Solid residue from the second filtration stage is repulped with reclaim water to 35% solids, and is pumped to our EXAMPLE Conditioning and Argentite Flotation Circuit which is a process for the recovery of this Silver Mineral.
A large portion of the silver in the ore will not have been leached in the initial grinding and leaching circuits. It is therefore the intent of this circuit to recover as much silver mineral (Argentite) as possible by producing a high grade silver flotation concentrate and delivering it to the Argentite Leach Circuit for further leaching.
Froth flotation separates different minerals by using the surface characteristics of the different minerals. The degree of preferential non-wetting or wetting of a mineral surface by water allows easily wetted minerals to separate from non-wettable minerals. Air bubbles, created by introducing air into a bubble diffusing mechanism, attach themselves by surface tension to the non-wettable minerals. The strength of the attachment is regulated by the mineral surface characteristics and can be enhanced or hindered by the use of chemical reagents.
For effective silver recovery in the flotation circuit, the flotation pulp must be thoroughly conditioned prior to entering the flotation cells. Two large flotation conditioning tanks provide this initial flotation pretreatment. The first conditioning stage is used to reduce the residual cyanide concentration in the pulp to less than 5 parts per million (ppm) total cyanide. This cyanide destruction process consists of oxidation of cyanide using a mixture of air and sulphur dioxide. The overall reaction being as follows:
CN(total) + 02 + S02 + H20 > CNO + H2S02
The second conditioning stage is used to condition the flotation pulp with flotation reagents. The flotation reagents used in this circuit consist of: potassium amyl xanthate (PAX), as the collector; Aerofloat 208 (A-208), as the promoter; methyl isobutyl carbinol (MIBC), as the frother; lime as the pH modifier, copper sulphate as the activator; and sodium silicate as the slimes dispersant.
The Argentite Flotation Circuit is designed to provide one rougher stage and two cleaner stages of flotation. The rougher flotation circuit is designed to recovery the maximum possible amount of silver (and remaining gold) in the secondary filter residue and the cleaner flotation circuit is designed to maximize the silver grade of the final flotation concentrate.
The Conditioning and Flotation Circuit is designed to the following criteria:
Feed Slurry CN Concentration —- 5 ppm maximum
Rougher Flotation Time ————20 minutes
Cleaner Flotation Time ————-15 minutes
Pulp pH ———————————- 8
Detailed Process Description and Control
Repulped slurry from the filtration circuit is pumped from the conditioning pump tank to the first conditioning tank by the 5 HP 4″x3″ conditioning feed pump at a rate of 48.7 cubic meters per hour and 35% solids.
The first conditioning tank is 4 meters diam x 4 meters and is equipped with a 50 HP agitator. This tank is primarily used to destroy cyanide according to the reaction given in the introduction of this section. Air is sparged into the bottom of the tank and the dual blade agitator dissolves oxygen into solution. Copper sulphate is added to the tank to assist the cyanide destruction reaction and sodium metabisulphite is added to provide the sulphur dioxide required to destroy the cyanide. Lime is added to this tank to maintain the protective alkalinity of the slurry, consequently neutralizing any acid generated from the destruction process. A hydrogen cyanide detector probe located in the vicinity of the first conditioning tank, will alarm at the mill control panel HCN controller if the level of toxic HCN gas near the tank reaches 30 ppm. An exhaust fan vents any gasses from the closed tank to the outside of the mill building. The size of the tank provides one hour retention time for the destruction reaction to occur. Slurry exits the first tank through a 6” riser pipe and flows into the second conditioning tank. The process air flow to both conditioning tanks is monitored by individual rotameters, and a single pressure gauge.
