What is a slurry? The dictionary definition of slurry is a thin mixture of an insoluble substance, such as cement, clay, or coal, with a liquid, such as water or oil. The word thin when referring to the mixture is important as it is used to separate a slurry from a paste. A paste is a thick mixture of an insoluble solid with a liquid.
So when is a slurry too thick to be called a slurry and would better be referred to as a paste? The general rule is if it flows such as the coal slurry shown on the image below does, it is a slurry.
If the product does not flow under its own weight, such as the photo of the soldering paste below, it is referred to as a paste.
A slurry can be described in many ways, but when selecting a pump, it is first important to define why you are pumping a solid. Here, there are generally three options:
- You are trying to relocate a liquid that may be contaminated with unwanted solids.
- You are trying to move a slurry of a fixed percent solids content.
- You are trying to use a liquid to relocate a solid.
A good example of the unwanted solid slurry would be the dewatering of a mine. Pumps in this type of application are normally made of cast iron, and every effort is made to minimize solids content. When the solids content is very low, generally less than 2% slurry, pump manufacturers refer to this as a dirty water application (not a true slurry application).
The second category, where you are trying to move a slurry with a fixed percent solids content, is most often associated with an application that is part of an ongoing process. Often, these are non-settling solids or ones transported at such a rate as to prevent settling. A good example of this would be the transportation of pulp stock in the pulp and paper industry.
The third category is the toughest. Here, you are trying to move as much solids as possible, generally with as little liquid as you can. An example of this type of application would be the relocation of mine tailings. Here, every effort is made to maximize solids content so as not to waste money/energy moving a liquid that is not required other than as a carrier for the tailings. Depending on the specific gravity of the dry solid slurries of 70% solids (by weight) are not uncommon and still flow well.
Hevvy/Toyo Pumps, while making all types of slurry pumps, concentrates mostly on the last category discussed today. Within this category, there are a host of parameters that are used to help describe a slurry. Things like particle distribution, particle shape, particle hardness, SG of the dry solid, bulk density, percent solids content and PH of the liquid just to name a few. These are all subjects for another day, but I will get to each of them soon.