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Investigation into the mechanisms controlling the formation of high density sludge

Bullen, Christopher James. 2006. Investigation into the mechanisms controlling the formation of high density sludge. PhD Thesis, Cardiff University.

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One of the legacies of mining is the generation of minewater which can be toxic to the environment and requires treating prior to discharge. This can be treated by either passive or active systems, with the latter being most common for high volume, high load minewaters or those that are difficult to treat. Conventional minewater treatment using pH adjustment can precipitate a large volume of voluminous sludge that is expensive to dispose of. This led to the development of the High Density Sludge (HDS) process in the 1960s, which has now become accepted as the best practical method of treating minewaters whilst minimising the volumes of sludge generated. The mechanisms controlling the formation of HDS are not fully understood and as a result there are a number of misconceptions adversely affecting the use of the HDS process. The primary objective of the present research was to explore in greater depth the mechanisms controlling HDS formation and hence dispel some of these commonly held misconceptions. The current understanding of the HDS process was initially established by undertaking a review of information reported in the technical literature. The key operating parameters and concepts were then identified by undertaking a performance review of the 440l/s HDS plant at the former Wheal Jane tin mine in Cornwall, UK. A series of laboratory batch tests and continuous pilot trials were undertaken to assess the importance of these concepts and generate an understanding of the sludge characteristics and properties. The sludge characteristics were investigated by reviewing the sludge settlement (settlement velocity and the ability of the sludge to self compact) and dewatering characteristics. The sludge properties examined were: mineralogy (by X-Ray Diffraction, XRD), morphology (by Scanning Electron Microscopy, SEM, and Transmission Electron Microscopy, TEM) and surface electrical potential (by . measuring the zeta potential). The research has shown that the dominant mechanism for the formation of Type II HDS involves establishing a pH in the Stage I Reactor that gives a negative charge to the recirculated solids. Physical adsorption then ensures heterogeneous nucleation. Control of the pH in the Stage II Reactor ensures removal of the final trace of metals from solution and return of sludge that is capable of achieving the desired pH in the Stage I Reactor. A review of the relevant literature suggests that too high a pH in the Stage I Reactor will favour homogeneous nucleation and hence inhibit HDS formation. The research has also shown that HDS can be formed using non calcium based alkali reagents and that iron is not required for its generation. Synthetic zinc and manganese minewaters produced HDS with the best settling characteristics. The sludge characterisation showed that there is no requirement for the HDS to be crystalline in nature. The presence of species such as calcium and magnesium cations can 'swamp' the process and slow down the formation of HDS, though they do not prevent its production. Finally, the research undertaken has shown that the HDS process enhances (by over forty times) the dewatering characteristics of the sludge generated during minewater treatment, the original purpose for which the process was developed.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
ISBN: 9781303205279
Date of First Compliant Deposit: 30 March 2016
Last Modified: 09 Jan 2018 22:17

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