ADVANTAGES
ADVANTAGES. Those ores which have garnet for a ganguemineral will in general be found particularly amenable to flotation processes unless a large amount of calcite is also present. Barite gangue is also separated in many instances with ease. Felspar ores are treated with excellent results. Schistose ores, which produce a large amount of slime, will also be found amenable to at least one of the processes. Copper ores, where the sulphides are so finely disseminated as to require crushing to extreme fineness to free the sulphides from the gangue, are the special province of these processes, and they will provide the greatest scope for the future extension of this method of treatment. The ores of the Great Fitzroy mine and the Caucasus Copper company are of this type. Many of the ores where the valuable constituents are in the form of silver sulphide, accompanied by pyrite in a quartz gangue like the ore of the San Pedro mine, Guanacevi, Mexicogive an excellent separation with high-grade concentrate and good recovery. This class of ore deserves close study in connection with these processes. An ore from Japan, containing gold apparently locked in stibnite, gave good results. This ore yielded scarcely any of its gold content by amalgamation, gravity concentration, or any chemical method. This ore required very fine grinding to free the stibnite from the quartz.
An ore from Bolivia, in which the gangue is quartz and garnet, and the valuable content in tetrahedrite, yielded 93% of the gold and silver in small tests in a high-grade concentrate.
BROKEN HILL. In 1904 there was estimated to be in the dumps at Broken Hill the following by-products :
Mine. Material. Tons. Zn. Pb. Ag.
% % oz.
Proprietary tailing . . 2,500,000 17.0 4.0 6.0
Block 10 tailing . . 520,000 22.0 6.0 9.5
middling . . 137,000 22.5 7.5 10.5
slime . . . . 165,000 18.0 10.0 9.0
Central tailing . . 1,000,000 22.0 7.0 6.0
slime . . . . 600,000 21.0 17.0 20.0
South tailing . . 620,000 19.7 7.2 4.2
slime . . . . 100,000 18.4 14.6 6.4
Block 14 tailing . . 250,000 16.5 7.0 6.5
British tailing . . 250,000 17.0 4.8 5.0
slime . . . . 125,000 21.0 8.9 10.0
Junction tailing . . 10,000 19.0 16.0 10.0
tailing . . 60,000 12.0 7.0 7.0
slime . . . . 25,000 13.0 12.0 12.0
Junction North tailing . . 95,ooo 12.4 10.0 9.0
slime . . . . 5,ooo 13.0 n.o 12.0
North tailing . . 100,000 17.0 8.0 5.0
slime . . . . 20,000 14.0 n.o n.o
Total . . 6,582,000
The annual production of zinc concentrate by flotation processes since the above estimate was made has been as follows :
Tons.
1904 . . . . . . . .57,602
1905 . . . . . . . .103,532
1906 . . . . . . . . 102,664
1907 . . . . . . . .236,251
1908 . . . . . . . .275,932
1909 . . . . . . . .373,906
1910 . . . . . . . .468,627
1911 . . . . . . . .500,000
Total estimated . . . . . . . .2,118,514
The ratio of concentration on this material has been about 3.6 to i, so that 8,000,000 tons of material has been treated. There still remains untreated sufficient material to keep the present plants going until 1919, when the accumulated tailing should be exhausted. It is evident that the 1904 estimates of tonnage available were far short of the truth. It is not probable that there will be any further great increase in the annual production of zinc concentrate at Broken Hill. It seems more probable that the pinnacle of tonnage was reached in 1910, and that the production will decrease from now on until the accumulated tailing is finished. There should be no large drop in the total tonnage produced, because the Zinc Corporation, the Amalgamated Zinc, and the Broken Hill Proprietary have reserves of tailing that will last for several years to come. One large plant last year ceased operating, but its place was taken by two smaller ones. A steady slow decrease in production may be expected, because most of these companies, during their early struggles with the vagaries of the processes, used their best material to make ends meet. They will probably make as much money out of their lower-grade material as they did out of the higher-grade material. When these immense piles are exhausted, which can be definitely predicted for 1919 or before, there will be a sudden drop in the zinc production of Broken Hill, which should considerably disturb the world's zinc market. The present production of Broken Hill zinc represents just about 20% of the total world's production. It is a remarkable fact that this enormous increase in the production of a district, which seven years ago produced practically no zinc, has caused very little disturbance of the price. This was due to the fact that the highgrade Broken Hill zinc concentrate took the place of low-grade ores from many places in the world, outside of the United States, and these latter low-grade mines simply ceased production because the low-grade material could not compete with the high-grade at the same metal price.
The material treated by these processes at Broken Hill is of comparatively high grade, averaging at present close to 18% zinc, 6% lead, and 7-5oz. silver per long ton. In spite of these good assays, the profits are not fabulous. After charging working cost, royalty to process owners, smelters' charges, and management, there is left on an average close to i2s. per ton treated. In the case of some of the companies this is further decreased by the prices paid to the mining companies for the tailing. The average price for all the tailing and slime sold by the mining companies to the treatment companies averages about 6s. per ton, leaving on an average not over 6s. per ton profit to the treatment companies when zinc is 23 per ton. These are not the actual results of any one company, but are a fair average of the whole. Some of the companies increase this profit in favourable cases by anything up to 45. per ton by the re-treatment of the flotation-concentrate, by means of which re-treatment a good lead concentrate is taken out of the flotation-concentrate. This lead concentrate represents about 50% of the lead in the flotationconcentrate. These figures mean that if the price of zinc dropped to 20 per ton the flotation plants would have to close down, or else reduce the working cost or economize in some other quarter.
The present selling contracts to the German zinc smelters represent a handsome profit to the latter. It seems certain that every ton of concentrate, which mostly goes to Germany, represents a clear profit to the Germans of at least 405. per ton of concentrate or us. per ton of the original material. From the standpoint of the treatment company, this seems a rather liberal share of profit to the zinc smelter, and it speaks well for the acumen of the Germans that they tied up this large production under long contracts.
Some of the contracts under which this concentrate is sold are represented by the following equations :
(1.) Payment on board cars at Broken Hill = (Zn-8) (P-5%) + (Pb-8. 5 ) (is. 3 d.) + (Ag-5oz.) (P'/2) R Where Zn is percentage assay in zinc. Pb lead. Ag ounces per ton of silver. P is average price of zinc over six months following delivery. ,, P' is average price of silver over six months following delivery. R is a smelting and freight charge, which is 5 when the price of spelter is 17 IDS., and increases 35. per ton for each i increase in the average price, and decreases 33. per ton for each i decrease in the average price. There is a penalty of is. 3d. per unit for every unit of lead under 8%, but the lead penalty is never to exceed the payment for silver.
(2.) Payment on board cars at Broken Hill = (Zn-8) (P-5%) + (Pb-8) (is.) + (Ag-5oz.) (P'/2) R Where Zn is percentage assay in zinc. Pb lead. Ag ounces per ton of silver. P is average price of zinc during six months of delivery. P' is average price of silver during six months of delivery. ,, R is a smelting and freight charge which is 5 175. 6d. with spelter at 23, and increases 2s. 6d. per ton for each i increase in the average price, and decreases 2s. 6d. per ton for each i decrease in the average price. There is a lead penalty of is. 6d. per unit for each - unit under 5%.
