134 The NJZ. Journal of Science and Technology. [June 
no insuperable difficulty to any one familiar with the cyanide process. 
Sodium hydrate (caustic soda), 76 per cent., is quoted in the Mining 
Magazine, October, 1920, at £32 per ton, and sodium sulphide £46 per 
ton. The sulphide is probably commercial sulphide, containing about 
56 per cent, anhydrous sulphide. The above prices are therefore equivalent 
to caustic soda, 100 per cent., £42 per ton ; sodium sulphide, 100 per cent., 
£82 per ton. The quantity of solution recommended is 50 lb. for every 
pound of mercuric sulphide, or, say, to every pound of mercury recovered. 
100 lb. mercury would therefore require 5,000 lb. solution, containing 
2001b. sodium sulphide, £7 10s.; 50 ib. caustic soda, £1: total, £8 10s. 
The solution could be used repeatedly, but allowing 10 per cent., which 
appears reasonable, for loss in tailings and for decrease in strength by 
oxidation, the cost of chemicals for every 100 lb. of mercury recovered 
would be one-tenth of this amount—47s. ; or a fraction over 2d. per pound. 
On a ton of ore assaying 0-75 per cent, mercury this would be 3s. Other 
costs would bring the amount up to at least 10s. 
Electrolytic Precipitation .—With a voltage of 2*5 and a current-density 
of 0-05 amperes per square centimetre the precipitation of the mercury 
is practically complete. This means that each pound of mercury precipi 
tated per hour would require 3 square feet of electrode surface, and would 
consume 0-3 kilowatt of power. The cost of this, assuming 30 per cent, 
efficiency, would not exceed 0*3d. The electrode surface may be reduced 
to 1 square foot per pound of mercury recovered by employing a stronger 
current, but the precipitation is not so complete. As the same solution 
would be used repeatedly, the mercury would, however, be ultimately all 
recovered. 
The estimate of cost is little more than a guess, and the total, working 
on a moderate scale and exclusive of mining costs, would be from £2 to 
£2 5s. per ton of ore treated, or 2s. 9d. to 3s. per pound of mercury 
recovered from an ore assaying 0-75 per cent, mercury. The costs in 
California in 1917 were approximately 3s. 6d. per pound of mercury 
recovered, including development and amortization. The ore averaged 
0-38 per cent, mercury, and w'as treated by furnace methods; In Texas 
the costs were lower, as the ore treated assayed 1-43 per cent, mercury 
(Mineral Industry, vol. 27, p. 632). 
With furnace treatment crushing of the ore is eliminated, and the 
mercury is recovered directly in a single operation. A modern furnace of 
the Scott type is provided with ample condensation pipes and chambers, 
and is, I understand, very efficient. Another type of furnace coming into 
use is the rotary cement clinker furnace. In this there would be no 
difficulty with fine material. 
The data obtained will enable a comparison of the furnace and solution 
processes to be made, and local conditions may render the adoption of 
the sodium-sulphide extraction advisable. In such a case the electrolytic 
precipitation would require to be developed from a laboratory process up 
to one capable of being used on a commercial scale. 
The experimental work done shows— 
(1.) That oil concentration can be successfully applied to Puhipuhi 
cinnabar-ore : 
(2.) That the Thornhill process can be used for treating the concen¬ 
trates : 
(3.) That electrolytic precipitation could probably be applied to recover 
the mercury : 
(4.) That, provided mining costs are not excessive, the cost of the 
process should leave some margin of profit. 
