In mineral processing operations, water is very important and therefore water recovery plants have an important position. According to CRC Care 2013, every year more than 10 billion tonnes of tailings are produced by mining activity.
Thickeners can be used as a water recovery system, to meet requirements in the recovery and reprocessing of water. Eventually, this can decrease the water used by the environment.
This environmental requirement can be encountered by an increase in the density of the underflow of thickeners. An increase of 1-2% in underflow density can return large amounts of water to the operating installations.
The increasing density is also important to avoid accidents in tailings dams, which can collapse when there is too much liquid pumping into the dams.
A thickener in Morocco, where the SDM is installed
Compared to other technologies, you can process high volumes of slurry at a quite low cost by sedimentation using thickeners. The two outputs, underflow and overflow, are not often prioritized at the same time.
In tailings applications where water recovery is required, underflow will be targeted. But in concentrate applications, the overflow measurement is essential. Otherwise, the important material would be returned to the concentrator and lose recovery.
Mineral recovery is done using the so-called product thickeners. Thickeners are very important to the processing plant, because if the material fed into the thickener is not getting out of its underflow, this leads to less overall metal recovery.
First, we will explain how product thickeners, in this case concentrate thickeners, operate and then we will highlight the advantages of density measurement for process control.
In concentrate thickeners, the feed usually comes from the flotation circuit. The concept of the flotation process is that desired particles attach to air bubbles and are carried to the surface and removed from the cell, while the particles that are not attached to air bubbles remain in the liquid phase.
If the same event occurs in the thickener tank, foam could carry valuable materials into the overflow.
When these valuable materials are not recovered, this will lead to a lower overall recovery of concentrator metal at the processing plant. This event can also lead to extra reagent costs, damage to pumps and valves and costs for cleaning process water tanks where the solids could eventually deposit.
Flocculants are chemicals by which fine particles are caused to clump together into a floc. These flocs of particles weight more than one fine particle itself, therefore the effect of gravitational forces is increased, and so is the sedimentation rate.
Inefficient use of flocculants leads to extra reagent costs and a less effective operation. Therefore, the operator sets a target density in the feed of the thickener. Process operators want to target the highest solids percentage by weight of the feed slurry at which free settling of particles can occur in the thickener.
Why density measurement is important
Measuring the density of feed slurry in the thickener is important for process control, to improve flocculant use and efficient control of the mixing process. The feed slurry is monitored in real-time by an in-line density meter to “see” if the density value complies with the target density.
The Slurry Density Meter (SDM) can be used in many applications to measure the density of a slurry. A typical application for the mining industry is a thickener. With a thickener, the solid-liquid separation is done by sedimentation. A thickener has several layers, the upper layer is clarified water.
Below the clarified water is the interface, this is solids and water. Under this layer is mud and the lowest layer is heavy mud. Sedimentation is used to produce two phases by thicken the incoming feed stream. In the ideal situation, the overflow contains minimal solids and the underflow contains minimal liquid.
That is why it is very important to measure the density.