Hypochlorite leaching at pH 7 evolves very little chlorine, if run outdoors there is no need to seal the leach vessel.
As in all leaching you have to get movement of the liquor through the particles of ore.
This can be done by agitating the pulp physically or by moving the liquor through the pulp solids as in a vat leach.
Any metal coming into contact with the leach will be attacked so you are looking to have all wetted surfaces made of plastic, this includes any pump internals.
Usually in a vat leach you would have a peristaltic pump with plastic hosing pumping through a plastic canister containing activated carbon.
If using mechanical agitation either a recirculating peristaltic pump with a carbon screen or air agitation is used, carbon rates are around 5 grams per litre in an agitated system or around 10 times the carbon volume as clean liquor in a vat system.
This means that if you had 1 litre of carbon in a canister you could pump 10 litres of liquor per hour through this carbon, this is scaleable.
Silt and other fines present no problem with this type of leach.
Concentrate particle size can present a problem if the particles encapsulate the gold, if the leach cannot contact the gold it cannot leach it.
Always keep in mind that if you have sulfides present in the ore you have to keep a close eye on the leach pH.
Litmus papers will work as well as a pH meter for this type of leach.
If using a pH meter keep in mind that the reading will be depressed by about 1 unit by the salt, so if the meter is reading pH 7 the actual pH is 8.
If using a meter you usually run the leach at a reading of pH 6 - 7, this gives an actual leach pH of 7 - 8.
pH 8 is about the top end of your leaching range.
Hypochlorite is usually added dry as calcium hypochlorite unless there are sulfides present where the formation of gypsum in quantity would present a problem.
Liquid sodium hypochlorite contains much higher proportional quantities of caustic than dry calcium hypochlorite, so is not commercially used because of the extra acid required to get the pH back into range.
If you do not have a high consumer of hypochlorite in the ore then you just add around 5 grams per litre of dry calcium hypochlorite to start the leach.
If you do have a high consumer of hypochlorite present you would start around 10 grams per litre and keep a closer eye on the pH.
The obvious consumers of hypochlorite are sulfides and organics. The pH shift from organics is much less than from sulfides.
The level of hypochlorite in the leach is usually monitored with an Eh meter, this is usually a single meter with separate probes for pH and Eh.
Start with your known level of hypochlorite in the leach at ph 7 and take the Eh reading, this is around the value you want to have.
If you do not have an Eh meter you can approximate the level by smell, the level of free chlorine is very small and presents little danger if sniffed carefully.
This is not a recommended method but is used in emergencies.
Usually the pH and Eh move together over extended periods of time.
In the short term a drop in pH will lead to the evolution of free chlorine to the point where the Eh is very high.
While this chlorine is being evolved the Eh will be very high but as the available chlorine level is depleted the Eh will drop substantially to the level where gold in solution levels cannot be maintained.
Sodium hypochlorite is sold by weight percent, do the maths to get the equivalent grams needed.
Sodium hypochlorite degrades fairly rapidly in storage, unless the supplier moves a lot of well rotated stock you may not get what you expect.
Generally charcoal will have some gold adsorption properties, it will not be anywhere as good as specialised activated carbon.
It also has the unfortunate property of being brittle and will attrition readily.
As the gold is actually reduced to metal on the outer surface of the carbon any attrition losses of carbon result in the higher than expected losses of gold.
If you do have to use charcoal make sure that you pre-attrition it before use to lessen the losses.
Deano
