Comparison of grinding media - Cylpebs versus balls

Cylpebs are slightly tapered cylindrical grinding media with a ratio of length to diameter of unity. The manufactures have made conflicting claims regarding the milling performance of Cylpebs in comparison with balls. One major point of interest is which one grinds finer at the same operating conditions. The difficulty in comparison is due to the shape difference. The two grinding media have different surface area, bulk density and contact mechanisms in grinding action. Comparative tests were conducted using the two types of grinding media in a laboratory Bond ball mill at various conditions of equality such as media mass, size distribution, surface area and input specific energy. The laboratory results indicate that at the same specific energy input level the Cylpebs produce a product with slightly less oversize due to their greater surface area, but essentially the same sizing at the fine end as that produced with the balls. The reason may be that the advantage of greater surface area is balanced by the line contact and area contact grinding actions with the Cylpebs. A new ball mill scale-up procedure [Man, Y.T., 2001. Model-based procedure for scale-up of wet, overflow ball mills, Part 1: outline of the methodology. Minerals Engineering 14 (10), 1237-1246] was employed to predict grinding performance of an industrial mill from the laboratory test results. The predicted full scale operation was compared with the plant survey data. Some problems in the original scale-up procedures were identified. The scale-up procedure was therefore modified to allow the predicted ball mill performance to match the observed one. The calibrated scale-up procedure was used to predict the Cylpebs performance in the full scale industrial mill using the laboratory tests results. (C) 2004 Elsevier Ltd. All rights reserved.

Temperature changes in dental pulp associated with use of power grinding equipment on equine teeth

Objective To quantify the temperature changes in the dental pulp associated with equine dental procedures using power grinding equipment. Design A matrix experimental design with replication on the same sample was followed to allow the following independent variables to be assessed: horse age (young or old), tooth type (premolar or molar), powered grinding instrument (rotating disc or die grinder), grinding time (15 or 20 seconds) and the presence or absence of water coolant. Procedure Sound premolar and molar teeth from a 6-year-old horse and a 15-year-old horse, which had been removed postmortem, were sectioned parallel to the occlusal plane to allow placement of a miniature thermocouple at the level of the dental pulp. The maximum temperature increase, the time taken to reach this maximum and the cooling time were measured (n=10 in each study). The teeth were placed in a vice and the instrument used on the tooth as per clinical situation. Results Significant differences were recorded for horse age (P < 0.001), instrument type (P < 0.001), grinding time (P < 0.001) and presence or absence of coolant (P < 0.001). There was no significant difference for tooth type. Conclusion Thermal insult to the dental pulp from the use of power instruments poses a significant risk to the tooth. This risk can be reduced or eliminated by appropriate selection of treatment time and by the use of water irrigation as a coolant. The increased dentine thickness in older horses appears to mitigate against thermal injury from frictional heat.

Potential of Sr isotopic analysis in ceramic provenance studies: Characterisation of Chinese stonewares

We compare the trace element and Sr isotopic compositions of stoneware bodies made in Yaozhou and Jizhou to characterise these Chinese archaeological ceramics and examine the potential of Sr isotopes in provenance studies. Element concentrations determined by ICP-MS achieve distinct characterisation for Jizhou samples due to their restricted variation, yet had limited success with Yaozhou wares because of their large variability. In contrast, Sr-87/Sr-86 ratios in Yaozhou samples have a very small variation and are all significantly lower than those of Jizhou samples, which show a large variation and cannot be well characterised with Sr isotopes. Geochemical interpretation reveals that Sr-87/Sr-86 ratios will have greater potential to characterise ceramics made of low Rb/Sr materials such as kaolin clay, yet will show larger variations in ceramics made of high Rb/Sr materials such as porcelain stone. (c) 2005 Elsevier B.V. All rights reserved.

