The Best Method of Controlling Thickener Underflow

Depending on the installation, elevation of the thickener and outlet piping the thickener may be controlled with either a valve or may need to be controlled by a pump. In a situation where the head of the thickener delivers enough pressure to deliver the flow to the desired destination then a valve may be your best selection for thickener underflow control. However when there is not sufficient head to deliver the desired flow rate to the end destination then the slurry will need to be pumped. In this article we will discuss different pumping and valve technologies and their benefits and weaknesses of various types of technologies. Obviously whatever technology is utilized the ultimate goal is accurately controlling the thickener to deliver correct density of material from the thickener. If the material is not thick enough, the process will cost more in filtration. The purpose of this article is to offer ideas on the best technologies to increase the efficiency of your thickening process.

Pinch valves are a natural for thickener underflow control and isolation valves. The pinch valve has no pockets or cavities for material accumulation and is also ideal for abrasive high slurry content. The same rule such as limiting the pressure drop across a thickener underflow control valve applies to the thickener as well. If the valve will be modulating or controlling the flow rate then cavitation will damage the valve sleeve. The best method to prevent this from occurring is to limit pressure drop across the valve. A J tube or a slight elevation of the piping as it leaves the valve with a vacuum break at its highest point will accomplish this. On/off non control pinch valves do not require a limitation of the pressure drop since they are not controlling flow and will not cavitate. So J tubes and pipe elevations are not required for on/off valves.

If the driving head of the thickener is not sufficient to deliver the slurry to ultimate destination then a pump is needed. There are many good slurry pumps that can pump highly thickened mineral slurries but some may serve you better. For example a centrifugal pump will certainly perform the duty. However there are some negatives regarding centrifugal pumps. First of all is that they require clean gland seal water. Gland seal water has a significant cost, and it can penetrate the pump gland seal and end up in the slurry that you just spent time thickening.  Also gland seal water enters into the pump gland seal and ends up back into the slurry that you just spent time thickening. Also centrifugal pumps have a pump curve you must keep the pump on to insure pumping accuracy. If you slow your centrifugal pump down so that it falls off the pump curve then you may not be able to control the pump accurately and lose control of your thickener. Finally if you turn off your centrifugal pump and don’t shut an isolation valve at the same time you will keep flowing past the centrifugal pump.

Figure above shows a LPPT-80 peristaltic hose pump that replaced centrifugal pumps on thickener underflow. The peristaltic pumps have been much more reliable and less costly with regards to spare parts usage and repairs.

One pump that solves many of these problems is a peristaltic pump. Peristaltic pumps are often forgotten for this application but can be very effective devices to control a thickener density and pump the thickened slurry onward. Some of the reasons peristaltic pumps have been ignored for this application is that in the early days of peristaltic pumps they were limited in their hose lifetime, pressure capability and flow rates. Today there are new designs of peristaltic pump which have a single roller which compress the rubber hose once per 360 degree revolution. Many people believe that the slurry abrasion is what determines the lifetime of a hose in a peristaltic pump. In reality the number one factor that determines how long a hose last in a peristaltic pump is how many times the hose is compressed. Older peristaltic pump designs used and still use either two metal shoes or multiple rollers to compress the hose. These types of designs will produce hose life that is at least ½ the lifetime of single roller designs.  In many mines the hose lifetime has proven to last 3 – 4 times longer on single roller designs due to the limited friction and heat generated on the hose.  The old designed peristaltic pumps have shoes which compress the rubber hose two times per revolution. This design will create much more heat and friction, wearing the hose out much faster. 

The newest peristaltic pump incorporates a single heavy duty roller which rolls over the hoses instead of rubbing against the hose.  It requires less energy and also uses only a fraction of the glycerin compared to the multiple shoe or multiple roller designs. In many cases you can run a roller design at a faster revolutions per minute and still provide significantly longer hose life than shoe or multiple roller designs. As well you can typically utilize a pump 1 size smaller when using a single roller design because it moves more material per revolution and compression. 

Peristaltic pumps are very accurate to control. You will get fixed displacement of liquid every revolution. So simply by speeding up or slowing down the revolutions per minute you can accurately control the pump. Peristaltic pumps when shut down do not require an additional check valve or isolation valve as it does not allow the flow to continue to flow when the pump is off. They also do not require any gland seal water. One negative of a peristaltic pump is that if you have a very large thickener there may not be a large enough pump to perform the required flow rate. They are limited in delivery capability.

Since we are talking about thickeners and flotation there is one other final note. Many types of pumps can be utilized to pump flocculants or reagents. However you should use a low shear pump for flocculants. Flocculants consist of long polymer chains and if you use a shearing pump you will have to use perhaps two times as much flocculant to perform the same work.  If you utilize a peristaltic pump your flocculant usage will be minimized since it is a low shear device.

Peristaltic pumps for flocculant or reagent metering to flotation cells.

Previous trends in the mining industry would have you select a dart valve for flotation cell control and a centrifugal pump for thickener underflow. However more recent trends suggest that pinch valves for flotation cell control and peristaltic pumps for thickener underflow are very effective options to maximize your process efficiency.