Information on the spray drying process which gives the ability to control powder form and thereby an advantage to powder producers.
Spray drying process - The use of spray drying process in ceramic manufacture began in the mid 1950's for white wares, e.g. floor and wall tiles. There followed a growth in electronic ceramics (ferrites, titanates, nitrides) and hard metals (carbides).
Spray drying process - In recent years we have seen the emergence of advanced ceramics for new and varied applications including heat engines, turbines, and high capacity batteries even for artificial bones, joints and heart valves.
Many of these novel applications have stringent requirements not just in terms of particle form but also in terms of purity cross-contamination and containment more commonly associated with the pharmaceutical industry.
This page outlines the developments of the spray process to meet these new challenges.
PROCESS OVERVIEW
Many products use the same basic manufacturing technique:
- Raw Materials
- Pre-Treatment
- Wet Milling
- Screening
- Drying
- Press
- Fire
This process may take place in an aqueous medium or in the case of non-oxides degrade by water then in the presence of a solvent. It is normally a pre-requisite of the spray drying process that powder properties should be controlled with respect to:
- Bulk density
- Particle size and distribution
- Flow characteristics
- Moisture Content
- Compressibility
- Shrinkage
Pressing operations also require a specific press body quality to overcome product sticking in dies, which leads to non uniformity in ceramic surfaces.
THE SPRAY DRYING PROCESS
The spray drying process comprises four steps that influence the final product form:
- Feed preparation
- Atomization/Hot air contact
- Evaporation, particle shape formation and drying
- Dried product separation from drying air and discharge
Read more about the spray drying process.
FEED PREPARATION
The first requirement is that the slip must be pumpable, homogenous, and free from impurities. Typically the slip may contain some or all of the following:
- Organic Binder, e.g. PVA to hold the material together after pressing and prior to firing
- Lubricants, e.g. glycerine or ethylene glycol to soften the particles, to aid pressing
Spray: As the binders and lubricants are held in solution within the prepared feed each spray dried particle has an even coating of binder and lubricant upon its surface enabling binder concentration to be reduced to a maximum recommended concentration of 2% and lubricant to 1%. As these elements are burnt out in the kiln it is important to minimize their concentration in order to minimize shrinkage during the firing process.
- Defloculating agents allow the feed solids concentration to be maximized up to 85% solids in the case of stearite
Increasing feed solids concentration improves the thermal efficiency and plant capacity of the spray drying operation and has a strong influence in the bulk density of the final powder.
Deflocculants may be organic or electrolytic however care must be taken in the latter case as the salts within the water used to prepare the slip in combination with the electrolytic defloculant, which remain in the press body may lead to powder sticking in the dye.
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