Making the Most of In-Bin Blenders

In-bin blenders, also known as gravity, tube or silo blenders, play an important role in industries that need to homogenise large quantities of material to meet specifications, minimise the effects of separation or blend a variety of other components. Typically, in-bin blenders are day bins or silos fitted with internal blending devices. They rely on the difference in retention time of the material from filling to discharge through the mixer, and they require recirculation of the "heel" or volume of material initially located below the blending unit.

Recent developments have increased the efficiency of the unit and expanded the range of materials that can be processed. With the right design, in-box blenders can be an economic investment, offering maintenance-free operation. These units are best suited to the use of free-flowing components that do not contain friable or degradable particles or have a low melting point.

Strategies to achieve in-bin blending include

First-in, first-out (FIFO) flow patterns to address any radial composition variations.

Using internal or external perforated vertical tubes to draw material horizontally from different bins and recombine it at the bottom to form a mixture; and installation of blending cylinders to impart different velocities to different levels of particles, resulting in different retention times.

 Bin Blender     

The only way to design an effective in-bin blender or to determine the effectiveness of an existing design is to test the material flow characteristics of the mixed components against key criteria.

Gravity flow tubular blenders require very free-flowing (FRI > 100) and uniformly sized materials with a low sliding angle on the bin wall surface (HI), such as plastic pellets. If the material exhibits cohesion (AI > 0.2 ft, RI > 0.3 ft), the tube will clog.

In-bin blenders with cylinders require low to moderately high cohesive solids (0.2 ft < AI < 3 ft and 0.3 ft < RI < 10 ft) and perform best with materials that are somewhat free-flowing and non-fluidisable (FRI) > 100).

FIFO Hoppers

Mass flow is a condition where all material in a hopper moves in an uneven manner during discharge.FIFO is an extreme case of mass flow and can be achieved by using steeply tapered or chiselled hoppers. Uniform flow is a function of the hopper level. As the level approaches the steeply tapered hopper, an uneven velocity gradient occurs, with a faster flow rate in the centre of the hopper and slower material movement near the transition from the straight edge to the cone.

The FIFO flow pattern represents true piston flow - each horizontal slice of material exhibits a uniform velocity and therefore appears at the outlet within the same time frame. This reduces or eliminates radial composition gradients due to certain separation mechanisms or due to loading different components at eccentric positions.

There are two commonly used methods for assessing the quality of the mix.

1. observing the mixture through the mixer wall while the mixer is running. This provides a good first subjective test to determine whether further blending is helpful, required or detrimental. Unfortunately, most industrial blenders are made of metal, which makes this method impractical and therefore requires an indirect measurement of the blending quality.

2. Small samples are taken at equal intervals as the contents of the mixer are discharged onto a belt conveyor or as the mixer is emptied. This allows visual identification of the results, including the effects of the emptying process. However, to assess the quality of the mix, the mixture must be subjected to sieve analysis or flow characteristic measurements such as the flow rate index (FRI) and the ratio of FRI to bin density index (BDI) or chemical analysis. This last method quantifies the results but requires more time, cost and effort.

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