Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2001-05-29
2004-11-23
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S710000, C210S730000, C210S731000, C210S734000, C210S735000, C210S738000, C210S747300, C423S121000, C423S122000, C405S128750
Reexamination Certificate
active
06821440
ABSTRACT:
The present invention relates to the treatment of mineral material, especially waste from the Bayer alumina process, including the treatment of red mud wastes in order to render it more easily disposable.
Processes of treating mineral ores in order to extract mineral values will normally result in waste material. Often the waste material consists of an aqueous slurry or sludge comprising particulate mineral material, for instance clays, sand, grit, metal oxides etc. It is normally necessary to dewater the waste slurries or sludges before disposal in a suitable location.
In some cases the waste material such as mine tailings can be conveniently disposed of in a mine to form a backfill. Generally backfill waste comprises a high proportion of coarse large sized particles together with other smaller sized particles and is pumped into the mine a slurry where it is allowed to dewater leaving the sedimented solids in place. It is common practice to use flocculants to assist this flocculation process.
For other applications it may not be possible to dispose of the waste in a pit or a mine. In these instances the waste material may be transferred to lagoons, heaps or stacks. There is a great deal of environmental pressure to minimise the allocation of new land for disposal purposes and to more effectively use the existing waste areas. One way is to load multiple layers of waste onto an area to thus form higher stacks of waste. However, this presents a difficulty of ensuring that the waste material can flow over the surface of previously rigidified waste within acceptable boundaries and allowed to rigidify to form a stack and that the waste is sufficiently consolidated to support multiple layers of rigidified material, without the risk of collapse or slip. Thus the requirements for providing a waste material with the right sort of characteristics for stacking is altogether different from those required for other forms of disposal, such as back-filling.
In the Bayer process for recovery of alumina from bauxite, the bauxite is digested in an aqueous alkaline liquor to form sodium aluminate which is separated from the insoluble residue. This residue consists mainly of particles of ferric oxide and is known as red mud.
The red mud is washed in a plurality of sequential wash stages, in which the red mud is contacted with a wash liquor and is then flocculated by addition of a flocculating agent. The supernatent liquor is further processed to recover aluminate. After the final wash stage the red mud slurry is thickened as much as possible and then disposed of. Thickening in the context of this specification means that the solids content of the red mud is increased. The final thickener may comprise settlement of flocculated slurry only, or sometimes, includes a filtration step. Alternatively or additionally the mud may be subjected to prolonged settlement in a lagoon.
The mud can be disposed of and/or subjected to further drying for subsequent disposal on a mud stacking area. To be suitable for mud stacking the mud should have a high solids content and, when stacked, should not flow but should be relatively rigid in order that the stacking angle should be as high as possible so that the stack takes up as little area as possible for a given volume. The requirement for high solids content conflicts with the requirement for the material to remain pumpable as a fluid, so that even though it may be possible to produce a mud having the desired high solids content for stacking, this may render the mud unpumpable.
EP-A-388108 describes adding a water-absorbent, water-insoluble polymer an aqueous liquid to a material comprising an aqueous liquid with dispersed particulate solids, such as red mud, prior to pumping and then pumping the material, allowing the material to stand and then allowing it to rigidify and become a stackable solid. The polymer absorbs the aqueous liquid of the slurry which aids the binding of the particulate solids and thus solidification of the material. However this process has the disadvantage that it requires high doses of absorbent polymer in order to achieve adequate solidification. In order to achieve a sufficiently rigidified material it is often necessary to use doses as high as 10 to 20 kilograms per tonne of mud. Although the use of water swellable absorbent polymer to rigidify the material may appear to give an apparent increase in solids, the aqueous liquid is in fact held within the absorbent polymer. This presents the disadvantage that as the aqueous liquid has not actually been removed from the rigidified material and under certain conditions the aqueous liquid could be, desorbed subsequently and this could risk re-fluidisation of the waste material, with the inevitable risk of destabilising the stack.
Thus there exists a need to provide a process that more effectively and conveniently allows a material to be pumped readily as a fluid and which on standing will rigidify to provide a stackable solid waste. There also exists a need to achieve this by substantially reducing the amount of aqueous liquid contained in the material.
A process according to the present invention provides a process in which material comprising an aqueous liquid with dispersed particulate solids is pumped as a fluid then allowed to stand and rigidify and the rigidification is improved whilst retaining the pumpability of the material by combining polymeric particles with the material during or prior to pumping the material,
wherein the polymeric particles comprise water soluble polymer which has an intrinsic viscosity of at least 3 d/g.
The addition of the polymeric particles, comprising water soluble polymer to the material enables the material to retain its fluidity and to be easily pumped but upon standing forms a solid mass that is strong enough to support layers of subsequent rigidified material. We have surprisingly found that the presence of water soluble polymers applied in the form of particles actually enables the material to remain fluid and pumpable during the pumping stage but results in rapid loss of fluidity and rigidification on standing. Furthermore this treatment desirably results in aqueous liquid being released from the material on standing. It appears that application of the particulate water soluble polymer to the material results in a gradual increase in viscosity but not so significantly that would prevent the material from being pumped. It is also surprising that the addition of the water soluble polymer does not result in more immediate dewatering of the material prior to the location for disposal, which could result in blockage of the pipe-line.
Suitable doses of polymer range from 10 g to 10,000 grams per tonne of material solids. Generally the appropriate dose can vary according to the particular material and material solids content. Preferred doses are in the range 100 to 3,000 grams per tonne.
The process of the invention is suitable for treating material wherein the dispersed particulate solids have very small particle sizes, for instance, substantially all having sizes less than 100 microns, and even for materials wherein substantially all of the particles have sizes of less than 50 microns. It is of particular value where at least 90% of the particles have sizes less than 20 microns, especially where the proportion of liquor in the waste stream cannot be easily or economically reduced.
The material particles are usually inorganic and/or usually a mineral. Although it may be useful for other materials requiring to be pumped and then stacked especially for materials which are filter cakes, tailings, thickener underflows, or unthickened plant waste streams, for instance other mineral tailings or slimes, including phosphate, diamond, gold slimes, tails from copper/silver/uranium ore processing, coal or iron ore, the major use of the present process is in the treatment of the final thickener or wash stage of a Bayer process. The red mud may thus be the solids settled from the final thickener or wash stage by the addition of flocculant alone, or, optionally, the mate
Adkins Stephen
Dymond Brian
Gallagher Michael Gerard
Hooley Anne Frances
Smith Hal G.
Ciba Specialty Chemicals Watertreatments Limited
Hruskoci Peter A.
Stevenson Tyler A.
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