Action of Oxygen in Precipitation

In regard to this question some interesting facts have been noted in connection with the recently devised method for determination of oxygen in working solutions already described in the chapter on " Testing and Analysis of Solutions," page 48. In the course of the article by H. A. White referred to and the discussion thereon it is shown that (1) the oxygen contained in the solution entering the zinc boxes is rapidly eliminated by the reducing action of the hydrogen formed and that the rate of precipitation increases as the oxygen content is diminished, and (2) that the formation of white precipitate seems to be a function of the amount of oxygen present, and that when the solution reaches a point where the oxygen has been practically eliminated, no white precipitate is formed even under conditions normally favorable to its production. The first observation tends a priori to demonstrate the advantage likely to be derived from a mechanical removal of the dissolved oxygen from the solution before precipitation while the latter suggests the possibility that by such a preliminary elimination of the oxygen the formation of white precipitate might be entirely prevented.

The Crowe Vacuum Process. It is interesting to note that almost at the same time that the action of oxygen in precipitation was attracting attention on the Rand, Thomas B. Crowe at the Portland Mine, Cripple Creek, was perfecting a process for the mechanical removal of dissolved oxygen from solutions, arguing that precipitation being a reducing process the less oxygen present the more favorable would be the conditions for the reaction. The invention consists in spraying or otherwise disintegrating the solution in a vacuum chamber by which means the dissolved oxygen is removed before the zinc dust is added, resulting in a reduction of the zinc consumption and also in some instances a reduction in the free cyanide loss which accompanies zinc precipitation. The process which is at its best when used in combination with zinc dust has shown savings in zinc amounting to two-thirds of the former consumption. In the case of zinc shaving precipitation its benefit is not so marked on account of the extreme rapidity with which the solution re-absorbs oxygen, but even here in the few instances in which it has been tried it appears to have resulted in a saving of zinc coupled with the possibility of obtaining good precipitation from solutions too low in free cyanide to precipitate under normal conditions.

The Zinc-lead Couple. The principle of coating the zinc with a film of lead to increase the activity of precipitation was patented by MacArthur in the early days of the process but it was not till the occasion arose for precipitating the exceedingly dilute solutions obtained from slime treatment on the Rand that the process attained to commercial prominence. When the decantation process of slime treatment was first mooted the impossibility of precipitating the solutions by zinc was assumed and the Siemens and Halske electrolytic process was considered indispensable. This had its drawbacks, however, and in the search for a substitute it occurred to W. K. Betty to try the zinc lead couple. He found that by its use, combined with a drip of strong cyanide solution into the head of the box he could precipitate satisfactorily solutions carrying as low as 0.005% in cyanide. The ordinary zinc shavings were dipped in a solution of lead acetate containing from one to five per cent, of lead, thus coating them with a deposit of spongy metallic lead. This zinc was then placed in the extractor box and covered with solution as soon as possible. A barrel containing concentrated cyanide solution was placed at the head of the box, and as soon as the pregnant stock solution was turned in a small drip of this strong solution was allowed to run in with it, and maintained for the first twelve hours. Julian and Smart1 state that this feature was later on abandoned.

When working with this process even under the most favorable conditions it was not found possible to maintain effective precipitation for more than five or six days after starting, and when the action became sluggish the zinc was transferred to the sand plant precipitation system and replaced by new.

Solutions Containing Copper. These sometimes give trouble by forming a continuous and adherent skin over the zinc and stopping further precipitation of the gold and silver. This effect is more often observed when the solution is low in free cyanide, and in strong solutions copper interferes very little with the proper working of the boxes. When trouble occurs from this cause the zinc lead couple may be found a valuable aid in counteracting it. Park1 states that its effect is to keep the copper in solution allowing only gold and silver to be precipitated but at the Creston Colorada mine in Sonora, Mexico, a system was in vogue, about the year 1900, devised by Mr. Higgins, the metallurgist, of precipitating the gold and silver with a minimum of copper, out of the strong and rich solution on ordinary zinc shavings and passing the weak and low grade solution through boxes in which the zinc was heavily coated with sponge lead, for the purpose of throwing out the copper and preventing its accumulation in the stock solution, which it was said to accomplish very satisfactorilv.

Should the presence of copper be found an insuperable obstacle to precipitation by zinc shavings, the zinc dust method will usually not be open to the same objections. The electrolytic process also may sometimes be effectively employed, either for recovering the gold, silver and copper together in the form of base bullion or for cleaning the stock solution of its copper content after recovering the gold and silver by the zinc dust method.

There is an interesting process in use at Dayton, Nevada, for precipitating the solution resulting from the treatment of the accumulated coppery tailings from the old pan amalgamation mills of Virginia City. The solution after the necessary contact with the ore is heated to about 200 deg. F. and strengthened with fresh cyanide before the zinc dust is added. By this means not only is no trouble experienced about precipitating the silver, but a large proportion of the cyanide combined with copper is said to be regenerated.