A thesis submitted to the Faculty of Engineering and The Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering Johannesburg 12 September 2016 / Comminution and leaching unit processes play a major role in extracting valuable minerals from ore. Most of the research reported in the literature has focused on optimising individual unit operations rather than on integrating the whole process. This thesis develops an integrated approach to mineral processing systems and flow sheets and is intended to create a methodology for process synthesis that can be applied throughout the extractive metallurgical industry. This could lead to improved efficiency in the overall process by obtaining optimum recovery and, most important, a reduction in energy and material costs. In order to illustrate the methodology a particular example was chosen, namely optimizing the joint comminution and leaching of a particular gold ore. In this investigation laboratory scale grinding and leaching profiles for a gold feed sample (1700–850 μm) were measured. In a laboratory mill various combinations of grinding media, filling level and ball size were investigated, and of the three ball sizes used (10, 20 and 30mm) breakage was most pronounced for the 20 mm. Thus for instance it was also established that when using a higher filling ( =30%) and a ball size of 30 mm, more energy was consumed but less liberation occurred, thus a lower amount of gold was extracted during a 24-hour leaching period. Finally, the breakage kinetics of the gold ore was looked at. Using a standard population model the breakage and selection function parameters were successfully calculated. An investigation into the dissolution kinetics of gold ore in a solution of NaCN was also done. These were found to depend on the stirring rate, reaction temperature, particle diameter and the concentration of the leachant. The rate increased with the stirring speed, reaction temperature and leachant concentration, but decreased when the particle size was greater. The activation energy for the dissolution was estimated at about 3 kcal/mol. Furthermore, the linear relationship between the rate constant and the reciprocal of the square of the particle size is a strong indication that the gold dissolution process is diffusion-controlled. The experimental results were well-fitted to a shrinking core model. In attempting to understand the results, the researcher carried out a number of experiments that involved an investigation into the relationship between comminution and leaching in terms of energy usage and particle size, the former to establish the most efficient application of energy, and the latter to identify the degree of fineness that would ensure optimal recovery. The Attainable Region (AR) method was then used to establish ways of finding the leaching and milling times required to achieve minimum cost (maximise profit). No work on utilizing the AR technique to minimise the cost of milling and leaching on a real industrial ore has previously been published. The investigation aims to show how the AR technique can be used to develop ways of optimising an industrial process that includes milling and leaching. The experimental results were used to show how this method could be successfully applied to identifying opportunities for higher efficiency when performing these operations. The approach however is general and could in principle be used for any two or more unit operations in determining how the product from one unit should be prepared to feed to the next unit so as to optimize the overall process. / MT2017

No description available.

Thesis (doctor)--Technische Universiteit Delft, 1997. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2006. / Melkote, Shreyes, Committee Chair ; Zhou, Min, Committee Member ; Liang, Steven, Committee Member ; Thadhani, Naresh, Committee Member ; Haj-Ali, Rami, Committee Member. Vita. Includes bibliographical references.

Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (M.Eng.Sc.) - University of Queensland, 1996. / Includes bibliography.

No description available.

Size-reduction of materials is an important operation in many chemical industries. The purpose of size-reduction are: (a) for specific mesh size product; (b) grinding for release, or unlocking grinding for purposes of surface exposure or locked-in products for chemical or physical attack; (c) specific surface grinding for product; and (d) size modulus, where limited mesh size is needed for specific processing operation. Size-reduction is accomplished by (a) impact, (b) differential movement of adjacent surfaces and (c) by the combination of both. Most size-reduction of a combination of both, whereas swing hammermill grinding is due mainly to impact. Rittingers’ and Kicks’ laws are empirical formulas which do not make an attempt to resolve the forces as applied to grinding into their component parts. Possibly the application of the component forces as applied to impact grinding would give a key to the multiple factors controlling size-reduction. If this can be worked out, the general formula might be obtained that would apply to all phases of size-reduction. The purpose of this investigation is to study the factors as applied to swing hammermill grinding; also, to determine if it is possible to formulate an equation which will hold for the size reduction of various materials. / Master of Science

The grinding models developed previously are concerned with size reduction only. Although they have proven to be useful in the simulation and design of grinding mills, they do not provide information on liberation which is the main objective of most comminution operations. In the present investigation, a population balance model describing the combined processes of size reduction and mineral liberation has been developed for batch grinding operation. The model parameters include conventional breakage rate and breakage distribution functions, along with a new parameter i.e., liberation function that is used to describe changes in particle composition. These parameters have been determined experimentally by examining mill products under optical microscope using a SEM-IPS image analyzer. The areal assays, obtained from the image analysis of monosized particle mounts, have been found to correspond quite closely to the actual chemical assays. It has been found that the method used to prepare particle mounts is critical in achieving accuracy. In the present work, it has been shown that the breakage characteristics of component minerals can be determined by examining the mill feeds and products using an image analyzer. The model parameter analysis has shown that while the breakage rate functions are sensitive to the grinding environment, breakage distribution functions are independent of it. Furthermore, the breakage distribution functions have been found to be normalizable with feed size, thus reducing the number of parameters that must be estimated. The study has also shown that both the breakage rate and the liberation function suggest a preferential breakage of sphalerite over dolomite gangue. The model has been validated by simulating the batch grinding of a sphalerite ore from ASARCO's Young Mine in eastern Tennessee. The model can predict the product size distributions for the total ore and its components, including gangue, sphalerite, and composite particles. An excellent agreement between the model predictions and the experimental results has been observed for both monosized and multisized feed materials. The model is capable of handling multiple classes of composite particles for a binary ore; however, the model has been verified against experimental results by considering only two composite classes. The method of determining liberation functions has also been discussed. The liberation function has been found to be useful for analyzing the liberation mechanisms of composite particles. / Ph. D.

The implications of comminution practices on the planning of a typical open pit mine was investigated in this study by means of computer simulation. The objective was to assess the effects of mining costs as well as processing costs on the production plan of a typical open pit mine. For the purpose of the research, MineLib, an open library of ore body models was consulted. This led to the selection of a copper-gold ore body named “Newman1” for use in the strategic mine optimisation. Various scenarios were considered in order to highlight the contribution of comminution costs to the mine plan. In all the simulated scenarios, the objective function was to maximise the Net Present Value (NPV). And in terms of simulation setup, the comminution costs and cut-off grades were systematically varied from 70 % to 140 %. It was hence possible to investigate their effects on the NPV of the Newman1 ore body using SimSched, a freeware for mine optimisation and planning. Results showed that there is a great opportunity to increase the NPV of the Newman1 block model by adjusting the contribution of processing costs in general and comminution costs in particular. This can be achieved for instance by controlling the policy of cut-off grades, lowering production costs, and increasing throughput. / Civil and Chemical Engineering / M. Tech. (Chemical Engineering)