Rheology of Sludges and the Positively Torque Limiting DBS Thickener Drive
The study of the viscous properties of liquids (including slurries, sludges, and pastes) is called Rheology. The relationship between the shear stress and shear rate for a particular liquid is examined for a range of shear stress conditions. The shear stress is an indication of the force being applied to a liquid to get it to flow and the shear rate is a measure of how much the liquid actually flows. The ratio of the shear stress to shear rate is known as the liquid viscosity. Some definitions associated with the subject are listed below.
Newtonian Fluid: With Newtonian fluids, shear rate is proportional to shear stress. Since viscosity is defined as shear stress/shear rate a Newtonian fluid is said to exhibit constant viscosity.
Pseudoplastic Fluid: As soon as shear force is applied to a pseudoplastic fluid, there is an apparent thinning of the liquid and a reduction in viscosity. In other words there is an increase in shear rate under steady shear flow.
Thixotropic Fluid: With thixotropic liquids, shear rate will increase with time for a fixed shear stress. The more the material is worked the lower its viscosity becomes. Slurries exhibiting this property are common, for example titanium dioxide, red mud and several other slurries containing inorganic solids. Thixotropic slurries recover viscosity once the shear stress is removed unless they suffer from shear breakdown or rheodustruction. Substances can also exhibit negative thixotropy.
Dilatant Fluid: A dilatant liquid thickens up or becomes more viscous as it flows. There is a decrease in shear rate under steady shear flow.
Bingham Plastic: A Bingham plastic exhibits near zero shear rate until a yield shear stress is reached, at which point shear rate starts to increase proportionally with shear stress. For example, tomato ketchup and mayonnaise.
Know Your Sludges
The properties of settled sludges that form in the lower regions of thickeners and clarifiers as a result of sedimentation affect the performance of the unit. In particular more viscous sludges generate a greater torque reaction in the rake and drive than do less viscous slurries. The viscous or rheological properties of these sludges are determined by many factors including solids concentration, solids particle size distribution and mother liquor density. When processing a slurry in a gravity thickener or clarifier, the two functions of the rake mechanism are to transport settled solids toward the outlet (usually the center) of the tank and to "work" the sludge to increase the solids concentration.
With conventional drives, if the rake drive stalls due to a high torque load, then both the objectives described above cease to be met. However with the positive torque-limiting feature offered by DBS, a drive never actually stalls. Torque can still be applied to the rakes even though a maximum pre-set torque has been exceeded. In this state a DBS drive ceases to be a constant speed drive and becomes a constant torque device. This means that the settled sludge is still being exposed to shear stress. If the sludge is Thixotropic, a Bingham Plastic or Pseudoplastic, the applied shear force will cause a viscosity reduction. With this reduction in viscosity and perhaps some other measures (e.g. increased underflow sludge removal rate), it is likely that the rake will once again start moving and the two main objectives will once again be met.
Torque Gauge Provides Added Insight
In addition to the positive torque limiting feature, all DBS drives have an accurate torque gauge. This can provide a new insight into the rheological properties of settled solids. This understanding frequently leads to improved process performance and removal of bottlenecks in the sedimentation step of many processes.