New found gold HEAP LEACHING AT HOPE BROOK

Hope Brook Gold’s heap leach operation was put in place principally to generate early cash flow in the order of $21 million throughout 1987 and 1988. In fact, production from the heap leach has already commenced — more than a year before the mine is to officially open. “Early cash flow was important for us,” Mine Manager Julian Bennet explains. “The reason is that Hope Brook is not the most robust of operations. Grade, at 0.13 oz gold per ton, is relatively low.”

In fact the size of the open pit has been increased so that 1.1 million tons can now be extracted (originally the pit was to provide 754,000 tons). This will help supplement the ore which is to be processed during the first 4-6 months of underground development.

“The mill circuit is designed so that both heap leaching (a carbon- column process) and conventional carbon-in-pulp milling can be carried out concurrently,” says General Manager Bill Fotheringham. “One reason for this is so we can draw revenue from the heap leaching while the mill is being commissioned. Another reason is so that any lower grade ore from the open pit or from underground development can be heap leached, if we choose to do so. Of course, that will depend on the price of gold and on the availability of such lower grade ore.”

About 48,840 tons of 3/8-in material have been laid on the heap leach pad. This serves to cushion the impervious, high-density polyethylene liner from heavy equipment which loads the 1/2-in material. The 294×272-yd pad is designed to handle 744,800 tons, and the heap leaching is scheduled to last a year and a half (afterwards crushing, grinding and cyanidation will be done in the mill). However, the pad has been designed for continual use, should the company decide to prolong the heap leaching.

“The ore here has a number of microfractures in the rock and no clay whatsoever, a condition which lends itself well to heap leaching,” says Larry Connell, superintendent of operations. “Fortunately we can control the size of the cyanide droplets which are deposited onto the ore from the sprinklers. Because of the high winds on the property, coarse droplets are preferred.”

After the gold has been dissolved in the solution, it is pumped to a series of carbon columns. There, the dissolved gold-cyanide complex is absorbed onto the granular carbon. The carbon is moved counter- current to the solution flow and the loaded carbon is moved into the stripping gold recovery section. The carbon is acid-washed and the gold is stripped back off the carbon using a strong cyanide caustic solution in a high-temperature, high-pressure stripping tower. The gold-bearing strip solution is then passed through an electrowinning cell where the gold is recovered on wire wool. The wool is acid-digested in sulphuric acid, the residue is dried and smelted down to cast a 600-oz bar worth more than $300,000. The carbon is regenerated and recycled.