WORKING COST
The working cost is difficult to calculate. In the Broken Hill Proprietary Co.'s half-yearly report for the period ended December 31, 1910, the tonnage is given as 179,713, and in the working account 45,213 i8s. id., or 55. o|d. per ton, is charged to the zinc plant for ' wages, salaries, and expenses.' There are, however, items of electric light, machine shop, assaying, electric power, experimental account, fire account, fuel and fluxes, water, locomotive traction, and horses and drays, a portion of which should be charged to the zinc treatment. The previous half-yearly report gave 103,792 tons treated at a cost for ' wages, salaries, and expenses ' of 44,999 195. 4d., or 8s. i id. per ton treated. The same list of other items was also given. It is a hopeless task to obtain the actual working cost of the process from these reports. We can, however, arrive at one useful deduction, namely, that 3olb. of acid was used per ton treated. The average market price of acid in Broken Hill is about one halfpenny per pound, so that the cost of acid was is. 3d. per ton of material treated. The sum of 4,846 was spent for water during this period. As water costs 55. per 1,000 gallons at Broken Hill, this means 19,384,000 gallons of water was used in the operations. About 100 gallons of water per ton is the loss at Broken Hill, and these figures fit sufficiently well to justify the inclusion of 6d. for water. The heating of the pulp to 82 C. is also a charge which can be accurately deduced from a knowledge of local conditions, and amounts to nearly 6d. per ton. The power charge is practically confined to the items of pumping solutions and elevation of pulp. This charge cannot be ascertained from the published reports ; an estimate would be 3d. per ton. Tabulating these and other items, the following table gives the items which concern the actual process itself, excluding all the expense of tramming, de-sliming, and grinding :
The acid would actually cost the company less, but is charged at Jd. per Ib. for the purpose of comparison.
In the report for the last half of 1910 figures are given from which a fair idea of the recovery can be obtained. To do this it will be necessary to assume the metal content of the material treated. There are no authoritative figures relating to this, except in the Annual Report of the Department of Mines of New South Wales for 1904, where the Proprietary company is said to have 2,500,000 tons of tailing assaying 17% zinc, 4% lead, and 6oz. silver per long ton. This, it is believed, is too low. Using these figures, the recovery and products are as follow :
For the six months ended November 29th, 1911, 137,249 tons was treated, yielding 3,136 tons of lead concentrate assaying 54% lead and 28 oz. silver, and 48,192 tons of zinc concentrate assaying 45% zinc, 7% lead and 13oz. silver. The concentrate being produced now assays 47% zinc.
