Alternative tailings disposal technologies

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Workshop on Environmental managenent in the foldfilds.

Kalgoorlie, May 19-20, 1987.

ALTERNATIVE TAILINGS DISPOSAL TECHNOLOGLSS

by Dr Allan Thomas Slurry Systens Pty Linited, Perth.

1. INTRODUCTION

The conventional nethod of disposing of gold tailings generally involves sinply punping then to a tailings dan at the density at which they exit the process plant. This density is often quite low. Upon discharge around the perimeter of the dan the coarser, grannular particles rapilly segregate fron the flow, forning a beach. The finer particles flow to the centre of the dan where they slowly consolidate. Clear water is generally decanted from the top of the dan and pumped back to the plant. Because of the large surface area a large proportion of the water is lost due to evaporation. The centre portion of the dan containing the finest particles is continually under water. The settled solids can therefore not lose water by surface evaporation. The settled solids concentration can only increase by drainage of moisture into the subsoil and by consolidation. The degree of consolidation largely depends on the height of the dan. When the mine eventually ceases production the tailings deposit consists of a retangular shaped mass with coarser, grannular particles around the perineter edges and fine, near colloidal sized particles in the centre. Evaporation can now begin from the top surface of the centre portion of the dan. This rapidly results in a crust forming which then hinders further evaporation. Conplete drying may take ten or twenty years. During this period rehabilitation is difficult since machinery cannot operate on the surface. An alternative disposal method is the high density slope disposal systen. This involves thickening the tailings to a higher density than nornal. Toon disposal they form a homogeneous, sloping deposit. This paper discusses various aspects of this approach including economic and environmental advantages.

2. STATE OF THE ARE OF HIGH DENSITY DISPOSAL

- The concept and developnent of tailings disposal by the thickened discharge nethod was first developed and tested by E.I. Robinsky, Canada (Robinsky 1978). It has since been applied in numerous industries, e.g. Robirsky (1981), Wood and MeDonald (1984) and Yoenans (1986). Robinsky investigated the relationship between percent solids of a tailing pumped and the resulting disposal behavior. Conventional tailing slurry densities resulted in segregation of solids on disposal and the resulting low slopes required high containment embankments. Increasing the slurry density to achieve a threshold slope of 2 percent and above resulted in norsegregation, with significant improvement in the environmental quality of the disposal of the tailing.

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Robinsky investigated the relationship between percent solids of a tailing pumped and the resulting disposal behavior. Conventional tailing slurry densities resulted in segregation of solids on disposal and the resulting low slopes required high containment embankments. Increasing the slurry density to achieve a threshold slope of 2 percent and above resulted in nonsegregation, with significant improvenent in the environmental quality of the disposal of the tailing. Robinsky s pioneering work provided the impetus for development of processing and pumping alternatives to econonically achieve the desired high density tailing slurries and to establish correlations and understanding to predict slope performance from pilot plant test samples.

Slurry Systems in association with Robinsky conducted extensive pilot plant tests and established technical feasibility of high density disposal by combing coarse and fine disposal fly ash in the power generation industry and disposal of coarse and fine refuse in the coal mining industry. This work allowed correlation to predict slope of deposits from testing of small (? litres) representative samples of tailings.

Laboratory testing of samples provides a basis for establishing both technical feasibility and design of the disposal slope as well as the punping and pipeline requirements for any system.

3. HIGH DENSITY DISPOSAL

The high density disposal schene involves thickening of the tailings prior to disposal. The thickener overflow water is returned immediately to the plant. At the disposal site the thickened tailings do not segregate but form a

discharge is from a single spigot the tailings will forn a conical shaped Deposit slopes depend on the rheology of the slurry and the discharge rate from the spigot. Typically slopes will be from? to 5%. The hill formed by the deposit unifornly spreads any runoff caused by rain falling on the deposit. The runoff does not channel to forn gullys since the slope of the deposits is less than 10 percent. Rain falling on freshly deposited high density tailings will runoff with little entrainment of solids. The slurry properties preclude ready mixing of the solids and the rain water. 4 small 1 to 2 metre perimeter embanknent on the downstream side of the tailings deposit is installed to contain only low density off-specification slurry and to store rainfall runoff. The perineter embankment volume is snall and can be constructed at minimal cost. The high density disposal concept relies on there being sufficient area such that successive layers partially dry out before the next layer is deposited over the top. Discharge fron a single spigot or number of spigots in the centre of the tailings area will achieve this automatically. This occurs since the flow tends to flow in one sector until the height increases sufficient to divert the flow to another sector and so on.

