GB2478781A

GB2478781A - Siphoning and neutralisation of contaminated mine water

Info

Publication number: GB2478781A
Application number: GB201004577A
Authority: GB
Inventors: Justin Robert Daglish
Original assignee: PROCESS ENVIRONMENT Ltd
Current assignee: PROCESS ENVIRONMENT Ltd
Priority date: 2010-03-19
Filing date: 2010-03-19
Publication date: 2011-09-21
Anticipated expiration: 2030-03-19
Also published as: GB201004577D0; GB2478781B

Abstract

A process for neutralising contaminated mine water wherein the abstraction of the mine water and the return of the treated water to a water course is by means of a siphon. The process utilises a sealed siphon system, whereby borehole pumps 1 are used to prime the system. The process may comprise the addition of lime to the contaminated mineshaft water in a sealed reaction tank 3 and excess air can be vented from the tank by a mechanical air valve. The limed water can then be aerated 5, and a coagulant may then be added to the water by inline dosing 6. Aerated and coagulated water can then be passed to a static mixer 7 and then preferably to a bank of hydrocyclones 8 which separate solids from the water. Advantageously, a heat exchange device 9 may be located after the hydrocyclones to remove heat from the mine water. A system for neutralising contaminated mine water is also claimed.

Description

Improved Mine Water Process

Description

Acidic mine water is produced in large volumes by the majority of closed mineral mines. The rate of production is a function of rainfall, local geology and the extent of the mine. In the UK a proportion of mines were closed in the early 1980's and mine water has been filling the mines since the dewatering pumps used for operations were turned off. If measures are not taken to control the mine water level, water can eventually spill out of the mine polluting local water reserves as metals contained within the mine water are carried into the receiving watercourse. To alleviate this problem, known techniques, described further below, are employed to treat the mine water removing metals and other contaminants The highest single financial operating cost for existing lime based Mine water treatment is the power consumed pumping the high volume of water from the mine to the treatment plant inlet located above the mine. This power consumption also has an associated and significant carbon footprint or cost.

The solution to this problem is detailed in the description of improved process provided below. The system utilises a sealed neutralisation process which significantly reduces the energy requirement in pumping to the plant in exchange for a small increase in energy consumption costs through the neutralisation treatment plant. The sealed neutralisation process employs a siphon; that is a continuous tube that allows liquids to drain from a reservoir through an intermediate point that is higher or lower than the reservoir, the flow being driven (at least in part) by the difference in hydrostatic pressure.

Accordingly, it is necessary that the final end of the tube is lower than the liquid surface in the reservoir.

Figure 1 shows the conventional set up employed.

Figure 2 shows the setup of the invention.

The prior art process is typified by the following outline and showed further in figure 1.

Stage 1 Water is pumped from a depth within a mine shaft to the inlet of the treatment plant, which can typically be some height above.

Stage 2 That contaminated water is gravity fed through the plant. Lime is added (sometimes in combination with re-circulated sludge) to raise the pH.

Stage 3 The output of stage 2 is then mixed in a tank for enough time to allow metals to precipitate out in solution. The liquid is then fed under gravity to a clarification tank. A coagulant such as poly aluminium chloride is added at this stage to allow the solids to reach settlement size.

Stage 4 Clarification tanks typically operate under gravity with solids settling to the bottom and the treated water discharged to a local water course from the top of the tank under gravity. Solids are taken from the bottom of the tank in the form of sludge and disposed of.

As explained above the drawback to the existing process is that large amounts of energy are consumed in pumping water from the mine up to the inlet of the treatment plant. This energy has associated financial and carbon costs.

The modified process according to this invention would include the following stages to allow the natural siphon effect to be utilised to minimise power requirements.

Stage 1 Borehole pumps are used to prime the sealed system, negate the effects of frictional losses and provide kinetic energy to separation devices. Labelled as [1] in Figure 2.

Stage 2 Water is fed through the plant predominantly by natural siphon. Mineshaft water is mixed with recirculated sludge before it is mixed in a static helical mixer [2]. The mine water and recirculated sludge mixture is then fed into a sealed reaction tank [3]. Lime is then added to the mixture at the required rate, controlled by pH. Excess air is vented from this tank by a mechanical air valve [4].

Stage 3 Limed Water then passes to two inline tanks from which the limed water is pumped around a small loop over which aeration occurs [5]. At this stage a coagulant may be added by inline dosing [6]. Recycle rate within this loop is determined to allow coagulation of precipitants and aeration of the mixture.

Pressure controls on the inline tanks ensure the main siphon is not broken Stage 4 Aerated and coagulated water passes though a static mixer [7] to ensure well mixed liquid and precipitant mixture enters the bank of hydrocylones [8] where solids are separated from the water. The Hydrocylones are sized to ensure the correct degree of separation at the pressure and volumetric flow. To cope with variation in flow over time, the number of hydrocyclones available will be controlled with flow.

Stage 5 Sludges are removed from the base of the hydrocyclones and then disposed of. Water is discharged via a sealed pipework manifold connecting all hydrocyclones to a low point either via the mine to a point which is at the lowest available clean water receiving point [11]. (The lower the level of the receiving point the greater the effect of the siphon across the plant and the greater the energy and cost saving). A sealed treated water tank [10] may also be present before the discharge point to allow water to be sampled for quality before discharge.

Stage 6 A heat exchange device [9] may be located after the hydrocyclones and before the discharge point or treated water tank to remove heat from the main siphon stream. The heat removed via the heat exchange device would heat a separate closed stream which would be used to transfer the heat to the point at which it was needed. The use of a heat exchange device would provide a low carbon heat source for reuse and also prevent discharge of mine water at temperatures too high for the receiving water course.

Claims

Claims 1. Use of a process for neutralising contaminated mine water wherein part or all of the method for mine water abstraction and return to water course after treatment is by the use of a siphon.
2. Use of a process described in claim 1 wherein use is made of a sealed reaction tank with a mechanical air vent.
3. Use of a process described in claims 1 or 2 wherein use is made of an aeration system and coagulation system which operates on a recycle connected into the main siphon stream. Water is recycled at pressures which do not interrupt the siphon.
4. Use of a process described in claims 1 or 2 or 3 wherein use is made of a bank of hydrocyclones which separate the water from solids at the pressure of the main siphon stream.
5. Use of a process described in any of the preceding claims wherein use is made of a heat exchange device to remove heat from the mine water and transfer it to a separate stream.
6. A system for neutralising contaminated mine water wherein priming borehole pumps are connected to a sealed siphon system.
7. A system according to claim 6 wherein the system includes a sealed reaction tank with mechanical air vent.
8. A system according to claims 6 or 7 wherein the system includes a recycle aeration system which aerates and coagulates limed water independently of the main siphon stream. The recycle aeration system controls the pressure in the main line to prevent interruption to the main siphon.
9. A system according to claims 6 or 7 or 8 wherein the system includes a hydrocyclone bank to separate treated water from precipitated solids after coagulation. The hydrocyclone bank is controlled by an automated control system so that the number available match the flow in the main siphon.
10. A system according to any of claims 6, 7, 8 or 9 wherein the system includes a heat exchange device to remove heat from the mine water and transfer it to a separate stream.