Rhosonics has interviewed Sandro Marino about the flotation circuit. He obtained a master’s degree in mineral processing at the University of Utah. In this second Q&A blog you can find the questions and answers about this subject.
Q: Can you tell us more about the flotation circuit?
A: Flotation is really important, because after you spent a lot of money on reducing the particle size, you don’t want to lose these valuable particles. At most of the mines the tailings of the flotation process goes to the waste. Basically, flotation is a probability process. The particles will have a certain residence time in the flotation process. If you do not recover the particles that you want, these are going to the waste. Most of the mines calculate the efficiency of the circuit based on what is feeding the flotation circuit and what is being send to the waste. That math has to give an indication of the recovery rates.
Q: So if you know the amount of material sent to the waste, from the tailings of the flotation cell, and you know the feed rate of the flotation circuit, you can calculate the amount of particles that has been recovered?
A: Yes, but you have a specific throughput. The variance of the throughput feeding the flotation is not really significant. But if there is a change in the particles, a difference of pulp density feeding the flotation, it will affect the residence time. That information has to be corrected immediately because the residence time can vary from one minute to 10 minutes. You can imagine that with 5,000 tons per hour, in 10 minutes time, if you don’t correct or operate the flotation under the specific pulp density that you need, the amount of material you are going to lose is huge.
Flotation is really depending on the particle size, pulp density, residence time and the capacity of the flotation of recovering these particles in the volume that is available in the flotation cells. If there is a high pulp density and a large volume fed into the flotation, the flotation circuit can’t be expanded. There is no possibility to put one more cell or a taller cell as addition to the circuit within five minutes. So, it has to be adjusted with water and the flow of air. Those are the only two variables that can actually be used for corrections when the pulp density is changed and that process is really sensitive and quick. When there is a variation of pulp density in the flotation cell, that has to be corrected immediately (by adjusting the airflow or adding more water). The pulp density is on the hands of the operators, they need to make a quick decision whether they have a maximum recovery or they are going to lose material to the tailings.
Q: So, it is important to know how to control the density using dilution water if the amount of material in the flotation feed is too much to be handled in the flotation cells. They add water to make the settling of the particles more efficient, together with the parameter of air injection, but also chemicals?
A: Yes, in the flotation cell you have a three-phase system. There is water, minerals and air. You want to make the particles available for the air, to collect these particles and move these to the froth phase. Therefore, the exact amount of water is needed, to make that particle available. The extreme scenario would be a high pulp density situation in the flotation cell. These particles will be attached to each other and the product will not be visible to the air or reagents. Adjusting the pulp density in the flotation cells makes the process efficient, because then the particles are available to be collected. If there is a low pulp density, the materials will be floating around. This means the flotation circuit would not reach its maximum throughput or efficiency.
Sometimes the valuable mineral that should be collected is being pulled down by the tailings (gangue) to the underflow. The best way to correct this situation is by adding water. By adding water, you make these particles available, either to be collected or separated or to be actually collected by the reagents and the air in the flotation.
Q:So water is very important for the flotation process and the amount of solids that is needed for efficient settling can be calculated. So they need to have calculations done by engineering departments before they are actually going to control the process, to know what the perfect conditions are. Or at least they have a certain range?
A: Yes, the process management will actually work with the engineering companies to say that we have to operate the hydro cyclone in this specific pulp density. If something changes in the process, for example the ore type changes, they will change a little bit of this target control parameter. This is the same for flotation. For example, the cyclone will be fed with 55% solids and the flotation with 35% solids, then there is a small room to work on changes. Most of the mines understand this concept, they do give a lot of credit and value to the pulp density.
Q: Is it also important to have a good homogeneous mixture in the feed, with a lot of water to keep it mixed, so that the particles don’t stick to each other?
A: Yes, with flotation you need the particles in suspension. You don’t want the particles to be drown and you don’t want the particles on the froth face. So these particles have to be in suspension in the collecting zone.
And what happens is flotations probability, so that’s why we have residence time. There are three types of probability in flotation.
1) collision, the bubbles have to collide with the particle that you want to collect.
2) attachment, after these two (the particles and bubbles) collide, they have to stick and get attached to each other.
3) detachment, after collect and attach, the particles can detach (this depends on the turbulence of the circuit). So if it’s a probability process, you have to keep the variables at its optimum vibe, which means you need the right amount of particles. These have to be in suspension and these have to be available for the probability of the design of the circuit.