Coal Preparation » Gravity Separation
The Australian coal industry has been progressively adopting dense medium cyclone (DMC) processing technology for the last 40 years. By the late 1970’s the use of 710 mm diameter units was very common, while since the mid 1990’s large diameter (1000 mm diameter and larger) have been almost universally employed in new and upgraded plants. Consequently the use of large diameter dense medium cyclones has become widespread in the Australian coal industry and in the future is likely to be the dominant processing unit. There is little readily available material information on the operation of these types of units. Access to such information will allow the optimisation of the design and operation of plants that include these units. As a result, ACARP has funded a number of studies in this area.
DMC circuits typically treat 60-85% of the plant feed. Therefore any small defect in DMC performance is quickly reflected in a corresponding loss in plant yield or an adverse product quality result.
Properly conducted audits on DMC circuits will result in the identification of poor operations or opportunities for better management, and hence provide the potential for significant operational improvements. It is essential that audits are carried out so that the data is reliable in a statistical sense, and all relevant information is collected.
In addition, the advent of large diameter DMC has been accompanied by the introduction of high capacity multi-slope (banana) screens. Combined, these technologies allow for large feed rates to be handled in modules containing large single pieces of equipment. Recent design trends have seen plant footprints decrease, so the end result is compact plants handling very high tonnages of coal with large volume flows within the dense medium circuits. Consequently, proper sampling has become quite difficult as getting representative samples from high volume and large mass flows is very challenging, and the large equipment sizes within relatively small confines greatly restricts access.
It is crucial that ‘best practice’ sampling procedures be employed. As well, many well planned and executed audits have been ruined by poor sample preparation and analysis techniques in the laboratory. Considerable logistics planning is required to prepare the sampling sites, extract the plant operational information, collect and analyse the samples, and appropriately assess the data.
The parameters used to characterise the separation efficiency of a dense medium cyclone are traditionally the separation relative density (RD50), Ep, and misplaced material through the spigot or vortex finder. Recent work on large diameter DMCs (Swanson and Atkinson, 2007) has identified the need to more carefully assess cyclone performance, particularly for the -4 mm fraction, by looking at additional factors such as the high and low gravity tails, Error Area, and cut point drift. These parameters can only be obtained from a properly constructed complete partition curve. A partition curve is critical to identify any loss of product in the lower density fractions, and to identify any contamination from higher density particles.
This guide has been developed in response to a need identified by ACARP for a handbook on the best practice for dense medium cyclone sampling. Appreciable effort has been directed towards improving the design and operation of such equipment and the proposed handbook is essential in ensuring that the maximum return is obtained from plant sampling, be it part of an R&D program or normal plant audit work. This resultant handbook outlines the background of sampling theory and reasons for seeking other operational information, as well as providing a guide to best practices and a check list to assist in setting up a DMC plant audit.
This project has been a joint effort between QCC and CSIRO. Personnel from these organisations have had extensive recent experience in plant sampling and testing, and have been actively involved in R&D into the operation of large diameter DMCs.