The removal of metal values from cyanide solutions, fortunately, can be done in several ways. The oldest practiced method is, cementation with zinc, or Merrill-Crowe.
The original patent for recovery of gold and silver from cyanide solutions described precipitation with zinc dust. Most early practitioners, however, used granules of zinc in a successful attempt to avoid payment of patent royalties.
The essentials of their design are still used today. They are:
a) Clarifying filters,
b) Vacuum degassing,
c) Addition of sine dust,
d) Filtration of the product.
Today the clarifying filtration is done with every kind of filter imaginable. Most of these filters are precoated with diatomaceous earth, DE, to produce extremely clear solutions. The design criterion of these filter vary with turbidity of the leach solutions.
Vacuum degassing is done in a packed tower. The clarified pregnant solution is introduced into the top of the tower and percolated down through the packing. Personal experience has shown that 6.75×10 4 Pa (20 inches Hg) gauge vacuum is sufficient for effective precipitation at 1500 m (5000 feet) MSL.
Zinc feeders are the nemesis of the Merrill-Crowe operators. The angle or repose of zinc dust is in excess of 90 degrees, it will “rat hole” and bridge. The cures for this problem are legion! In small package units the feeders are tapered bottomed boxes with augers or springs to advance the zinc to a discharge port. The rate of discharge is adjusted by varying the speed of the auger. To keep the zinc flowing the entire feeder is vibrated and/or finger springs are set to “click” along the auger.
It is apparent that one mole of zinc will precipitate one mole of gold ( or silver or copper ). Therefore, one gm of zinc should precipitate 3.02 gm of gold or 1.64 gm of silver or 0.97 gm of copper.
Experience has shown that as the pregnant solution becomes weaker the more excess zinc must be added. Loosely stated above 100 ppm should require less than 10% above stoichiometric. At 5 ppm approximately 200% of stoichiometric and 1 ppm may require as much as 1500%.
Metallurgical considerations dictate that the zinc must be added in an approximately constant manner. In small systems this can be done with screw feeders and variable speed drives. On larger systems belt feeders can be used.
It is important to keep the zinc dry and covered. Provision must be made to keep the zinc flowing. The angle of repose of zinc powder is 90°.
A word of warning is in order about product filter feed pumps. These pumps MUST have submerged seals. There is a tendency to try to cut costs by not not placing this pump under barren solution. This is especially true with small “shoe string” operations and in some snail packaged units. The problem surfaces when the seals start to leak, and they will leak. Because the seals are at the center of the pump, they are in a negative pressure area and air leaks into the pump. A small air leak can quickly undo everything accomplished in the vacuum tower.
Most product filters are of the plate and frame type. They are used because of the ability to blow them down to remove a substantial amount of the water from the precipitate. Some plants, where filter plugging of the product filter has been a problem, have found sock filters to be acceptable.
Systems which use sodium hydroxide, rather than lime to prevent Calcium plugging of the product filter, may be generating sodium silicate which will very effectively blind off the clarifying filters. It is also true that most floculants work better in lime systems.