Considering that primary grinding circuit has no problems, the mill supervisor must be focused on the gold flotation circuit. Probably, the simplest case is related to gold-silver ores and the performance of the process is basically influenced by the criteria employed by the supervisor or operator to modify some parameters. In this way, the points and dosages of flotation reagents are a key aspect. Sometimes, the dosages used in the testwork program must be adjusted and probably, the dosage of collectors tends to be modified many times until the froth and gold concentrate look good. It is important to control the level of collectors due to they are the main responsible of the precious metal recoveries. The key parameters for operation of gold-silver flotation circuits are pH, dosage of depressant, addition of collector, level of activator, quantity of frother and mode of circuit operation. Good control of the operation parameters is fundamental to get a stable flotation circuit and high gold and silver recoveries.
Most the time gold-silver ores are processed at natural pH, but sometimes is important to increase the pH of flotation in order to have froth with appropriate viscosity. This aspect is overlooked by many people. The quality of the alkali is important, otherwise will be difficult to provide a stable pH. With more strict control and supervision, the optimum pH can be obtained. For example the optimum pH for rougher flotation is usually 7.5-8.0. The addition of lime or sodium carbonate is important because they provides stable froth and improves rate flotation. The restriction to add lime is related to the possible depression of gold mineral flotation; however, this is relative and depends on mineralogy. In this way, if the natural pH is 4.5-5.0, the addition of lime until pH 8 improves the flotation and froth texture. If the pH is near 11 the depression of silver sulphosalts is possible and must be performed the changes carefully.
Depressants are added to modify the surface of some minerals that interfere with the flotation process. It is important to mention that depressant only minimize the problem and not solve it completely. If the operation uses two or more depressant is good idea to change one dosage and wait for results. If we increase or reduce all additions, it will be difficult to know what reagent was necessary to modify. In other words, it is important to avoid the introduction of too many variables into the flotation circuit.
The addition of collector must be modified according to froth texture and precious metals recovery. The main problem with collectors is to add very small dosages. Basically, by panning gold concentrate and gold tailings will be feasible to estimate the quality of concentrate and recovery. Obviously, the operator must have enough experience to recognize the minerals and color of gold concentrate. This exercise is not simple and the operator and supervisor have to match assays and panning product. If there is not enough experience, the alternative is to take samples and perform some quick assays. There are several collectors in the market and the most used in gold operation is potassium amyl xanthate (PAX). This collector is good, but if the addition is excessively high, the froth will disappear and the recovery of gold and silver will be extremely low. The changes must be done considering the froth viscosity, texture, movement velocity and color.
Activators are added to help the collection action of collectors. It is believed that copper sulphate or lead nitrate improves the flotation of gold and silver, but this is not totally true; there is a positive effect on some gold ores, but with others, there is not a significant change in flotation performance. The addition of frother must be controlled in order to avoid overfrothing. If the addition is in excess, bubbles will be large and the transport of valuable minerals will be poor. Other negative effect is related to transport of gangue into the rougher concentrate and probably, these particles will be difficult to eliminate in the cleaning circuit. All modifications done in rougher flotation have an impact, positive or negative in the cleaning circuit.
Many times, the design of the flotation circuit is not simple and designers are used to employ scaling factors no appropriate for the flotation circuit required to recover gold. For example, a significant problem in some gold operations is associated with the time considered for the cleaning circuit. It the time is too long, it will be difficult to maintain a stable flotation circuit and more material has to be pulled from the gold rougher flotation circuit to compensate the excessive flotation time and diluted slurry. Most flotation circuit designers have no much experience on flotation plants and overlook many aspects that only a experienced operator or supervisor is able to describe.
The pulling rate and froth depth must be maintained constant in each flotation circuit. Rougher and cleaner circuit does not work under the same criteria or parameters. Each gold ore has special conditions and the parameters employed in other gold flotation plant are only a reference. Also, the type of flotation cell has an impact on the control required to get a stable operation. In this way, air addition is other variable in the performance of flotation cells.
Visual inspection of gold flotation circuit