Nitric acid digest

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Definition and application

A NAD is a nitric acid digest and it is used to determine the best crush size of the ore for heap leaching once amenability testing has been successfully completed.

Method

Parcels of ore are crushed to different crush sizes then subjected to a timed NAD. The nitric acid dissolves any water soluble and acid soluble metals as well as leaching any exposed sulphides. Once complete, the recoveries are determined and a decision is made as to the optimal crush size of the ore in the heap leach. Often there is very little difference in metal recovery between two crush sizes. When deciding the crush size of the ore there needs to be a substantial increase in recovery to justify an additional stage of crushing. The NADs are used as indicators of crush size rather than of recovery in the heaps. In the past, the recovery from operations has been significantly higher from a heap than from the NADs as the ore decrepitates with time.

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