Refiners of lead by the Parkes process have always been solicitous of recovering the zinc used in the desilverization, and justly so, as the loss in zinc constitutes one of the heavy costs in this method of refining. Part of this loss is due to the absorption of zinc by lead, and in the present state of knowledge is not recoverable as metallic zinc. The remainder of the zinc, which finds its way into the precious-metal-bearing crust, is recoverable and naturally engages the operators’ most exacting attention.
Careful elimination of the impurities in base bullion has enabled the refiner to produce a zinc crust better adapted to retorting, which reflects favorably in the zinc recovery; and to produce a richer crust, thereby lowering the zincking cost, as the increased efficiency of the zinc results . in less of it being in circulation.
It is obvious that if the ratio of concentration be increased, a lessening of the amount of crust formed will ensue, throwing less work on the retorts and diminishing the amount of retort metal sent to the cupels, thus making an actual money saving in all these operations.
In the effort to obtain a rich crust, the metallurgist has increased the heat to a considerable degree, upon the assumption that at the higher temperature the zinc-silver alloy could be squeezed free from the adhering lead.
A further possibility for improvement suggested itself in the freezing-point curve of Prof. H. C. Carpenter, upon zinc and silver. Study of this curve disclosed the existence of a definite chemical compound, Zn3Ag2, with a freezing point of 665° C., a promising compound for practical application.
In the experiment undertaken it was endeavored to force this compound into the crust, hoping thereby to obtain a higher concentration than formerly. It was recognized that in bringing a large kettle of lead to such a heat, more fuel would be used, the life of the kettle would perhaps be shorter, and other disadvantages would be experienced. As a commercial operation, the project would have to stand on its own feet, so the records were carefully kept to balance the advantages against the disadvantages.
The first experiment, however, showed such refinement unnecessary, as it was conclusively indicated that the melting point of the zinc-silver compound was so modified as to nullify its feasibility as a commercial factor.
In the first experiments the bullion in the kettle was heated to 705° C., 40° above the melting point of the zinc-silver compound to be formed, and the zinc stirred in as rapidly as possible. Great care had to be taken in adding the zinc to the red-hot bullion, as in more than one instance the zinc took fire and spoiled the experiment. The work about the hot kettle was very trying to the men, and had we been successful in the endeavor to make a rich crust, it is doubtful if we could have held our kettle crew to the work. After the zinc was stirred in, the kettle was cooled, the crust removed in approximately 55° C. stages with the Howard press, and squeezed at a pressure of 90 lb. per square inch. The crusts removed at the various stages were retorted separately, a dip sample being taken from the retort metal and carefully assayed. All conclusions are based upon the assay of the rich bullion remaining in the retort after distillation, and referred to as retort metal. Reasonably accurate sampling of the rich zinc crust is practically impossible.
It will be seen in the following experiments that, in every instance, the crust removed at the highest temperature, that is, just below the freezing point of Zn3Ag2, furnished the lowest retort metal.
Dore contents of bullion, 191 oz. Zinc stirred in at 705° C.
Dore contents of bullion, 253.8 oz. Zinc stirred in at 705° C.
Dore contents of bullion, 315.5 oz. Zinc stirred in at 705° C.
These tests clearly demonstrated the impracticability of removing the crust at a very high temperature. Further experiments were made to determine what beneficial result, if any, would be obtained by the
greater heat. In these experiments, two kettles of metal of the same dore contents were desilverized, one heated to 705° C., and the first press removed at 535° C.; the other treated according to the normal procedure, the zinc being stirred in at 535° C. and the pressing begun immediately.
Dore contents of bullion, 263 oz. Zinc stirred in at 705° C.
Same bullion stirred at 535° C., pressed at 535° C. = 3,210 oz.
Dore contents of bullion, 315 oz.
Stirred in at 705° C.; pressed at 535° C.; retort metal, 4,165 oz.
Stirred in at 535° C.;.pressed at 535° C.; retort metal, 4,465 oz.
Dore contents of bullion, 318.4 oz.
Stirred in at 680° C.; pressed at 535° C.; retort metal, 5,011 oz.
Stirred in at 535° C.; pressed at 535° C.; retort metal, 5,505 oz.
In all cases where the greater heat was used for stirring and pressing, the dross and blue powder formed, especially the former, were far in excess of current practice.
The accompanying curves (Fig. 1), to some extent, show the effect of the temperature. The bullion desilverized in the various experiments was of different dore contents. It is a well-known fact that a richer crust will result from high-grade bullion upon desilverizing than from low grade. I have endeavored to compensate for this, in the curve, by converting the dore contents of the retort metal, in proportion to the assay
value of the bullion treated. A sudden drop in the grade of the retort metal at the lower temperature is due to the increase of lead contents, the lead solidifying too quickly to be squeezed from the zinc-silver compound.
The reasons for the fact that desilverization at high temperature gives a lower retort metal are not quite clear. Alder-Wright and Thompson refer to the solubility of the zinc-silver compound in lead, also its dissociation by means of lead. Kremen and Hofmeier in a paper upon Ternary Systems of Lead-Zinc and Silver-Zinc Compounds, base their investigations upon the old freezing-point curve of Petrenko. They overlooked, however, the point vital to the practical application, namely, the solubility of the zinc-silver compound in lead, or its desilverization by means of lead. A full explanation of the results obtained in the above experiments would require an extensive laboratory research upon the behavior of Zn3Ag2 with lead at varying temperatures. While this would be interesting, we could hardly alter the facts.
The conclusion to be drawn is, that as a commercial project, the slight increase in dore contents of the crust, which might be obtained from zincking at the greater heat, is more than counterbalanced by the attendant disadvantages. The current practice of to-day, dictated more or less by the question of fuel economy and better life of kettles, has been justified as a practical and efficient procedure from the standpoint of concentration. For comfort of operation, effect upon the men, etc., all arguments are in favor of zincking at a lower temperature.