Image 3. Slurry pump and solids for impact wear testing
Before testing, there was some debate that material strength might be more important than hardness in these conditions. In fact, hardness proved most important in all but the most brittle forms of tungsten carbide, although the relative difference between materials fell to about half of that seen under normal wear tests.
As a part of this study, the interactions of the solids within the pump were also modelled using discrete particle computational fluid dynamics (DEM CFD) software, in which individual particles are tracked and their interactions with the pumping surfaces and with each other are determined. This has led to improvements in pump design for reduced impact wear effects.
In many mining applications, the ore contains small amounts of metal. For example, many copper mines contain less than 1 percent copper. The process of extracting the valuable metals requires extensive grinding and processing of the ore at the rate of thousands of tons per day.
Once the concentrated metal bearing minerals are removed for smelting, 97 percent of the solids may remain for reclamation into the environment. As mentioned above, these solids should be placed at the highest concentration possible to ensure a geologically stable landscape and conserve water resources. These are often finely ground, sized to 100 microns or less, and form a thick paste with non-Newtonian fluid properties. Often, transportation must extend several kilometers and friction losses in the pipeline can be high.
Image 4. Comparison of various impact wear samples
The paste from each mine is different and only testing in the lab can define its parameters precisely enough to ensure a successful system design and protect the large investment involved in construction of the pumping system and reclamation area. The slurry pump hydraulic lab regularly tests such slurries for mining engineers, but is also interested in determining the effects of these slurries on pump performance.
In some ways, these slurries act like viscous liquids, but also often have what is known as a “yield stress,” or a value of force that must be applied before the liquid will shear. This behavior is exhibited in everyday liquids like mayonnaise and peanut butter. In pumping applications, unusual flow patterns can result, such as plug flow in pipelines, in which a central core of liquid moves as though it was a solid cylinder. In the pump, the situation becomes more complex and in some cases, centrifugal pumps fail to move these liquids for unknown reasons. In recent testing, slurries were being pumped at yield stresses up to 1,200 Pa, resulting in pipeline friction losses up to 4 feet of head for every foot of pipe, an extreme case from a practical (economic) standpoint, but useful for the development of slurry pump performance models.
The next step in this program will be the modelling of the complex fluid dynamics within the pump using non-Newtonian CFD modelling to correlate the measured effects against underlying flow patterns and gain insight for the design of improved paste-handling pumps.
Hopefully, these examples help explain the challenges and rewards of slurry testing. Although often less than exact, usually somewhat messy and frequently without established guidelines, slurry testing places the engineer in direct contact with complex fluid dynamics problems of practical economic interest and offers the opportunity to take a hands-on area of scientific study from the empirical to the theoretical.
Image 5. Typical high yield stress (but still pumpable) paste-type slurry