Size Reduction
Size reduction is a widely used process in industry. In the mineral industry it finds application on a large scale, but in other industries like the chemical, pharmaceutical and pigment industry, where it is performed on a much smaller scale, it is a big issue. There is even wider interest in the closely associated problem of attrition, inadvertent breakage, which occurs in almost all particle transport processes and can lead to extensive costs to control the fine dust produced, as well as interfering with the flow properties and subsequent processing of the main product.
Key questions arise:
- Can we understand what is happening during breakage?
- Can we quantitatively predict the breaking process?
- What can be done with the available knowledge in industrial situations?
These and other questions have been addressed in an extensive IFPRI program of research over the past ten years. This review attempts to bring together the achievements of this program and to identify what questions may now be posed or remain unanswered for any reason.
This review is divided into seven chapters, covering Breakage Phenomena, Problems and solution methods available, Material Properties, Breakage in Industrial Equipment, Industrial Requirements and concludes with a summary of achievements and remaining questions.
Fracture mechanics, Discrete Element Simulation, materials measurement as well as improved experimental techniques have done much to improve our understanding of the processes involved in Comminiution and Attrition. The IFPRI program has contributed a major element in this improvement. Attempts at quantification, whilst much improved have been unable to leap the barrier between laboratory measurement and industrial equipment. The main reason is that we have an inadequate knowledge of the forces developed in industrial machinery nor yet any successful method for predicting a successful comminution event in a multi particle situation in real machinery. Additionally our materials property assessment assumes uniformity of material property and idealised fracture behaviour, which is contrary to practical experience. Changes in property for nominally the same material brought about by a change in previous processing of raw material source will add to the variability of processing and unpredictability of output.
There are reasons, therefor, as a long-term requirement to improve on our ability to predict particle breakage from material properties. This needs to be extended to include agglomerated and porous materials and to define more precisely existing flaw structures, inhomogeneity and shape effects. The role of repeated collisions also need clarification, as do the breakage mechanisms brought about by shear and compression.
A major problem is the definition and description of the processes that occur in industrial equipment. This needs to include flow regimes, the occurrence (scale and frequency) of breakdown events, and the type of comminution or attrition event. At this time, the best way forward may be by simulation to provide a guide to what events to look for, followed by carefully planned experiments for confirmation. It remains an open question as to whether simulation can provide the hoped for guidance and the experimental program will not be cheap.
Until these further objectives are achieved, empirical methods will still be required, but even here improvements will be slow unless the problems posed above are tackled vigorously.
This final report covers research funded by IFPRI arising out of a French national research effort on fine grinding. Three relatively independent teams were involved in separate projects which were to be combined to give an overall view of fine grinding in air jet mills. The teams are:
- Pierre Guigon (Compiegne) Individual High Velocity Particle Impacts.
- Alain Thomas, succeeded by Marie-Noelle Pons (Nancy), Morphological Description of Debris.
- John Dodds (Nancy then Albi) Modelling Fine Grinding Processes.
Whilst much progress has been made in each of the separate investigations the ambitious objective of combining the three projects has not been fully achieved.
PVC ([CHZCHCI] n) powder with fine CaO and/or Ca(OIQ powder and PVDF ([CH$ZF&,) powder with NaOH powder were ground in air by a planetary ball mill to investigate their mechanochemical reactions. The ground mixture was washed with distilled water to remove soluble compounds in the ground product by filtration. Reaction yield was determined by measuring the concentration of halogen in the filtrate. In addition, for the system of PVDF-NaOH, the filtrate containing soluble organic compounds was regulated with HCl solution to decrease its pH to 2, then ethyl acetate was put in the filtrate to extract the organic compounds.
All the same, the grinding causes dehalogenating reaction, forming CaOHCl and polyetylene ([CH=CI&) from the former system, and NaF and organic phases such as [CH2-C=O], and [GC] from the latter one.
For the PVC and CaO/Ca(OH)Z system, the grinding enables to accelerate the dehydrochlorinating reactions between. However, the reactivity of CaO is superior to that of Ca(OHJ2, due to the formation of Hz0 in the PVC-Ca(OH)z system. PVC is transformed into partly dehydrochlorinated polymer, while CaO/Ca(OH)2 is changed into chloride form. The mechanochemically formed CaOHCl in the ground mixture can be removed out by washing with water. The dechlorination is improved with an increase in the molar ratio of (CaO/PVC) as well as grinding time. The impact energy of balls simulated by the PEM would be one of the important parameters governing the mechanochemical reaction between PVC and CaO/Ca(OH)2.
Mechanochemical reaction between PVDF and NaOH proceeds rapidly to transfer nearly half of total fluorine into NaF through the displacement of fluorine in PVDF by OH base. When both fluorine bases bound to the same carbon atom are replaced by OH-base, the dehydration takes place to generate water, which plays a significant role to cause the strong agglomeration of fine particles during grinding. About 90% of fluorine in PVDF can be transferred into NaF by grinding. Dehydration of polar CH2C(OH)2 base proceeds in two ways to form carbonyl (C=O) base and carbon double binding. This gives us water soluble and insoluble organic compounds formed in the ground product.