Silver Mineral Flotation reagents are added to the second conditioning tank: MIBC, A-208, and Xanthate, monitored by individual rotameters. Lime can be added to this tank to ensure a pH level of 8.0 respectively for flotation purposes. Sodium silicate can be added for a slime dispersant. An overload condition on either of the first or second conditioning tank agitators is alarmed at the mill control panel annunciator. The 50 HP agitator in the second conditioning tank is two speed and requires gland seal water. This tank can be used for cyanide destruction purposes if necessary; consequently, it is equipped with air sparge lines, copper sulphate, and sodium metabisulphite lines identical to the first tank. Under normal conditions the agitator will be run on the lower speed, and during cyanide destruction it will be run at the higher speed. The discharge from the second conditioning tank is pumped by one of two 7.5 HP 4″x3″ flotation feed pumps to the rougher flotation cells. In order to prevent the second tank being pumped dry and the agitator from being exposed, this tank is maintained at a high slurry level with the conditioned slurry overflowing into a separate pump discharge compartment. A high level switch will alarm at the mill control panel annunciator on excessive overflow or high slurry level. The pH of the second tank is monitored and is calibrated to alarm at the mill control panel on a pH of 8 or less. The instantaneous pH value is also monitored by a recorder located in the mill control panel. Any gases generated in the second conditioning tank are vented outside by an exhaust fan.
The flotation feed pumps deliver the conditioned slurry through a PSA sampler to the feed box of the first bank of rougher flotation cells. A cyanide detector probe monitors the total cyanide concentration in the slurry, which should be approximately 2 ppm. A 5 – 10 ppm cyanide concentration will alarm at the mill control panel annunciator.
The rougher flotation cells consist of two banks of four 100 cubic foot cells each. Each bank of rougher cells is equipped with two 7.5 HP motors and each motor drives two flotation mechanisms. The rougher cells are equipped with a feed box; prior to the first cell, a junction box; between the two banks of cells, and a discharge box; at the end of the last rougher cell.
The junction and discharge boxes are equipped with manually operated plug valves and are provided to control the overall pulp level in the rougher flotation cells. Reagents can be added to the feed box and/or the junction box, and feed rates can be controlled and monitored by rotameters.
Conditioned pulp is fed to the feed box of the rougher flotation cells and is drawn into the bottom of the first rougher cell by the flotation mechanism impeller. Process air from the flotation blower is delivered down the mechanism shaft to the impeller and is combined with the pulp. The air rate to each bank of rougher cells is adjustable and is monitored with rotameters. The impeller thoroughly mixes the air and pulp together and then expels this mixture against the diffuser blades which further mixes the air and the pulp. The aerated pulp is distributed over the bottom of the cell where the conditioned sulphide minerals attach to the air bubbles and are transported to the surface of the flotation cell; forming a stable froth layer on the surface of the pulp. The tailings material from this first rougher cell overflows a manually adjustable tailings weir and is drawn into the second rougher cell and the flotation process of mixing the air and the pulp is repeated. Reagent addition at the feed box and junction box of the rougher cells is provided to allow maximum operating flexibility. By the time the flotation pulp reaches the last rougher flotation cell, the majority of the silver and other sulphide minerals will have been recovered in the froth of each flotation cell. The froth from each bank of cells overflows into a common froth launder and flows by gravity to the 2 HP 2″ vertical rougher concentrate pump. Water is added at the froth launder to aid in transporting the concentrate.
Silver Mineral Rougher concentrate is pumped to the feed box of the first stage of cleaner flotation cells at a rate of 2.6 cubic meters per hour and 25% solids. The first stage of cleaning consists of 3-30 cubic foot cells. The flotation process of the cleaner cells is identical to that of the rougher cells and likewise, flotation reagents can be added to the feed box of the first cleaner cells. The air rate to the cleaner cells is also adjustable and is monitored with rotometer. The froth from the first cleaner cells is swept off the pulp surface of each cell into the common froth launder with a rotating paddle. The first cleaner concentrate flows, with the help of spray water in the froth launder, directly into the single 30 cubic foot second stage cleaner cell. The concentrate from the second and final cleaning stage is collected and flows by gravity to the 2 HP 2″ vertical Argentite leach feed pump.
The tailings from the second cleaner cell overflow into the first cell of the first cleaner stage and the tailings from the first cleaner stage are recirculated back to the feed of the rougher stage with the 2 HP 2″ vertical cleaner tails pump.
The tailings from the rougher flotation stage flow by gravity from the rougher cells discharge box through the tailings sampler and into the mill tails box. The tailings from the Argentite Flotation Circuit are the main tailings from the processing plant.