The diaphragm Thickener Underflow Pump is essentially one for low lifts and is particularly adapted to handle metallurgical pulps economically and with minimum attention. The Dorrco diaphragm pump was developed primarily as a means of controlling the density of the underflow from the Dorr thickener in the first continuous treatment of cyanide pulps. It has been proved the most satisfactory method of accomplishing this because of its positive, uniform displacement, which can be regulated at any rate of flow within the limits of its capacity. When the underflow from the thickener is too thin, a simple regulation of the pump will decrease the rate of discharge until its density is correct. Once the pump has been regulated, it is seldom necessary to change the adjustment unless there is a decided change in the feed to the thickener. Cord diaphragms constructed on the same principle as cord tires give appreciably longer life than the original fabric construction. Diaphragm pumps of different types are available from several manufacturers.
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The diaphragm pump has on various occasions been challenged by the centrifugal pump, but the characteristics of the former as mentioned above show advantages in its favor. When a thickener becomes overloaded, the centrifugal pump tends to “pack up” and remove less pulp; the diaphragm pump increases its delivery in terms of solids and so tends to compensate the overload condition. On the other hand, the centrifugal increases its output as pulp density decreases and this is obviously undesirable.
Describing the history of the diaphragm pump, Luther B. Eames, well- known engineer and an early associate, writes:
To our knowledge the earliest use of the diaphragm pump in metallurgical plants was for recirculating pulp in the loading tank of a Moore filter at the Lundberg, Dorr, and Wilson mill at Terry, S.D. This was of the type sometimes called a pitcher pump. It had flap-type valves which, however, caused irregular operation due to the presence of wood chips and tramp oversize in the pulps discharged from thickeners.
As a result, pumps were designed more suited to use in connection with thickeners. At the Hollinger mill valves were developed which discharged around the whole periphery of the valve seat and were so designed that the center of gravity of the valve was below the valve seat. Also seats and valve disks were of soft rubber so that any chips or tramp would not cause leakage and would be washed off the seat at the next stroke. This is of importance, particularly in countercurrent washing, where it is essential to keep the capacity of all pumps constant and equal.
Ball-type valves have also been used. The balls are of rubber weighted with steel cores and operate against circular rubber seats. This type, however, has been used more generally in the industrial than the metallurgical field.
Cyanidation and concentration of gold and silver ores
For more complete automatic control the set point of the flow-controlling loop can be governed by underflow density with an override to increase the underflow as rake torque rises. In the absence of automatic control coupling, the control loops are closed through the operators.
To avoid plugging of the underflow line, it is advantageous to make the section between thickener underflow and underflow pump suction as short and straight as possible, but this presents a problem. It is not good practice to install an electrically-driven pump in a pit under the thickener, so in many cases the pump is located at ground level alongside. Where it is evident that over-thickening will be severe, and particularly for segregating pulps containing a lot of sandy material, it is good practice to elevate the thickener so that the underflow pumps can be located under it. In at least one alumina plant the thickeners are carried on columns. In a nickel plant they are constructed on a fill. Underflows are into tunnels, open on both ends, passing under the thickeners.
In any case, underflow lines should be provided with high pressure water connections and with drains so that they can be flushed out when they become plugged. Means should also exist for adding a small stream of water (if the process permits) at the inlet end of the underflow line to dilute overthickened sludges to a more pumpable consistency. Actual arrangements will vary from case to case and from layout to layout. Figure 28 shows the sort of thing needed.
Unit Operations Handbook: Volume 2
Thickener tanks are a time and space-saving way of de-watering slurry, removing the waste silt and allowing the water to be recycled.
In the quarry environment, thickener tanks are used at the end of the wash plant process, after the cyclones. A lagoon on site can serve the same purpose, but these take up space, require maintenance, and are slower as it takes a long time for the silt to settle out.
Thickener tanks are becoming more popular and are now regularly seen on inner-city sites, wharfs, quarries and sites that are no longer able to use their lagoons.
Thickener tanks are circular in design with a conically shaped-centre. The tanks vary considerably in size.
Fluid enters the thickener tank via an overhead pipe and down into the centre of the tank.
At the bottom of the tank rotating rakes help the solids settle and draw the silt towards the centre of the tank.
The water meanwhile flows over the edge of the tank and into an overflow before exiting at the side via a pipe. This is pumped away and reintroduced to the wash plant process.
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The silt eventually exits the tank at the centre point underneath and is pumped away.
While a light duty slurry pump can be used to pump the water away from the thickener tank, there are three types of pump that can be used to remove the thickener underflow:
Currently the most common pump to be used to remove the thickener underflow, a centrifugal pump is not necessarily the best option. From a spares perspective they are the most cost effective pump however using a centrifugal pump carries a number of issues.
Firstly, the slurry has to be watered down to go through the pump, which is somewhat counter-productive. Also, as it is a batch process, solids can settle in the pump when it is switched off. Stray solids can also jam the pump; if blockages occur, they are very difficult to rectify.
These pumps are not self-priming, meaning they have to be located directly below the tank to ensure suction, this makes access more difficult. Seal leakage is also a common issue due to back pressure which can in turn cause premature bearing failure.
Not as common as centrifugal pumps, screw pumps are the most cost effective of the three types of pump we feature here and unlike centrifugal pumps, the slurry doesn’t have to be watered down.
The pumps however carry the shortest lifespan while spares can be very expensive. The pumps also have narrow tolerances so extraneous solids are catastrophic.
Peristaltic pumps are by far the best pump option to use with this type of application.
Although reasonably expensive to purchase, they are the most cost effective option, are long-lasting, easy to repair and spares are relatively cheap to purchase.
Unlike centrifugal pumps, peristaltics can be located further away from the tank and the slurry doesn’t need to be watered down.
Peristaltic pumps also have the ability to run backwards allowing blockages to be cleared, they can pump a higher solids percentage and the flow rate of the pump can easily be controlled.
Furthermore, peristaltic pumps offer the lowest energy consumption of the three types of pump we feature.
For an overview of how a peristaltic pump works, please see here.
Danish based LSM Pumps are one of the world’s leading peristaltic pump manufacturers and Atlantic Pumps are the companies’ exclusive UK distributor.
The company produces a range of different sized peristaltic pumps and the pumps are known for their low energy consumption, low and easy maintenance and their exceptional build quality.
LSM pumps are also now available for hire. Our hire fleet gives you a low risk and capital free way of trialing the cost and downtime savings that are available to you.
A full overview of the LSM range, can be found here.
For all enquiries regarding pumps for thickener tanks, please call 0808 196 4938.
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