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ELECTRIC AVENUE

TECHNICAL NOTE

Colin Jones and John Lamont-Black explain electrokinetic dewatering of tailings and sludge.

Extremely large volumes of mine waste are produced annually and some mine tailings lagoons are among the largest geotechnical structures in the world.

Many mining operations produce extremely fine tailings that dewater very slowly under self-weight consolidation. This results in a number of problems, including potential instability if the containment system is inadequate, loss of valuable storage space because the insitu densities are very low and difficulties with rehabilitation and closure because of the compressibility and low shear strength of the tailings, even many years after deposition.

The usual method of transport of mine tailings, sometimes over considerable distances, is by pumping (Figure 1).

There are significant economic and technical benefis gained from dewatering tailings before disposal. These include reduced capital expenditure and operational costs of transportation by substituting conveyors for pumps; more economic use of storage space; greater stability of the tailings; and major environmental benefits associated with water recovery. However these advantages are often elusive because the materials can be very difficult to dewater.

Concept Electrokinetic (EK) dewatering (GE October 2001) provides a potentially attractive technique for tailings both as part of the disposal process or insitu after disposal. The method involves the application of a potential difference between electrodes which causes the flow of water to the negatively charged cathode.

There are five recognised EK phenomena relating to the relative movements of solids, liquids and particles under voltage gradients. Electroosmosis is one of these phenomena and describes the movement of liquids through a static solid matrix. Not all materials support electroosmosis.

The hydraulic flow of liquid through a solid is described by Darcy's law:

e = voltage gradient.

While hydraulic permeability is controlled by the grain size of the material, for example gravel compared with clay, the coefficient of electroosmotic permeability is relatively constant among materials that support electroosmosis.

In terms of equivalence, K e approximates to K h for materials of fine sand to silt size and the relative benefi of electroosmosis over hydraulic flow increases as materials become finer and the hydraulic permeability decreases.

Electroosmosis has had some limited use in civil engineering applications, including the stabilisation of slopes, excavations and embankments, increasing pile capacity and increasing the strength of clays.

Widespread use of electro osmotic treatment has been hampered by poor durability of the available electrodes and the subsequent high cost in electrical power. However these problems have been overcome by the development of electrically conductive geosynthetics (EKG) by Electrokinetic Ltd, a Newcastle University spin-out company.

Applications EKG materials act as electrodes while retaining the established functions of geosynthetics such as filtration, drainage and reinforcement.

As geosynthetics they can be produced in a range of forms including sheets, tubes, bags or tapes.

The method of application of electrokinetic dewatering depends on the nature of the material to be treated, Table 1.

Table 1 shows the potential EK methods available using different EKG material forms. The EK belt press is a continuous method of dewatering, electrokinetic prefabricated vertical drains (ePVDs) are used insitu and EK bags and tubes are suitable for batch processes.

The applicability of the different methods depends on the volume and uniformity of the tailings or material to be treated and the rate of supply, Table 2.

Thus the dewatering of diamond mine tailings is particularly suited to EK belt fi lter press treatment as they occur in large continuous volumes and are homogeneous, while the occasional dewatering of sludge in a water treatment plant or the inhomogeneous arisings from tunnelling are candidates for batch treatment using EK tubes or bags.

Dewatering of diamond mine tailings

Electrokinetic Ltd is working with DeBeers Consolidated Mines to determine whether a process based on EK dewatering is suitable for treatment of diamond mine tailings as a precursor to transportation by conveyor.

Within this, additional aims are to determine whether an electrokinetic belt filter press has the potential to achieve the desired level of dewatering and to see whether EK treatment is effective on unthickened mine tailings. The study is being conducted in two phases.

In phase 1 (now complete), EK dewatering laboratory tests were conducted on tailings from the Kimberley, Orapa and Premier Mines in South Africa. The test's objectives were to characterise mine tailings' behaviour with decreasing moisture content as an indicator of the ease of transportation by conveyor, to determine the suitability of the materials for EK dewatering and characterise the dewatering performance of a simulated belt press conguration using standard electrokinetic test cells. Phase 2, which is ongoing, is a full scale EK filter belt trial in Kimberley.

Results of diamond mine tailing tests

The tests showed that all three tailings materials were suitable for electrokinetic dewatering. The following results were based on 15mm thick samples in an electrokinetic test cell (Figure 2):

Kimberley tailings improved from 57.2% dry solids to 73.5% dry solids

Orapa tailings improved from 38.5% dry solids to 73.0% dry solids

Premier tailings improved from 59.7% dry solids to 79.0% dry solids

A percentage dry solid content greater than 65% is required for transportation by conveyor belt.

The coeffiients of the hydraulic and electroosmotic permeability of the tailings are shown in Table 3.

In the tests, K e values were determined by having an 'open' or irrigated anode, which was supplied with water at zero hydraulic head.

Therefore any ow of water noted in an electroosmotic permeability test is caused only by electroosmosis.

The slump test was used as a simple method of identifying the suitability of the tailings for transport by conveyor. An example of the relationship between water content and slump for the Kimberley tailings is shown in Figure 3. The spacing (pitch) of the conductive elements woven into the lter belt were varied and different thicknesses of the tailings were considered to provide a matrix of potential conductive belt arrangements.

Figure 4 shows that, for any given target solids content, there are a number of different combinations of parameters which can combine to produce the desired dewatering.

Part of the on-going field trial is to verify the interrelationship between these parameters and to assess their influence on the critical performance objective of throughput.

Dewatering waterworks sludge

Electrokinetic Ltd is also working with CA Blackwell (Contracts) to determine the potential use of electroosmosis for the dewatering of settling pond sludge in electrokinetic tration containment (bags and/or tubes).

The sludge is a by-product of primary clari cation of river abstraction water (Figure 5). This is an example of the need to dewater a medium quantity of sludge on an irregular basis and is therefore suitable for batch treatment, Table 2.

Conventional treatment involves pumping the sludge, after the addition of water, to a temporary lagoon and allowing it to drain/evaporate before transport to a nal disposal site.

Due to the nature of sludge, dewatering is relatively slow and can be in enced by weather. An additional constraint is the location of the settlement pond in a river ood plain which invokes a strict window of opportunity for the treatment.

The viability of electrokinetic filtration containment treatment rests on its ability to accelerate the rate of dewatering and achieve this in a confined space. This can be determined by hanging bag trials using electrically conductive ltration bags. Figure 6 shows the arrangement of the trial.

Before the trial, tests were done to see if dewatering of the sludge could be accelerated by the addition of a occulent. These proved unsuccessful and the bags were lled with untreated sludge taken directly from the pond.

Three bags were used to permit the application of two voltages (12V and 36V) provided by conventional batteries and a control (0V). The EKG bags acted as the cathode with a steel anode located in the centre of the sludge.

Results of the EK hanging bag tests

The results of the trial are shown in Figure 7. In accordance with basic electroosmotic theory (Equation 1) the 36V trial produced dryer material than the 12V test. The conclusion of the trial is that EK dewatering of the sludge is possible either using an EK tube or insitu with electrically conductive wick drains (ePVDs).

Acknowledgements

The assistance of C A Blackwell (Contracts) in the waterworks trial is gratefully acknowledged.

The results of the diamond mine trials are published with the agreement of DeBeers Consolidated Mines.

Colin Jones is emeritus professor of geotechnical engineering, University of Newcastle and director of Electrokinetic Ltd.

Dr John Lamont-Black is operations director at Electrokinetic Ltd.

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