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Pumping Lime-calcium hydroxide

Quarried lime is kiln-burned to produce calcium oxide, a highly caustic powder which can cause irritation to the skin, burns, blindness or respiratory problems. The calcium oxide is added to water (slaking) to produce calcium hydroxide which can be utilized in a great number of applications.

As a fluid, lime chemical slurry is a suspension of the powder within water. It has a silt-like nature in that it does not dissolve and falls out of suspension easily. It is commonly kept in a tank under constant agitation with a dynamic mixing unit preventing a build-up of sediment in the storage tank. Many of the applications for lime are due to its alkaline properties. 

Applications

  • Lime chemical provides an economical solution for clean water, waste water and sewage treatment processes as a flocculant, for pH control of acidic sewage and base water. At clean water sites it is used for the mineralization and/or decarbonisation of fresh water as well as removing impurities.
  • Not only does lime chemical treat the public water and sewage network, it is commonly used in industrial plants where waste effluent must be neutralized prior to be returned to the water course.
  • As a fungicide and pesticide, lime chemical solution is sprayed on to crops, typically citrus fruit and grapes.
  • In food production, lime chemical is used extensively to assist clarifying liquids. 
  • Paper production uses lime chemical for both the production of sodium hydroxide onsite and as part of the bleaching process and neutralization of acid water.
  • Lime chemical is also used as a reagent and neutralizing acidic substances in chemical processes.

Lime pumps & systems: An overview

Peristaltic pumps are especially suited to this type of fluid as it is extremely resistant to abrasion.

The fluid is moved by a rotor compressing a hose, causing a fluid-tight seal which draws the fluid and is pushed out of the discharge port. There is no surface for the abrasive particles to wear so the hose wears with the number of compressions and is simply replaced periodically.

The pump can be linked to an inverter and analytical probe for corrective dosing.

Thanks to its screw-channel open impeller, the working principle of the pump moves flowable slurry very easily with minimal wear. This makes it extremely useful for circulating the lime suspension in a storage tank. 

If sedimentation does occur raising the viscosity, a screw-channel pump will be able to handle this, discharging  the slurry to the top of the liquid and into the mixer impellers, maintaining the even concentration.

Pumping considerations

Lime is hazardous to health when in contact with skin and eyes – It therefore important to contain the fluid. Lime chemical is strongly alkaline and very abrasive as the powder chemical format is insoluble in water and the particles will have a wearing effect on moving parts where there is friction and velocity.

Commonly, the answer to pumping lime is a) to keep the solution moving constantly to maintain the chemical in suspension and b) as the lime solution is usually dosed, a working principle which allows the accurate delivery of a measured quantity.

Lime as a suspension

In storage the suspension is maintained with a dynamic mixer. It is important that the mixer is specified so the turbulent flow is a force channeled throughout the tank with no dead spots where sedimentation can occur (usually in the bottom peripheries). Where sedimentation does occur, the build-up can incur site costs with chemical costs, extra tank cleaning and inspection and the pump having to work harder with more viscous slurry being pulled through. 

As a solution it is recommended to install a pump in the system to act as a circulatory device. An example of this is to install a screw-channel pump to extract the more viscous slurry at the base of the tank and to deliver it in to the top of the tank where the mixer can distribute it and maintain a more even suspension.

The screw-channel pump has a large open impeller which can handle a much more viscous fluid. This method is sometimes used on water or sewage treatment sites where a large quantity of the fluid is handled in 1 or possibly 2 tanks where a dynamic mixer on its own would be insufficient.  

The pumping system and discharge point are recommended to be close to the tank to prevent settling of the powder in the line, plugging the flow path.

Peristaltic pumps

To accommodate the abrasive nature of the fluid and to deliver an accurate flow of chemical, a peristaltic pump or metering pump is used.

The peristaltic pump is particularly effective due to the hose design, open flow path, suction capabilities and that the flow rate is directly proportional to the speed. 

The movement of the abrasive suspension is through the draw and push of the peristaltic rotor compressing and releasing a hose – which negates the effects of abrasion on the internal wet-side of the hose. The rotor compresses the hose, causing a seal which draws through the fluid, so the lime suspension is easily handled. Avoid the use of polycarbonate, polyurethane and bronze wet-side materials.

A metering pump allows a very accurate delivery and is also available with many wet-side materials. Metering pumps may suffer from blockages inside the pump, with the pump building up pressure and suffering catastrophic failure. However, there is a new generation of metering pumps which are more able to handle this type of fluid.