Segregation of bulk solids – causes and solutions

INTRODUCTION

Mixtures of solid particles often segregate (separate) while they are being handled in hoppers and conveying systems. This often results in costly quality control problems due to the waste of raw materials, lost production, increased maintenance and capital costs required to retrofit existing facilities. Segregation problems occur in many industries and with a diverse range of bulk solids. However given the value of pharmaceutical and cosmetic powders, segregation in these industries can have more drastic economic implications than in almost any other industry. Consider, for example, a single batch of pharmaceutical powder which may have a value in , excess of several hundred thousand dollars. Strict U. S. quality control standards dictate that some or all of the batch may have to be discarded if it is found that the amount of active ingredient or total weight of just five tablets in a batch varies outside narrow limits. Segregation problems are particularly pronounced with direct compression formulations in which there is a wide range of particle sizes and the active drug is at one extreme of the particle size distribution (usually the finer end). Even wet granulation formulations can exhibit segregation problems if the batch is dry blended after granulation or if the individual particles in the granulation do not all have the same percentage of active drug

EGREGATION MECHANISMS

Five primary mechanisms have been identified as being responsible for most particle segregation problems .. Of these five, only three typically occur with pharmaceutical and cosmetic powders. Each of these three will be described below along with conditions under which it occurs.

• Sifting

Sifting is a process by which smaller particles move through a matrix of larger particles. From experiments 121 and actual field observations, it has been found that four conditions must be present in order for this mechanism to occur: Sifting has been found . .. to occur with particle diameter ratios as low as 1.3:l. In general, the larger the ratio of ) particle sizes, the greater the tendency for particles to segregate by sifting. Below 500 microns (#35 U.S. mesh) the tendency to segregate by sifting decreases substantially. However, some sifting can occur down to and below a mean particle diameter of 200 microns (#70 U. S. mesh) for particle diameter ratios as low as 2:l. Free f- In order for sifting to occur it is essential that no agglomerates are formed, either between particles of a given size or particles of varying size. This generally requires the mixture to have a low moisture content and little or no fine particles. are essentially locked together and their tendency to segregate becomes almost nonexistent, even for materials which are highly prone to segregation. Thus, a velocity gradient through the flowing material is required. .. If particles are stationary or moving with a uniform velocity they It is essential that all four of the above conditions be present in order for sifting segregation to occur. If any one of the four is absent, the mixture will not segregate by this mechanism. Sifting segregation is common in pharmaceutical plants and, to a lesser extent, in cosmetics facilities. One place it often occurs is during filling of a bin or hopper. A concentration of fine particles develops under the fill point while the larger particles roll or slide to the periphery of the pile. Sifting segregation can also occur as a bin or hopper is being emptied. If a funnel flow pattern develops, fines may percolate (sift) from the flow channel into the stagnant region. 

• Air entrainment (fluidization)

Fine particles generally have a lower permeability than coarse particles and, therefore, tend to retain air longer in their void spaces. Thus, when a mixture of coarse and fine particles is charged into a hopper, it is not uncommon to find a vertical segregation pattern develop, caused by the coarse particles being driven into the bed as the bin is filled while the fine particles remain fluidized near the top surface. Segregation by this mechanism can also occur when there is a source of air introduced into the hopper. Segregation by air entrainment often develops with mixtures which contain a significant percentage of particles below 100 microns in size. 

• Entralnment of particles In an airstream

The finer the particle size, the longer it may remain suspended in an air stream such as might be set up upon filling of a hopper. This effect starts to become important around 50 microns and is very common below 10 microns. Thus, secondary air currents can carry airborne particles away from a fill point into outer areas of a bin, scattering them in a way that bears no resemblance to the calculated trajectories. 

SOLUTIONS TO SEGREGATION PROBLEMS

There are three main techniques that can be considered when segregation problems are present: change the material, change the process, or change the design of the equipment. Each of these will be addressed below. Change the material A common characteristic of most highly segregating materials is that they are free-flowing and, therefore, the particles easily separate from each other. Thus, one obvious change to decrease the segregation tendencies of a material is to increase its cohesiveness by, for example, adding water or oil. It must be recognized that this can be overdone since, if the cohesiveness is increased too much, flow problems such as arching or ratholing may develop and result in greater disruption to the process. Another technique that can be used is to change the particle size distribution. For example, if segregation is occurring by the sifting mechanism, lowering the particle diameter ratio below 1.3:l or decreasing the mean particle diameter below 100 microns will reduce or eliminate it. If all of the particles are of a nearly uniform size, differences in fluidization and particle entrainment tendencies will be minimal as well. Change the process There is often little that can be done in a pharmaceutical or cosmetics facility in this area. However if the powder is being pneumatically conveyed into a hopper, it is generally preferable to use tangential entry into the side rather than going in at SO" to either the sidewall or the top. This will reduce the tendency for vertical segregation to develop. Another technique which has been used with success is to carry a pneumatic line into the center of a bin and then direct it upward to a deflector plate. This will decrease particle velocity and allow a symmetric pattern when particles fall from this surface. Change the design of the equipment The flow pattern in a mass flow bin is firsl-ln, first-oul flow, whereas in a funnel flow bin it is first-in, last-out . Thus, if particles have segregated from side-to-side while filling a bin (e.g., by the mechanisms of sifting or particle entrainment), a mass flow pattern will tend to minimize segregation upon discharge, whereas a funnel flow pattern will make the segregation worse. Increasing the height-to-diameter ratio of the cylinder section of a mass flow bin above 1.0 usually results in a uniform velocity pattern across the top surface as long as the level is above one diameter in the cylinder. This lessens the effects of sifting segregation compared to using a short cylinder section or no cylinder at all. Mass flow will not correct segregation in which there are vertical striations in a bin. Such problems can only be overcome by first eliminating the vertical striations (e.g., by redirecting the entry of a pneumatic line). Then mass flow can be used if the resulting particle size distribution shows some side-to-side variation. An alternative to traditional mass flow bin design is to use a patented BINSERT@, which consists of a hopper within a hopper in which the velocity pattern is controlled by the position of the bottom hopper. It is possible to design such a system to provide a completely uniform velocity profile and thereby an absolute minimum level of Segregation.

References :DRUG DEVELOPMENT AND INDUSTRIAL PHARMACY, 14(18), 2749-2758 (1988)