Smelter in Canada, applying the metal for the first time in the form of a powder, and achieved a complete success. He later installed it at the O'Brien Mine in Cobalt.

When making tests on the cyaniding of the Nipissing lowgrade ore it was found that the arsenic and antimony dissolved out of the ore by the cyanide formed a harmful combination with the zinc derived from zinc precipitation causing a serious loss of dissolving power in the stock solutions. Consequently an alternative method of precipitation was sought for, and that in use at the O'Brien mine naturally suggested itself. The experiments were so successful that the process was introduced in the new mill at its start and the results fully justified the choice, the stock solution at the end of twelve months being more active for dissolving silver than freshly made up solution. The fact that aluminium does not replace the precious metals in the cyanogen compound renders necessary the presence in the solution of caustic soda. There are two possible equations to represent the reaction incident to precipitation.

The first is suggested by Moldenhauer: 6NaAg(CN) 2 + GNaOH + 2A1 = 6Ag + 12NaCN + 2A1(OH)3 , the aluminium hydroxide at once dissolving in the excess of caustic to form sodium aluminate, 2A1(OH) 3 + 2NaOH = Na2Al2 4 + 4H2O This would indicate that one part of aluminium should theoretically precipitate twelve times its weight of silver, but in practice the amount is about three times its weight only, so the following may more nearly represent the actual reaction, 2NaAg(CN)2 + 4NaOH + 2A1 = 4NaCN + 2Ag + Na2Al2 + 4H The method of working is somewhat similar to that for zinc dust but the action is slower in practice, and necessitates a receptacle in which to retain the emulsion under agitation until the action is complete. The dust also is not nearly so easily wetted as zinc dust and a special form of agitator is used to assist in effecting this. The reason why the action is so slow is probably a mechanical one, because if an excess of dust be used in a strongly caustic solution the reaction is rapid, about two minutes only being required for total precipitation, but in practice the amount of precipitant used has to be cut down to the lowest possible point for the sake of economy and consequently a proportionately longer time is needed to bring each molecule of the cyanogen compound in contact with the metallic surface of an aluminium particle. It follows from this, at least in the case of solutions of the average low metallic content, that the quantity of precipitant necessary depends more upon the volume of solution to be precipitated than upon its assay value, and therefore it is more economical to concentrate the precious metal into as small a bulk of solution as possible for precipitation rather than to precipitate large volumes of low grade solution.

One point in the chemistry of the process is vital to its practical success and that is that at the time of precipitation lime must be absent, as it decomposes the sodium aluminate to form insoluble calcium aluminate, (Na2Al2O4 + Ca(OH) 2 = CaAl2O4 + 2NaOH), which deposits in the press with the precipitate, making a large bulk of low-grade product which it is almost impossible to melt into bullion. At the Nipissing this condition of absence of lime does not present any difficulty because the slime settles well for the final decantation in the cyanide liquor with only the merest trace of lime, but difficulties are likely to arise with other ores from this cause and to meet such cases the writer devised the modified process described la'ter.

The reaction between the lime and the sodium aluminate is turned to useful account after precipitation in preventing an undue accumulation of the sodium aluminate in the stock solution, because when the barren solution next comes in contact with the lime added in the mill it is broken up, the aluminium being precipitated as the insoluble calcium compound with liberation of caustic soda.

There is usually a tendency to a formation of soluble sulphides as the result of precipitation by this process. It has been suggested that this is caused by a decomposition of the soluble sulphocyanates present, but laboratory tests made in weak solutions of KCNS (0.05% to 0.15%) have failed to show any signs of such a reaction. Sodium thiosulphate is easily decomposed by aluminium dust with formation of sodium sulphide, so where soluble sulphides are found after precipitation they are probably due to the presence in the stock solution of thiosulphates.

At the Nipissing mill the formation of soluble sulphides was considerable and a few drops of lead acetate added to a little of the effluent from the press in a beaker would turn it a dead black. As the addition of lead salts to neutralize this sulphide seemed to have a prejudicial effect on the subsequent extraction of silver from the ore, the expedient was adopted of installing an air lift in the barren sump, and a few hours agitation in this way served to remove every trace of soluble sulphide, rendering the solution perfectly efficient in dissolving power when next brought in contact with the ore pulp. At the Butters Divisadero mill only traces of soluble sulphide were ever found in the press effluent and none at all in the barren sump, and it was not found necessary to take any precautions to counteract their effect.