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The depth of fresh tailings deposited before the flow diverts to another sector is typically around 100mm. Once the flow ceases, evaporation begins. Capillary rise in this relatively thin layer aids the drying process and enables uniform moisture content to be achieved. The slurry density and spigot flow rates are designed so that sufficient time occurs in winter months for the deposited layer to achieve a reasonable shear strength before fresh slurry is deposited over that layer. Further noisture reduction takes place by consolidation. Shortly after nining ceases the deposit is sufficiently dry to enable machinery to operate on the surface. The evaporation rates during the winter months dictate the areas needed. Based on pilot plant data the areas required in the Kalgoorlie region will be sinilar to those currently occupied by conventional tailings dans. At the cessation of mining a conical shaped deposit will cover an area sinilar to the conventional deposit. Consider for example the particular case of 5. 4Mt. of tailings. Assume the residual density in the dan when mining ceases is 55% and the ratio of dam wall height to width is 0.3. For wall heights ranging fron 12 to 18 metres the area covered ranges from 50 hectares to 40 hectares. With high density disposal the residual density is higher, around 35%. The area occupied by a cone shaped deposit will depend on the average slope. For the sane 5.4 Mt the following areas and cone heights will apply: Slope (%) 6 3 Area (ha) 40.4 52.8 63.9

Height (m) 21.: 16.4 13.5

The slopes attainable will depend on the particular tailings, the degree of

thickening and the spigotting method. They are similar to the areas involved with conventional disposal.

At an existing mine high density disposal can be used to extend the life of the conventional tailings dan. Overlaying the existing tailings will consolidate and renove water from them. In some cases the high density tailings can ba allowed to spill over the dam walls. By this action the existing tailings dan is engulfed and converted to a cone shaped deposit. This may suit cases where an existing mine is undergoing major expansion.

The sterilisation of valuable ninerals by placement of tailings may be a long tern consideration, in the same manner as retreatment of tailings is a long tern option as technologies change. The high density tailings disposal system allows for renoval or reclain for retreatment of the tailings using conventional low cost mining equipment. This is in contrast to the conventional tailings dan, which cannot be renoved for many years or requires a dredging operation.

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Further, the small quantities around the shallow edges of a slope be readily removed. For example, by removing just 5.5% of the total material fron around the perineter the area can be reduced by one third.

4. IMPLEMENTATION

To implement the disposal schemel a method of thickening to high densities is required. Table I illustrates the capabilites of conventional equipment.

TABLE I

Typical Dewatering Processes for Tailings

Process or Equipment Typical % Solid Range

Direct from process 30 - 40 Conventional thickener 40 - 50 THICKENING High rate thickener 42-53

53

Range for slope disposal 45 - 55

75

Vacuum filters 75 - 85 Pressure filters 80 - 90 MECHANICAL DENATERING

The most common process for water recovery from a slurry is by static settling under hindered settling conditions in a thickener.

At the other extreme the slurry can be dewatered using mechanical equipment such as filters and centrifuges. These produce solid cakes with moisture contents in the range of 10 - 25 percent. These cakes are handled by conveyer for spreading and depositing using mechanical equipnent. The ideal density requirenents for slope disposal are seen to lie intermediate between those typically achieved in thickeners and filters. The conventional and high rate thickeners can produce a slurry suitable for slope disposal at the lower range of slopes. i.e. 2 to 3 %.To obtain higher slopes than these special equipment is necessary. Some options are as follows: Increase in Thickening area A larger thickener than normal will allow higher densities to be achieved.

