Executive Summary
A literature review of precipitation from the liquid state has been undertaken. Two areas of concern to precipitation technologies were highlighted i) factors controlling precipitate size, morphology and agglomeration, and ii) parameters influencing the crystalline phase precipitated.
WHAT IS KNOWN
A wide variety of materials have been precipitated as particles of uniform size in the micron and submicron size range. No limitations on preparing narrow size distribution precipitates appears to be imposed by the chemical nature of the material of interest. The importance of colloidal interactions in the control of size distribution cannot be over emphasized.
Equilibrium thermodynamic factors as well as solution and surface reaction rates are important in determining the phase and morphology of precipitates. An understanding of these factors at small length and short time scales is required for morphological control. Impurities and additives strongly affect particle growth rates and the phase precipitated. Today, shape and phase control are more art than science.
Many of the technologies currently available for the preparation of uniform precipitates have been developed at the bench scale and have yields in the milligram to gram per liter range. With the exception of silver halides and emulsion polymers, no work has been reported in the literature describing scale-up to industrial quantities. Poor understanding of surface reactivity of freshly prepared particles and their colloidal nature currently limits development of separation schemes when unagglomerated particles are required. Due to the lack of published information on large scale processes, no attempt was made to quantify limitations on post precipitaton processing steps.
Agglomeration of nanometer sixed particle nuclei appears to be a critical step in the formation of many uniform precipitates. Heterogeneous size distributions arise when aggregation/agglomeration is not halted at the required size.
Little information was found on the cost of generating uniform particles in the sub-ten micron size range. Much of this information resides in industry and not academia.
WHAT IS UNKNOWN
No general rules for controlling the precipitate phase or morphology have been developed despite the industrial need.
Growth mechanisms of precipitates are poorly understood for most materials. The chemistry occurring in supersaturated solutions far from equilibrium impacts strongly on precipitate phase and morphology but is not well characterized. This poor understanding limits attempts to scale-up current precipitation recipes.
Mechanisms governing nucleation and growth of uniform particles have seen surprisingly little study. In the early stages of precipitation, growth mechanisms are difficult to assign. Often, even the size of the growth unit is poorly characterized.
Scale-up of precipitation chemistries resulting in uniform particles has not been extensively studied. As a result little is known about the magnitude and size of the scale-up difficulties.
WHAT IS PROPOSED
Critical review of the current state of the precipitation of uniform particles has led to the formulation of two key areas of opportunity where fundamental research could lead to enhanced understanding and thus to better precipitation technologies. These opportunities are discussed on pages 26-29 of the review and are briefly described below.
Particles Fundamental Studies of Nucleation and Growth During Precipitation of Uniform Particles
While the chemistry of precipitation reactions is very complex, broad themes important to preparing uniform precipitates may be more general. This program would search for these themes. Three areas have been identified as being of critical importance. First, prenucleation chemistry and what species form nuclei would be explored. Secondly, the colloidal interaction of nuclei would be studied in order to establish factors which act to give colloidal stability to growing precipitate particles. Thirdly, growth mechanisms of the particles would be determined to establish the importance of aggregation on final particle morphology. The goal of this work would be to develop an understanding of the physical chemistry leading to uniform particles and to determine the generality of the mechanisms such that they can be applied to new systems.
Fundamental Studies in the Scale-up of Precipitation of Narrow Size Distribution Particles
Few attempts have been published detailing the scale-up of precipitation technologies leading to uniform particles. As a result, little is known about the difficulties in increasing the size or the yield of current precipitaton technique. The thrust of these studies would be: i) to explore various scale-up scherncs, ii) develop quantitative models from a basic understanding of the reaction pathways, and iii) provide a means of assessing the difficulties in scale-up operations. The general goal of the research would be to determine the difficulties in scaling up precipitation reactions that result in uniformily sized particles.