Clamp Down on Clumping

Poor flow is one of the most common problems encountered in handling or storing solids. With liquids you open the valve and (hopefully) material runs out. With solids you often have to pray before opening the valve. Wouldn’t it be nice to have an inexpensive well-established test method or procedure that would allow you to predict whether a fine powder will flow after a given time interval?

Often test methods take too long and generate results that are qualitative and very subjective. Also, material may be limited and quantitative test procedures are very expensive. If your plant has been working with solids you’re probably familiar with this scenario. The basic issue is a balance of cost versus usable results. Clumping is a complicated issue that’s difficult to quantify; so it’s no surprise that finding a meaningful test is difficult. In most cases, clumping is unpredictable. However, a generalized procedure can be used to solve a clumping problem after it has occurred.

By clumping we really mean unintended agglomeration. While some types of agglomeration are desirable, e.g., to reduce dustiness or make a material easier to handle, most clumping isn’t appreciated. The last thing you need in a pharmaceutical plant is for a bulk bag of acetaminophen to come in as a solid block (as has happened). Clumping is such a tricky issue due to its many causes. Before you can select a test method or procedure, you need to determine the root cause of clumping. Sometimes that can identify a solution without further testing.

Causes of Clumping
The 10 most common sources of agglomeration in bulk solids are:

1. Simple dissolution followed by drying of solids without any chemical reaction. This is the fundamental problem with a lot of storage systems. Bags aren’t sealed well enough or a solvent gets into the transport line. Several of the following causes also involve this process but in a roundabout way.

2. Chemical reaction between particles and gases in voids in the bulk solid. Most common is formation of hydrates, which changes particle density or creates bridges between particles. Oxidation or reduction of particles is less common but can release gases or yield condensable products that form a sticky film on the solids. Diffusion of soluble gases such as carbon dioxide can soften particles and make them susceptible to shear. In addition, particles can interact with wall material through abrasion, which can act as a catalyst to reduce the potential energy needed for a reaction to occur.

3. Change of phase. This is the most difficult problem to diagnose but often is easiest to prevent. Many people don’t realize that a polymorph could be present. About one-third of all organics have at least one polymorph [1]; lots of pharmaceuticals rely on chemicals that aren’t in their most stable form. While transformation upon storage may take years a small amount of change can result in a big effect on flowability. For crystalline solids the problem usually starts with a very small amount of excess solvent and a temperature change. The unstable form dissolves and then recrystallizes into the stable form with solvent release. The process will repeat as solvent moves from particle to particle. A similar process can occur with amorphous organics because the crystalline form is at a lower potential energy.

4. Recrystallization of solids during storage. Often particles can pick up excess energy prior to storage though handling or milling operations. The latter is a very common culprit because attrition raises the surface potential energy of the solids and creates very fine solids, which have a much higher charge-to-mass ratio. It’s rare for solid-state transformation to occur but it only takes a small amount of solvent to aid the crystallization process, similar to a polymorphic transformation.

5. Viscous films on particles. Interstitial solvent can prompt formation of such films. Heating or cooling solids can cause solvent to migrate and collect in one location. As the solvent partially evaporates, it leaves behind a sticky surface on the particle that can lead to bridging.

6. Impurities in solids. These can induce stresses in the particles, which can hasten chemical reaction, phase changes and recrystallization. Impurities can act as catalysts in a reaction or interact with wall materials to initiate one of the causes previously cited. Localized change in density due to an impurity can prevent normal transfer of shear force from particle to particle and put more stress on an individual particle, resulting in breakage. Location of the impurity — whether on the surface or interior of the particle — may even be critical and cause some batches of solids to behave much differently than others.

7. Particle size and width of the particle size distribution (PSD). These attributes often can’t be changed but contribute to clumping. Finer particles have higher specific surface area and more particle/particle contact, resulting in higher shear forces. In addition, the van der Waals’ forces increase rapidly with finer particles. The wider the PSD, the more likely voids around larger particles will fill in with fine particles and boost cohesion. While this is a major factor in clumping, it’s very easy to identify in advance of a problem.

8. Attrition. This is more of a contributing factor for the previously mentioned sources of clumping. Breakage of particles releases energy that’s confined to the solids’ surface. In addition, finer solids will have poorer flowability and higher electrostatic charge. The increase in fines makes the PSD wider and solids easier to bridge.