With conventional thickeners this necessarily means a proportional increase

ir cost. Most of this cost is in the vessel itself and the rake nechanism. far cheaper approach is to build a small dam or utilize an existing tailings dan as the settling area. An unmanned floating barge can then be used to recover the thickened slurry for high density slope disposal. SSPL has designed such systems for the diamond and coal industries. Increase Thickener Deoth An increase in the depth of a thickener causes greater compression and hence higher underflow densities. This approach is the basis of the Deep Cone thickeners developed in the UK coal industry and the Jamaican aluninium industry (Abbot et al, 1973: Akers, 1975)

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Special Filter Thickeners B.I. Robinsky is developing a special filter thickener in Canada purposely designed to achieve the internediate range of densities required. A prototype is currently being tested. Combining Dry Material with Tailings This approach is being inplemented for tailings disposal in the coal and mineral sands industries. The nineral sands industry mixes dry sand with thickener underflow slimes to achieve a high density slurry. In the coal industry it is economically attractive to purposely crush the coarse rejects for nixing with fine tailings thickener underflow to achieve the high density combined tailings. In some cases in the gold industry it may be feasible to nix overburden material with the tailings to achieve a high density slurry. Substrean Filtering This option involves splitting the tailings strean and using conventional filters or centrifuges to produce a low moisture cake. This is then recombined with the low density stream to produce the desired high density slurry. Chemical Treatment Additior of line or caustic to adjust pH conmonly results in a thinning of the slurry. Ine converse is also true - if the pl of the tailings is lowered by acid addition the slurry will thicken. pH is not the only factor which influences the slurry viscosity. The Boddington mine will rely on special chenical thinning agents. Similarly chemical thickening agents are available. By this approach a sinple chemical dosing plant could achieve the required density without a thickener being required.

5. SUMMARISING THE ADVANTAGES OF HIGH DENSITY DISPOSAL

Some environmental advantages of this method of disposal are:

Blinination of large tailings dams - visually oleasing cone shaped hills instead. b. 4 few months after mining ceases the deposit is conpletely dry and rehabilitation can begin. C. Cyanide dispersal depends on contact with the atmosphere. The cone shaped deposit together with the fact that no area is submerged mean naxinun contact with the atmosphere and therefore naxinun rate of cyanide dispersal. d. The continuous slope on the deposit means that sub surface drainage after rain will leach salts fron the top layer leaving an ideal surface for revegetation. The salt will appear around the perimeter but this area could be covered by scraping a thin, salt free layer from the deposit.

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Steep tailings dan walls, with their susceptibility to erosion and failure, are avoided. from an economic viewpoint the advantages are: No tailings dans walls required. b. No water recovery system, pumps and pipeline required. Consequently no power is required at the disposal site. Less pipe blockages due to the inherantly non-settling pulp. This reduces labour requirements for tailings disposal nanagenent. d. The ground beneath the tailings area is not permanently sterilized since the tailings can be moved if necessary. With renoval of only a small proportion of the material around the perimeter the area required can be less than that required for a conventional dam.

5. CONCLUSIONS

The high density slope disposal method of tailings disposal is an attractive alternative to the conventional tailings dam. It has obvious environmental advantages. There can also be significant econonio advantages. This concept has been successfully applied in many other industries including aluminiun, coal, copper, mineral sands, power generation and uranium industries. In Western Australia it is being used by Alcoa, Argyle Diamond Mines and Associated Minerals Consolidated. The technology exists now to inplement this scheme in the gold industry. Design methods developed in other industries can be utilized. Conventional thickening methods can be used to achieve the required high density slurries. There are also a number of other nethods available which have been utilized in other industries. These should be considered in any evaluation of the technology for gold application.

REFERENCES

Abbot, J, Dell, c.c., Denison, B, Knott, D and Hill, N.W., Sth International Coal Preparaion Congress, Paris, 1973. Akers, R.J., Chenical Engineer, n301, p511, 1975. Robinsky, 5.I. Tailing disposal by the thickened discharge nethod for inproved economy and environmental control, proc. 2nd Int. Tailings Symposiun, Vol?, Denver, May 1973. Robinsky, E.I. Uraniun tailing disposal by the thickened tailings discharge Fort Collins, October 1981. systen, Uranium Mill Tailings Management Workshop, Colorado State University,

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wood, K.R. and MaDonald, G.W. Design and operation of thickened tailing disposal systen at Les Selbaie, CIMM 36 the Annual General Meeting, Ottowa, April 1984. Yoemans, J.J., 1986, Tailings disposal at Kidd Creek Mines Ltd., Proc. Canadian Mineral Processors Annual Meeting, January, 1986.