Under what conditions is grinding unnecessary for liquid-solid leaching in mineral processing?

Question

I am a chemical engineering student and I am having trouble understanding a passage in a separation process textbook I am reading. The passage is about the preparing solids for leaching, such as grinding and crushing ores. At the end of the paragraph, the textbook states "Grinding of the particles is not necessary if the soluble material (the desired product) is dissolved in solution adhering to the solid. Then, simple washing can be used, as in washing of chemical precipitates."

I have tried researching this myself, and the only relevant information I found was if the desired product is on the surface of the solid, no grinding is needed. Is this what the textbook is trying to say? I am having a hard time understanding the wording of the textbook, especially about "solution adhering to the solid".

Reference I found: https://nitsri.ac.in/Department/Chemical%20Engineering/Leaching.pdf

Answer 1

It might help to consider this with the mind set of a primary metallurgist, not a chemical engineer. Primary metallurgist try to recover minerals, not chemicals, but they usually use chemicals in trying to do so.

Most metalliferous minerals that are processed by primary metallurgists are sulfides. Sulfides of such minerals usually do not dissolve in water. Some metalliferous oxides, chlorides and fluorides can dissolve in water.

Water insoluble sulfides are usually crushed and ground so they can be initially processed in a froth flotation plant to obtain an initial bulk concentrate. The size of grinding depends on the size of the mineral being recovered. Large minerals only require a coarse grind, whereas small sized minerals require a finer grind.

The slurry of crushed rock and water is placed into an agitator tank, a frothing chemical is added and a collecting chemical, usually a xanthate, is also added to the tank. The xanthate attaches itself to the sulfide mineral making it hydrophobic. Agitation keeps everything in suspension. Forcing air through the bottom of the tank, with the frothing agent produces stable bubbles in the slurry. The sulfides being hydrophobic, because of the attached xanthate, attach themselves to the air bubbles and rise to the surface to be scooped off.

Water soluble minerals don't need to be reduced to a specific size for them to dissolve, but the smaller they are the easier they dissolve. Such minerals can be recovered using counter current processing plants where dissolving water based liquor is pumped from one tank to another. In moving from one tank to the next the concentration of required elements increases.