Paste Thickeners

Paste Thickeners can help producers eliminate the need for settling ponds and other structures. They are meant to achieve the highest solids concentrations possible through gravity separation. The resulting mud, described as a paste, is often so thick that it becomes non-segregating  no further separation of water and solids can be achieved. As a guide, a paste material is measured with a Yield Stress of >150 Pa. Pastes are then moved to impoundment structures via positive displacement pumps.

Because of the high percentage of solids in the resulting space, producers can greatly reduce  and sometimes eliminate  impoundment size, structure and volume. They are also able to eliminate freestanding water or ponds. Not only does this create a safer working environment, it also helps with permitting and regulatory requirements.

Why McLanahan Paste Thickeners

McLanahan has provided Paste Thickeners both with and without pickets and rakes depending on the process outcomes sought by producers. By employing a series of proprietary design tools, McLanahan process engineers will test and design the Paste Thickener to achieve the appropriate value for each operation. They will utilize either their own workshop or a global subcontractor network for fabrication to ensure the logistic challenges can easily be met.

Each McLanahan Paste Thickener includes point level pressure sensors and bed level devices to accurately track sludge level and density. Programmed logic controls run the system and log data, such as feed rate, pH, sludge level and solids concentration. Hydraulic drives can give excellent indication of torque on the rake structure, and underflow discharge may be set to activate based on sludge density, downstream demand, sludge level or simple timers.

How Paste Thickeners Work

The upper section of a Paste Thickener is identical to a High-Rate Thickener and relies on achieving optimum flocculation conditions in the proprietary feed system. The optimum feedwell solids are commonly determined by testwork. Care needs to be taken to ensure exit velocities from the feedwell do not cause excessive shear and fracturing of flocculated particles.

On exiting the feedwell, flocculated solids settle according to their free settling rate to form an unconsolidated bed. The compression effects of the increasing height of the mud bed combined with the action of the rake and pickets to create dewatering channels consolidates the mud bed. In principle, the higher the side wall of the tank, the greater the compression effect and the higher the underflow density.

The high-density mud has a greater resistance to flow, therefore moving the material into the discharge zone requires more force. Thus, the higher the design underflow yield stress, the larger and more powerful both the drive and the rake structure needs to be. 

The floor slope on the tank is often increased to provide some assistance to transport the thickened paste, with 30 and 45 degrees being commonly utilized.

A compromise option of using a rakeless (ULT) design can be considered. In this case, the cost and complexity of a mechanical drive is traded off for a deep mud bed and steeper 60 degree floor slope picture . The ULT design will not achieve the same underflow density as the paste thickener with rake and pickets, but it can be more economical. A further benefit to this option is it can be married to the McLanahan M3H centrifugal Pump, relieving the operator of the challenges of using a positive displacement pump.

Popular Applications for Paste Thickeners

Paste Thickeners are used by mineral and aggregate producers, as well as environmental contractors, where the benefits of high-density underflow translate into savings in downstream processing costs, such as tailings storage facility costs, or lower volumes in leaching operations.

Paste Thickeners Features and Benefits

  • Produces the highest solids concentration of sludges achievable by gravity thickening alone
  • Maximum water recovery
  • Underflows often achieve paste consistency, meaning there is no further separation of free water
  • Rotating pickets provide pathways to release water from sludge and allow for further compaction
  • Paste sludges require less infrastructure for impoundments

Frequently Asked Questions

What is paste?

Paste can be defined as the rheology or resistance to flow of a material.This is usually measured by a rheometer. Rheometers can come in a wide range of types and sophistications, from Boger’s "50c rheometer", to a concrete slump cone, to cup and bob and vane viscometers. Industry standards usually express the rheology as the yield stress in Pascals as measured on vane viscometer. Paste is usually taken to be material with a yield stress of >150 Pa.

Does paste mean high density/high solids?

Not always. The complex interaction of surface chemistry driven by varying particle size and material composition means the relationship of yield stress to solids percentage is specific for each given substance. The higher the surface area (smaller particle size) will tend to cause a paste consistency at lower solids, eg. fine mineral sands slimes may exhibit paste characteristics at ~30% solids w/w, but gold tailings will not be paste until ~70% solids. Hence, a Paste Thickener is effectively designed to a rheology target rather than a density target.

Can Paste Thickeners operate at a higher rate (flux)?

No, not for the same input and output objectives. Although our McLanahan engineers are really smart, they can’t change physics. However, since paste thickeners are commonly chosen for applications where underflow density (rheology) is the key design criteria, alternative considerations, such as overflow clarity or flocculant consumption, may be less important and willingly compromised for lower capex or layout. Thus, Paste Thickeners are commonly run at a higher rise rate than a conventional or High-Rate Thickener (either using more flocculant or accepting more solids in the overflow), giving the appearance of having a higher capacity.

How much more than a High-Rate Thickener does a Paste Thickener cost?

A Paste Thickener normally uses an elevated tank design. To achieve the higher compression, an additional side wall is required. Supporting the additional load requires a much more robust tank design. Moving thickened mud of up to 10 times the yield stress requires a mechanical drive and rake structure capable of 10 times the torque. It is therefore not usual for a Paste Thickener to cost two to three times that of a High-Rate Thickener.

How is the extra cost of a Paste Thickener justified?

Being able to deposit non-segregating tailings in a much smaller footprint that can be rehabilitated much faster will have very significant savings on tailings storage facility management costs. Counter-current wash circuits or leach feed applications significantly improve efficiency, requiring less chemical or wash water by lowering liquid volume at each stage.

What is shear thinning?

The rheological properties of most pastes are affected by changes to shear conditions. Since flocculated tailings are created by a network of polymer strands, the flow properties (and therefore pumpability) can be improved by using a shear thinning mechanism. McLanahan Thickeners can be fitted with shear thinning loop when required.

How do I monitor and control a Paste Thickener?

The basic principles of Thickener control are common across all styles; however, the consequences of the Paste Thickener going outside of the control parameter are often worse. More attention should be paid to ensure the Paste Thickener has a full suite of instrumentation and control.