Milling of Organic Solids

Publication Reference: 
Author Last Name: 
Y. Ding, C.C. Kwan, K.J. Roberts, and M. Ghadiri
Report Type: 
Research Area: 
Size Reduction
Publication Year: 
Publication Month: 
United Kingdom

The aim of the project is to establish a relationship between the product properties and feed material and the mill functions for milling of organic solids. The specific objectives are:

a) to characterise the physical, mechanical, and thermal properties of model organic (feed) materials (material function) at the single particle level, and to examine the effects of temperature and humidity on these properties

b) to investigate the breakage behaviour of single organic particles under various conditions

c) to characterise the properties of milled products, and to correlate the product properties to material and mill functions Models materials that are planned and approved for use in the project by the TC of IFPRI include aspirin (low hardness, relatively high fracture toughness, ductile), á- lactose monohydrate (áLM, high hardness, high fracture toughness, semi-brittle), sucrose or sorbitol (high hardness, relatively low fracture toughness, regarded as brittle in the literature), starch (ductile and mechanical properties greatly affected by the strain rate), and microcrystalline cellulose (MCC, medium hardness, high fracture toughness, semi-brittle). These materials cover a fairly wide range of physical, mechanical and thermal properties, hence ensuring generality of the results to be achieved. This report summarises the work done over the first year on five model organic materials, á-lactose monohydrate (áLM), sucrose, sorbitol, starch and microcrystalline cellulose (MCC). The work includes extensive experimental investigation into the behaviour of both the single particle impact breakage and the bulking milling under various conditions, and preliminary mathematical modelling based on the distinct element method (DEM) and the population balance method. Also included in the report are some results of the milling of MCC and áLM obtained by the investigators before the project was started. The single particle impact experiments provide data of the breakage extent of single particles as a function of impact velocity, which is then used to infer the physical and mechanical properties of the tested particles by utilising the Ghadiri-Zhang model for semi-brittle materials. The results show that the modes of failure of the five tested materials agree well with that reported in the literature. Sucrose and sorbitol, which are generally regarded as brittle materials, demonstrate the highest extent of breakage. On ii the other hand, starch and MCC, two materials regarded as more difficult to mill, show the lowest extent of breakage. Sub-ambient impact experiments are conducted on áLM, sorbitol and MCC. The results suggest that the breakage propensity, thus the mechanical properties, of áLM and MCC are little affected by the temperature, but a lower temperature is seen to reduce the breakage extent of sorbitol particles. The bulk milling experiments are conducted in a single ball mill and the results are quantified by an analogy to the first-order rate process. The results show that milling of both MCC and áLM is little affected by the temperature, in agreement with the single particle testing results.

Attempts are made to relate the single particle impact breakage behaviour to the bulk milling behaviour. For MCC, starch and áLM, the milling behaviour expressed by the milling rate constant relates linearly to the single particle parameter containing the physical and mechanical properties of the particles. However, the relationships for sucrose and sorbitol are non-linear, indicating alternative methods should be explored for these types of materials. The relationships between the single particle breakage and the bulking milling behaviour are different for different materials. Preliminary efforts are made to unifying these relationships. This is achieved by quantifying the mill function with a term called ‘milling power’ predicted by DEM simulations of motions of both the milling media and the feed particles. The new bulk milling parameter is shown to relate to the single particle behaviour well and a unified relationship is obtained for áLM, starch and MCC. Preliminary modelling work using the population balance method has been carried out on the milling of sucrose in collaboration with Dr R Bertrum Diemer of DuPont. The results will be reported in the future.