Piece of cake

The coal industry is one of the most important sectors in the world being a sector that offers one of the highest employment levels. Though there has been a reduction in its production in some countries, such as the US, its use is still fundamental on a global scale. It is therefore of great importance that solutions are identified and put into action to combat its negative impact on the environment when it is mined and used as a source of energy.

Waste water treatment

One of the main issues in the mining industry is related to the use of water for coal mining, so waste water treatment is necessary to lessen its negative impact on the environment.

Clean water discharged from Matec filter press.

Waste water treatment in the coal industry involves the cleaning of water so that it can be reused in the production cycle, concentrating fine particles and dewatering them to reduce byproducts to be disposed of.

In the past, the use of environmentally-friendly waste water treatment systems was rarely taken into consideration, nowadays more strict regulations, high costs to build further settling ponds and a greater social responsibility perceived by companies has driven a great development in the green technologies.

Italian company Matec, which specialises in the design and development of waste water purification and filtration plants, offers a solution that involves the use of a filter press and thickener at the coal-fired power plant.

Matec water treatment plant in West Virginia.

The filter press and thickener solution is simple and effective, provided the sizing of the plant and the materials used for it are designed perfectly for each application. This can be achieved only through experience and customised designing.

The operation of a filter press and thickener plant can be briefly described as follows:

1. Process waste water is collected into a dedicated pit or tank, in which there is submersible pump.
2. The submersible pump sends the waste water to the thickener, vertical or horizontal, for the clarification of the water and the thickening of the mud.
3. Before entering the thickener, the waste water is mixed with a flocculant solution, to speed up the sedimentation process.
4. The waste water arrives into the thickener in the centre of it, where a metal cylinder for the collection of water is placed (the design makes the thickening more efficient, creating a column of water pressing on the solids at the bottom of the thickener).
5. Clarified water overflows from the top of the thickener and it is collected in a dedicated clear water tank.
6. Thickened mud is discharged in a buffer homogenising tank, which keeps the mud at the right density.
7. When the mud level in the homogenising tank is enough and the filter press is ready to receive it (when it is empty and closed) a high-pressure pump sends the mud in the filter press.
8. Inside the filter press, the mud is dehydrated through pressure; the actual filtration process occurs in the filter pack, consisting of a customised set of plates. The cloths and under-cloths cover the filtrating surface of the plate, which has small projecting cylinders forming drainage channels. Once the filter pack is completely sealed, the filtration process begins from the head plate with the feed pump injecting the sludge into the filter plates chambers. The cake will form uniformly all across the filtrating surface and is ready for discharging once the required moisture level has been reached.
9. The water is collected by a drainage system (with an open design to evaluate the quality of water and spot possible failures).
10. A probe detects the water from the mud dehydration; when it detects no more water, the pump stops after a safety delay, and the filter press can open the plates to discharge the mud cakes (average moisture is 15 - 20%).

The use of high pressure (up to 21 bar with the help of the Matec filter press) is essential to meet the high productivity standards of multinational coal production companies that cannot be satisfied with normal results.

Iron-ore mining application

The optimisation of the coal processing plant is achieved by eliminating settling ponds used for tailing. Settling ponds are an out-of-date solution that are expensive to maintain. Indeed, emptying them when they have reached their maximum capacity means stopping the processing plant, because there is no place where to stock waste material. On the other hand, the low humidity cake output of the filter press is easy and economical to dispose of.

Case Study: West Virginia

One of Matec’s latest installations in the coal sector is at a plant in West Virginia for clean coal.

Problem

Prior to Matec’s intervention, the large coal extraction and processing site in West Virginia had experienced high maintenance and running costs. For a site as big as this, those costs were very high as it used tailing ponds that needed to be emptied frequently, stopping the plant for long periods.

Furthermore, they had low productivity rates.

Solution

After running tests on samples and onsite, Matec decided to install two TERRAE 1500 x 2000 filter presses: one dedicated to tailings and one to clean coal, that is to say for the production of coal concentrate cakes.

Data

For the tailings, the filter press has 200 plates, whereas for the concentrate 171 plates.

Tailings (Coal)

• Water to be filtered: 15 000 gal./ min
• Solid: 95 tph

Concentrate (Iron-ore)

• Water to be filtered: 9000 gal./min
• Solid: 115 tph
• HPT and TT2 fast opening system has allowed Matec to reach 3-4 cycles per hour.

Result

Today, thanks to the help of Matec, the plant does not need to stop for long periods for maintenance and has an increased production rate. The main difficulties faced and resolved in this project included low productivity, the difficulties associated with using two materials: coal/iron-ore, the perception that the waste water could not be treated.

Read the article online at: https://www.worldcoal.com/special-reports/06032018/piece-of-cake/