Present Annual Report - 2000
The present annual report-2000 described results on the third years’ task in the theme of “Mechanochemistry of Materials” approved by the IFPRI organization. The task covers development of novel material processes by means of mechanochemical treatment and its relation with a ball mill simulation work. The simulation work plays a significant role to elucidate mechanochemical phenomena of materials. The present report contains considerable findings on the relation of mechanochemical phenomena and information obtained from the simulation.
Report Composition
The report composes of three parts:
- Mechanochemical treatment of EP dust, forming soluble vanadium (V) compound in water.
- Mechanochemical treatment of fluorescent powder, accelerating its structure change.
- Dechlorination of PVC by its mechanochemical treatment with inorganic material such as CaO.
Findings
Regarding the first example, the yield of vanadium extracted by water leaching is well correlated with impact energy of balls in the mill calculated from the result obtained by the Particle Element Method (PEM). As for the second example, the dry grinding the EP dust enables us to form a water soluble vanadium compound. The well correlation between the V-yield and the impact energy of balls is obtained, suggesting that the ball impact energy plays a significant role to control the formation of vanadium compound.
The third one is dechlorination of polymers such as PVC (poly-vinyl chloride), PVDF (poly-vinylidene fluoride) and PTFE (poly-tetra fluoro ethane) by their mechanochemical treatment with inorganic material such as CaO. This work has been presented at the IFPRI AGM 2000 held at Scheveningen, Netherlands. The present report described only the dechlorination of PVC and its correlation with the impact energy of balls in the mill calculated from the result simulated by the PEM.
All the approximately same, the impact energy of balls in a mill is a significant key to control mechanochemical effect and reaction. In such sense, the computer simulation based on the PEM regarding the ball motion during milling is a quite useful tool for determining the optimum operational parameters, mill design with scaling-up.
The increasing particle-particle-interactions with increasing fineness are an essential problem during wet comminution in stirred media mills. These interactions have an influence on the stability of the product suspension towards agglomeration and on the rheology. Experimental results show that during comminution the measured particle size increases again after reaching a product fineness of approx. 500 nm, although a further increase of the specific surface can be determined (BET-method). The reasons for this phenomenon are the increasing particle-particle-interactions and spontaneous agglomeration.
Possibilities of the stabilization of the product suspension in the stirred media mill are therefore mainly discussed in this report. With respect to this, first results of sample preparation are mentioned. It is shown that during the comminution changes of the pH value, the conductivity and thus the ionic strength as well as the zeta potential occur in dcpendencc of the materials of the grinding chamber lining and the stirrer discs as well as the grinding media material and the product material.
In further investigations the product suspension shall be stabilized electrostatically during the grinding process. The comminution progress as well as the electrochemical properties of the product suspension shall be characterized by online measurements. For this measurements a possible experimental set-up is introduced and discussed. Only after this steps the systematic investigations concerning the influence of the grinding media size are reasonable.
The increasing particle-particle-interactions with increasing fineness are an essential problem during wet comminution in stirred media mills. These interactions have an influence on the stability of the product suspension towards agglomeration and on the rheology. Experimental results show that during comminution the measured particle size increases again after reaching a product fineness of approx. 500 nm, although a further increase of the specific surface can be determined (BET-method). The reasons for this phenomenon are the increasing particle-particle-interactions and spontaneous agglomeration.
Possibilities of the stabilization of the product suspension in the stirred media mill are therefore mainly discussed in this report. With respect to this, first results of sample preparation are mentioned. It is shown that during the comminution changes of the pH value, the conductivity and thus the ionic strength as well as the zeta potential occur in dependence of the materials of the grinding chamber lining and the stirrer discs as well as the grinding media material and the product material. In further investigations the product suspension shall be stabilized electrostatically during the grinding process.
The comminution progress as well as the electrochemical properties of the product suspension shall be characterized by online measurements. For this measurements a possible experimental set-up is introduced and discussed. Only after this steps the systematic investigations concerning the influence of the grinding media size are reasonable.
Fundamental theoretical and experimental works about grinding and crack formation in solids has been done by Schönert [1]. Based on this work he estimates the possible minimum particle size for grinding purposes to be in a range of 10 to 100 nm depending on the physical properties of the material. Investigations of the comminution of fused corundum (Al2O3) should answer the question whether and in which particle size range a lower limit of the grindability exists. During the investigations the increasing particle- particle-interactions with increasing fineness are an essential problem. Due to this interactions the viscosity of the product suspension increases and often spontaneous agglomeration of product particles occurs.
This report shows the experimental setup which allows the measurement of the most important electrochemical properties and the analysis of the particle size distribution of the product suspension as well as an adjustment of the pH-value for stabilization during the comminution process.
First results for comminution of fused corundum with different grinding media materials and grinding media sizes are shown. In addition, the influence of the electrostatic stabilization on the grinding progress is discussed.