The Wonders Of Washing

Mick Naylor, Derek Parnaby Cyclones International Ltd, UK, considers the benefits of coal washing and outlines the key elements of two new washing plant installation projects.

Coal washing is considered by many as a last resort or at best a necessity. Due to the natural variations found in general mining conditions, unwanted material is removed as well as the desirable coal. These detritus materials are usually from the rocks forming the floor, as well as the roof of the seam or the rocks adjacent to it. Also, other material may have been erroneously included in the mining and mechanical handling of the material, as well as any inferior coal that may not be suitable for the desired purpose of the product.

In order to remove the wanted from the unwanted material, a physical difference between the two must be exploited. Usually, it is the variation in the physical density of the coal and discard that facilitates their separation, but for the very finest of coal it is normally the surface chemistry of coal particles that is used. By its very nature, coal washing must remove some of the ‘as mined’ material (even though it is not saleable) and throw it away. It adds capital and operational costs as well. However, the coal, once it has been washed, is far more saleable and the value after its enrichment should far exceed the cost of removing mine waste and the costs associated with the plant and its operation.

Derek Parnaby Cyclones International Ltd are a family-owned British company that has been selling coal washing and processing equipment since the 1970’s. The company has recently been awarded two more contracts in South America for washing plants, and this article aims to share some of these details of these two projects.

Modular plants

Based on the company’s proven range of modular dense medium cyclone plants, the new installations will have 80 tph and 100 tph capacities. Firstly, Parnaby was approached by an existing client for a new plant and an inspection of their existing plant, in order to determine whether it required updating. The second plant was requested by a new client to Parnaby, but one which had previously heard of the company via employees that had worked on some of its previous installations in South America.

For high yielding soft South American coals, nearly all of the modular plants supplied by Parnaby are of a dense medium cyclone design, which takes the best of the old DSM principles and is updated with more modern technologies. Using a wide range of home and internationally sourced equipment to provide a cost-effective installation that is efficient, modular, mobile and easily containerised, in order to minimise shipping costs, Parnaby aims to provide a cost-efficient solution for their clients. The two plants are very similar in principle with different equipment sizes to suit the different feed materials.

Basic design

The plants will consist of a conventional design with a nominally minus 50 mm raw coal feed. The minus 1 mm will be removed by wet screening on the de-sliming screen. The minus 50 plus 1 mm overflow from the screen is fed directly to the wing tank and mixed with the circulating medium. From here it will be pumped to the dense medium cyclone.

The selection, size, and parameters of the cyclone were adjusted to suit the two different applications. The cyclone will separate the coal from any discard that was included in the feed. With adjustments to the density of the medium, the quality, and, of course, the yield of the coal product is modified to maximise the coal recovery at the desired ash and sulfur contents. Coal and discard products report to their own medium recovery/dewatering screen, prior to centrifuging the coal product.

The minus 1 mm removed from the feed by wet screening on the de-sliming screen is pumped to a bank of classifying cyclones to remove the nominally minus 0.15 mm material. The minus 1 plus 0.15 mm is cleaned in a bank of spirals to again separate the fine coal from fine discard and each product is dewatered on a high ‘G’ dewatering screen. The fine coal is further dewatered in a fine coal centrifuge.

The minus 0.15 mm material is pumped to a bank of froth flotation cells to recover the very finest coal particles. An oil-based collector collects the fine coal, and this then attaches to a bubble created by the action of the froth cells and the frothing reagent that is added. This obviously is lifted by the bubble, and in turn makes a froth layer at the top of the flotation cell, which is removed and dewatered. Fine discard remains in the slurry and passes to the thickener. All effluents are collected and treated in the thickener/clarifier cell, so that the regenerated clarified water is reused throughout the plant as sprays, top up water and screening water, whilst the suspended solids that were held in the effluents settle to the bottom of the thickener. They are then pumped to a holding tank and dewatered in a multi-roll filter press. By this method, under normal operating conditions a closed loop effluent system is maintained, and no effluents are released.

The majority of Parnaby’s modular plants have galvanised structures and are fitted with wear linings as appropriate.

Site layout and installation

Both sites offer similar difficult challenges. Due to their locations in the mountainous regions of Latin America, the sites will have to be tiered. Therefore, Parnaby have looked at using the natural fall of the hillside to assist in the processing of the coal. So, typically, the raw coal preparation will

take place on the highest tiers, the washing plant will be located on the middle tier, and the product stockpiles, bunkers, etc. will be on the lowest level. This means that the product conveyors will not have to be excessively long or steep to achieve good working capacities in all the product storage facilities. To achieve a practical and maintainable layout, Parnaby worked closely with each client’s civil engineering experts and their operation and maintenance personnel. Several plans and provisional layouts were submitted before the final layouts were accepted and approved. This has involved a weeklong series of meetings in South America, where the final options were discussed and agreed.

Due to the ever-increasing costs of shipping, where possible, the plant will be containerised with a minimum amount of equipment being transported as deck cargo. This has necessitated a small amount of redesigning for the larger items of equipment, in order to enable it to be quickly and easily broken down into smaller parts that can readily be reassembled on site, without the need for specialist tools and equipment. Fabrication and procurement are currently ongoing, and the client’s ground breaking for civils installation is expected to commence in 4Q22.

