Whatever method of gold precipitation be used it is of great importance that the solution should be absolutely clear and free from suspended matter, and in practice it is almost impossible to obtain solution from slime treatment, either by decantation or by means of the various types of slime filter, that is sufficiently clean to give the best results. It is therefore almost always advisable to clarify it by filtration before precipitation.

There are special presses on the market for this purpose, or a cheap and efficient filter may be made by using submerged filter leaves of the Butters type, operated either by vacuum in an open tank or by pressure in a closed receptacle. Perhaps the simplest and cheapest device is a sand filter, or better a pair of such filters for alternative use. They may be circular tanks 20 to 30 feet in diameter and 3 or 4 feet deep with a filter bottom such as is used for leaching tanks, covered with six inches of clean river sand of from 20 to 40 mesh. The suspended matter will collect in a thin film on top of the sand, and when percolation becomes too slow it is only necessary to drain the tank dry, skim off the surface with shovels, and sprinkle a little more clean sand over it, the whole operation taking only a few hours.

To ensure the maximum efficiency and minimum of trouble with these filters it is advisable to run the solution first to a settling tank of about the same size fitted with a baffle board extending right across it, and from the top downward about 18 or 20 inches; this baffle gives a tranquil surface and good settling conditions. The solution is introduced on one side of the baffle and flows out at the opposite side into the sand filter, the tank remaining all the time full of solution. Most of the suspended matter will settle down to the bottom of this tank which is periodically decanted and the collected slime hosed out either to the agitators or slime filter. By this device a minimum of work is thrown on the clarifying filter and under ordinary circumstances it should need skimming only about once a month and may even run two months without attention.

Gold Precipitation with Zinc

Electrolytic Gold Precipitation

Aluminium Precipitation

Sodium Sulphide Precipitation

Charcoal Precipitation