Slurry engineering: what to consider

Extensive systems

When designing mineral processing facilities, BBA experts must deal with extensive slurry pumping systems. Ore beneficiation processes often use water mixed with comminuted material.

Once the material particle size can be pumped, usually after the grinding stage, it is carried by pumps throughout the plant as slurry, or flows by gravity through a specially designed, abrasion-resistant piping or launder system. In other words, slurry pumping is similar to material transportation or handling systems that work like conveyors for dry material.

Types of slurry

Slurry can be categorized into the following classes based on material particle size and concentration:

• Settling (heterogeneous) slurries
• Non-settling (homogeneous) slurries
• Mixed regime (industrial) slurries (this class has properties from both settling and non-settling slurries).

Once the type of slurry is known, apply a specific approach for system sizing.

Settling slurries

These slurries contain coarse particles that are dense enough to settle over time at the pipe bottom and can block transportation piping. Turbulent mixing and inter-particle collision support the particles in these systems.

Settling slurries are characterized by an increased concentration gradient as it approaches the bottom of a horizontal pipe; hence the name “heterogeneous”. Establish a critical (deposition) velocity before successfully sizing these systems. In most cases, piping size should, at the least, include flow line velocity that is 10-15% above critical.

Non-settling slurries

These slurries often exhibit non-Newtonian liquid characteristics. Some testwork is usually required to determine slurry parameters, such as yield stress and viscosity. Select the pump type using the resulting rheological data. If selecting a centrifugal pump as a driver, slurry should be transported in a turbulent regime, i.e., slurry velocity should be above both the transition (laminar to turbulent) velocity and the estimated deposition velocity.

Mixed regime systems

These systems contain a wide range of particle sizes and densities, where fine particles mix with liquid and form the carrier liquid (transportation vehicle) and coarser particles are supported by both the vehicle viscosity and turbulence. This type of system could be resolved by applying special software with minimum testwork performed on a slurry sample, or could be subjected to a loop test. The latter is performed by a specialized laboratory with a slurry sample in smaller than actual piping system size. Resulting data is then scaled up to provide the designer with sufficient information, such as velocity range and pressure loss gradient, to successfully size the system.

Pump selection

Pump selection is one of the most critical elements of a slurry system design. Basically, pump types can be either centrifugal slurry pumps or positive displacement (PD) pumps. This selection is based on the intended application and slurry type. Most in-plant systems traditionally use centrifugal pumps. Sometimes positive displacement pumps are used, such as peristaltic or progressive cavity pumps. A PD pump is used while pumping paste slurry for tailings deposition, including paste backfill.

Slurry transported over medium-distance systems, such as a conventional slurry tailings disposal system, could be delivered to the deposition site by centrifugal pumps, which are installed in series to compensate for friction loss along the pipeline and static head (elevation difference), if any.

Pumping slurry can be a very economical way of transporting the final product from the mine or concentrator to port, where conditions exist. PD pumps are used in these systems because the overall system pressure is high and, unlike centrifugal pumps, PD pumps can develop such pressures in a single stage.

Gravity flow systems

Gravity can also be used to transport slurry, as is the case for some in-plant systems and outdoor flumes (e.g., tailings). To size these systems, select an adequate flume or launder cross-section size and slope so slurry is transported in a rather uniform flow without any settling or excessive wear on the flume or launder. Needless to say, where applied, this type of slurry transportation has an impact on plant equipment elevation and should be properly evaluated in the early stages of the project.

Piping material selection and pipe connection methods are also subject to system design and should comply with industry best practices for the transported material.

Our knowledgeable experts

Sizing of slurry transportation systems is complex and mistakes are easily made, which can be costly. A combination of experience and comprehensive knowledge is the key to a successful design. BBA experts closely monitor new developments in slurry engineering in order to adopt the best engineering practices.