The plants are destined to be delivered later in 2022, and commissioning is envisaged to commence in 4Q22. The product will be utilised as prime coking coal and will be used in-house for their production of top quality coking coal. Both clients have chosen to use modern versions of ‘beehive coke ovens’, as both feel that it is best suited to the local coals and generates the best coke available. An expansion of the coke ovens on one of the sites has already commenced, so the new coal product is required as soon as possible. Again, the siting of the coke ovens is tiered to suit the existing ground conditions, but does present some logistical problems for the use and firing of the ovens.

Parnaby is looking forward to completing these plants and their commissioning. All of the major functions will be carried out in-house, with electrical engineering works being sourced out with the clients approval and to their standards. The plant controls will be programmable logic controller (PLC) based, with visual displays of levels and densities. All the plant equipment will have its start/stop sequences controlled and monitored, in order to ensure correct functioning. The switchgear will be housed in a ‘wardrobe’ style panel.

Figure 1. Typical Parnaby modular coal washing plant.

Figure 2. Typical dense medium cyclone installation.

Figure 3. Typical electrical MCC board installation.

History

Parnaby have been familiar with delivering coal washing plants worldwide for many years. Larger 400 tph modular plants have been supplied over the years; to Spitsbergen and to two clients in the UK.

Slightly smaller plants have also been supplied into several artic locations. 17 plus plants and parts thereof have been shipped over a period of numerous years to the Republic of South Africa, with different capacities and combinations of large and small coal dense medium systems. Plants and parts have been supplied to all parts of America (north and south), as well as Canada. Asia has been a constant market for Parnaby with plants located in India, Indonesia, Vietnam, and China. A 300 tph plant has recently closed in Poland after nearly 25 years of production, but this plant was of the natural medium system, utilising the fines in the feed to create a ‘natural’

medium that assisted the separation in the barrel and cyclones.

The structural design for the majority of these plants has been with a firm eye on it being modular. This eases its relocation if required. No one knows how a mine will develop and whether production will continue as it was anticipated at the time of planning the washing plant project. A case in point being that the Spitsbergen plant was expected to have a long life with the new mine coming on stream. However, a change in Norwegian government policy meant that although the new mine was more or less developed and proven out, it was closed. If the location of the plant had been further south and the logistics of shipping the plant were easier, it could easily have been relocated elsewhere. However, due to the harbours being frozen for six months of the year, it was very cost prohibitive to dismantle and relocate the plant.

Conclusion

Ideally coal washing or coal preparation should be assessed and reviewed as to whether it will be required at an early date in a mine’s life, not as an ‘add on’/‘retrofit’ when all else fails. If the review finds that in the early years, it is unlikely that coal preparation will be required, then great. If it indicates that it will be essential to produce a saleable product it should be assessed and designed into the project from the beginning; thus minimising handling, storage, and, ultimately, costs. Even though it will be many, many years before the stigma of it being a ‘necessary evil’ is overcome, it can and often does transform an ‘as mined’ unsaleable product into a high value, desirable, and highly saleable product. Nonetheless, the design and operation benefits hugely from a small amount of initial planning and forethought.

For the near future coal, washing is here to stay, but it deserves to be put higher up the list of planning requirements, in order to minimise its costs and overall impact.

Figure 4. 400 tph plant installed in Spitsbergen.

Derek Parnaby Cyclones International Derek Parnaby Cyclones International a company synonymous with coal washing - a company synonymous with coal washing - Derek Parnaby Cyclones International has a world-wide reputation for excellence. has a world-wide reputation for excellence. a company synonymous with coal washing - The market-leading company has manufactured The market-leading company has manufactured has a world-wide reputation for excellence. high quality plants for over three decades, and high quality plants for over three decades, and The market-leading company has manufactured it continually develops its natural and dense it continually develops its natural and dense high quality plants for over three decades, and medium washing and separation processes. medium washing and separation processes. it continually develops its natural and dense Its experience and expertise attracts contracts Its experience and expertise attracts contracts medium washing and separation processes. from across the globe. from across the globe. Its experience and expertise attracts contracts Applications include: Applications include: from across the globe. • Coal preparation • Coal preparation Applications include: • Effluent treatment • Effluent treatment • Coal preparation • Modular and mobile plants • Modular and mobile plants • Effluent treatment • Fines coal beneficiation via gravity spirals • Fines coal beneficiation via gravity spirals • Modular and mobile plants and hydrosizer and hydrosizer • Fines coal beneficiation via gravity spirals • Froth flotation • Froth flotation and hydrosizer • Dense medium and natural medium • Dense medium and natural medium • Froth flotation washing process washing process • Dense medium and natural medium • Plants of up to 5 million tonnes per annum • Plants of up to 5 million tonnes per annum washing process • Plants of up to 5 million tonnes per annum

Derek Parnaby Cyclones International Ltd Derek Parnaby Cyclones International Ltd

Chilton Industrial Estate, Chilton Industrial Estate, Derek Parnaby Cyclones International Ltd Chilton, near Ferryhill, Chilton, near Ferryhill, Chilton Industrial Estate, County Durham County Durham Chilton, near Ferryhill, DL17 0SH DL17 0SH County Durham United Kingdom United Kingdom DL17 0SH United Kingdom Telephone: +44 (0)1388 720849 Fax: +44 (0)1388 721415 Telephone: +44 (0)1388 720849 Fax: +44 (0)1388 721415 email: enquiries@parnaby.co.uk www.parnaby.co.uk email: enquiries@parnaby.co.uk www.parnaby.co.uk Telephone: +44 (0)1388 720849 Fax: +44 (0)1388 721415 email: enquiries@parnaby.co.uk www.parnaby.co.uk