LOW GRADE ORE.

Hauraki Plains Gazette, Volume XXXX, Issue 5465, 23 August 1929, Page 4

LOW GRADE ORE.

OHINEMURI AND THAMES. AMERICAN COMMENT. On the Waihi field, throughout the Ohinemuri district and at Thames, there is an enormous tonnage of ore that cannot be treated to profit owing to the high cost of mining and milling. The great bulk of this had to be discarded as unprofitable under the treatment methods followed in prewar times, and with the coming of the war, which increased the cost by several shillings per ton, the quantity of quartz that had to be “written off” reserves as of no commercial value was materially swelled. With apparently no prospect of a return to the low cost of the earlier days, the possibility of treating the quartz is still awaiting solution, and at the annual meeting of the Thames School of Mines the director, Mr H. Crawford, touched on the question of low grade propositions and the possibility of handling the ore with profit. A copy of the paper read by Mr Crawford was sent to the editor of the “Engineering and Mining Journal,” a monthly publication of standing in the United States, who comments on the subject a$ follows :— Large Scale Mining.

“The problem is complicated, but it is one that should not be judged offhand as beyond solution. ‘Engineering and Mining Journal’ has frequently urged that the gold industry of such a region should seriously consider the adoption of large-scale mining and the formulation of a simple and inexpensive method of ore treatment. The first consideration, however, is the determination of gold content and recoverable percentage, necessitating adequate sampling and research. Assuming that the result promises profitable operation, or at least indicates the possibility of success, after assuming certain mining and metallurgical costs, the question of ore recovery must be planned in detail and on a scale that is courageous enough to disregard customary practice elsewhere with higher grade material, in an cifort to duplicate the recent history of the metallurgy of copper. The metallurgical problem is perhaps the most important, because the success or failure of the enterprise may depend wholly on whether or not the ultimate recovery equals or exceeds the pre-determined recovery. Ball Mill Reduction. “No general article on the subject could be prepared that would be helpful. An intimate knowledge of the character of the ore is necessary before even a sound prophecy can be made that would hint success. However, one must accept as a foregone conclusion the abandonment of the stamp mill as a crushing unit, because of operating complexity and comparatively lbw capacity. Ballmill reduction will probably prove most economical for large tonnage output. A screened product is necssary ; sliming must be avoided, so a screen ball mill may be indicated, capable of large capacity at low power consumption. “To state what may appear to be a paradox, the metallurgical problem is mainly one of economics—to select the method giving the highest recovery at the lowest relative cost, initial and operating. The choice is narrowed to lie between simple amalgamation, oi’ amalgamation and cyaniding of the wet-crushed ore, or straight cyaniding of the dry-crushed ore.

“History records many examples of successful work on low-grade gold ore after the selection of either method, but courage or capital has been lacking that would indicate a determination to proceed on a tonnage scale similar to that which has brought prosperity to the copper industry. At the Wasp No. 2 mine, in South Dakota, an extraction of 70 per cent, of the gold by cyanide on dry crushed orc yielded a profit of 50 to 75c per ton (approximately 4s to 6s), with a cost of treatment of only 68c per ton (under 3s per ton). Faith and Capital. “The address provides interesting reading , A systematic account is given of the history of the Thames gold field, beginning with the original proclamation in 1867 and detailing all the parcels of ore treated and the tonnages and yields. The figures are impressive as indicative of the wide areal extent, of payable ore, and suggestive of further opportunity for expansion and the re-establishment of prosperity. Mr Crawford estimates that the low-grade gold ore in the locality amounts to about 40 million tons. He ".v-rs that ‘The present system of mining is too costly. . . . The methods w’ill need to be changed. The field lends itself to cheap methods of breaking and conveying the orc to the treatment plant. It could be worked wholly from the surface by means of benching and m : ng steam or electric shovels ’ “To ‘Engineering end Mining Journal” it does not seem improbable that the immense reserves of low-grade ore in the Thames district may be • treated at a profit, provided the probis attacked with courage and faith and adequate capital, and that technical experience -and engineering skill are utilised.” —Waihi Telegraph.