Exploratory modelling of grinding pressure within a compressed particle bed

With increasing industry interest in high pressure roll grinding (HPGR) technology, there is a strong incentive for improved understanding of the nature of grinding pressure that exists in the interior of a compressed particle bed. This corresponds to the crushing region of the HPGR. The relationship between applied pressure (stress) to the particle bed and induced pressure (stress) within particles and at contact points between particles is of particular interest. A detailed parametric investigation is beyond the scope of this exploratory paper. However, this exploratory investigation does suggest some interesting behaviour. The compressed particle bed within an 80 turn diameter piston has been modelled using Particle Flow Code for three dimensions. PFC3D is a discrete element code. The total number of simulated particles was 1225 and 2450 for two beds of different thickness. Particle diameters were uniformly distributed between 4 and 4.5 mm. The results of the simulations show that stress intensity within the simulated particle beds and within the observed particles increased with increase of the applied stress. The intensity of the average vertical stress in the selected particles tended to be comparable with the intensity of the pressure applied to the surface of particle bed and was only occasionally higher. However, the stress at contact points between particles could be several times higher. In a real crusher, such high stress amplification at contacts will quickly decrease due to local crushing and a resultant increase the size of the contact area. Therefore, its significance is likely to be relatively small in an industrial context. The modelling results also suggest that failure within the particle bed will progress from the crushing surface towards the depth of the bed. (c) 2006 Elsevier Ltd. All rights reserved.

Fracture load and marginal fit of shrinkage-free ZrSiO4 all-ceramic crowns after chewing simulation

The aim of this in vitro study was to evaluate the fracture load and marginal accuracy of crowns made from a shrinkage-free ZrSiO4 ceramic cemented with glass-ionomer or composite cement after chewing simulation. Thirty-two human mandibular molars were randomly divided into two groups. All teeth were prepared for and restored with shrinkage-free ZrSiO4 ceramic crowns (Everest HPC (R), KaVo). The crowns of group A (N = 16) were luted to the teeth using KetacCem (R) and group B (N = 16) were adhesively cemented using Panavia (R) 21EX. Measurements of the marginal accuracy before and after cementation were made using replicas and an image analysis system. All specimens were exposed to 1.2 million cycles of thermo-mechanical fatigue in a chewing simulator. Surviving specimens were subsequently loaded until fracture in a static testing device. Fracture loads (N) were recorded. All specimens survived chewing simulation. The mean fracture loads (+/- s.d.) were Group A, 1622 N (+/- 433); group B, 1957 N (+/- 806). There was no significant difference between the two groups (P > 0.05). The marginal gap values before cementation were (mean +/- s.d.): Group A, 32.7 mu m (+/- 6.8); group B, 33.0 mu m (+/- 6.7).The mean marginal gap values after cementation were (+/- s.d.): Group A, 44.6 mu m (+/- 6.7); group B, 46.6 mu m (+/- 7.7). The marginal openings were significantly higher after cementation for both groups (P < 0.05). All test groups demonstrated fracture load and marginal accuracy values within the range of clinical acceptability.

Ba0.5Sr0.5Co0.8Fe0.2O3-delta ceramic hollow-fiber membranes for oxygen permeation

Setf-supported asymmetric hollow-fiber membranes of mixed oxygen-ionic and electronic conducting perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) were prepared by a combined phase-inversion and sintering technique. The starting inorganic powder was synthesized by combined EDTA-citrate complexing process followed by thermal treatment at 600 degrees C. The powder was dispersed in a polymer solution and then extruded into hollow-fiber precursors through a spinneret. ne fiber precursors were sintered at elevated temperatures to form gastight membranes, which were characterized by SEM and gas permeation tests. Performance of the hollow fibers in air separation was both experimentally and theoretically studied at various conditions. The results reveal that the oxygen permeation process was controlled by the slow oxygen surface exchange kinetics under the investigated conditions. The porous inner surface of the prepared perovskite hollow-fiber membranes considerably favored the oxygen permeation. The maximum oxygen flux measured was 0.031 mol-m(-2).s(-1) at 950 degrees C with the sweep gas flow rate of 0.522 mol(.)m(-2).s(-1). To improve the oxygen flux of BSCF perovskite membranes, future work should be focused on surface modification rather than reduction of the membrane thickness. (c) 2006 American Institute of Chemical Engineers.

Effect of microstructure on material removal mechanisms in nano/micro grinding of tungsten carbide mould inserts

Nano/micro grinding of tungsten carbide (WC) mould inserts was performed. A form accuracy of 〜200nm (in PV) and a surface roughness of 〜7nm were achieved. Nanoindentation revealed that small chipping or cracking occurred even at a penetration depth of 38nm, which could hinder the further improvement of surface quality during grinding. It was found that when grinding was conducted at nanometric scale, the microstructure of the work material and the morphology of the WC grains should be taken into account to enable a fully ductile removal. Copyright 2005 by the Japan Society of Mechanical Engineers