Abstract:
A method of treating water to remove fluoride ions from the water, thereby to reduce the dissolved fluoride content of the water, is provided. The method includes, in a contacting step, contacting water containing dissolved fluoride ions with alumina, to cause the fluoride ions to react with and become bound to the alumina. In a regenerating step, the alumina, when spent, is regenerated to drive off fluoride ions bound thereto. The regenerated alumina is recycled to the contacting step where it is used to remove further fluoride ions from the water. The fluoride ions driven off the alumina in the regenerating step are passed on to downstream processing thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the claims priority under the Paris Convention for the Protection of Industrial Property and through the World Trade Organization to Patent Application No. ZA 2006/08539, filed Oct. 12, 2006 in the Companies and Intellectual Properties Registration Office of South Africa. 
       BACKGROUND OF THE INVENTION 
       [0002]    THIS INVENTION relates to water treatment. More particularly, it relates to a method suitable for, but not restricted to, the removal of fluoride ions in water arising from the production of aluminium from bauxite by a process wherein bauxite is reacted with hydrofluoric acid to produce aluminium fluoride, followed by the reduction of the aluminium fluoride to obtain aluminium metal. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    According to the invention, there is provided a method of treating water to remove fluoride ions from the water, thereby to reduce the dissolved fluoride content of the water, the method including: 
         [0004]    in a contacting step, contacting water containing dissolved fluoride ions with alumina, to cause the fluoride ions to react with and become bound to the alumina; 
         [0005]    in a regenerating step, regenerating the alumina, when spent, to drive off fluoride ions bound thereto; 
         [0006]    recycling the regenerated alumina to the contacting step where it is used to remove further fluoride ions from the water; and 
         [0007]    passing on the fluoride ions driven off the alumina in the regenerating step, to downstream processing thereof. 
     
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0008]      FIG. 1  is a schematic showing a flow diagram of an installation for carrying out the method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    As indicated above, the method is suitable for treating raw fluoride ion-containing water obtained during the production of aluminium from bauxite by reacting the bauxite with hydrofluoric acid. Accordingly, the alumina (Al 2 O 3 ) with which the raw water is contacted is conveniently in the form of bauxite. The contacting of the raw water with bauxite may be effected by passing the raw water through a bed, which may be a fixed bed, a completely mixed bed or, conveniently, a fluidized bed, of bauxite, which is thus in particulate form, with the fluoride ions reacting with the bauxite to produce aluminium fluoride, in accordance with Reaction (1): 
         [0000]      Al 2 O 3 +3H 2 O+6F − →2AlF 3 +6OH −   (1) 
         [0000]    According to Reaction (1), aluminium fluoride is produced and the water is rendered alkaline or at least provided with a raised pH. However, Reaction (1) preferably takes place with the water at a pH in the range 6.5-7.5, and the method thus contemplates adding a suitable acid, such as sulphuric acid, hydrochloric acid and/or carbonic acid (carbon dioxide) to the contacting step to keep the pH of the water in the range 6.5-7.5. 
         [0010]    The regenerating of the spent bauxite may be effected by heating it, the heating acting both to dry the bauxite and to drive off fluoride ions as HF gas/vapour, in accordance with the Reaction (2): 
         [0000]      2AlF 3 +3H 2 O→Al 2 O 3 +6HF   (2) 
         [0011]    It follows that, in accordance with the invention, the fluoride ions driven off in the regenerating step may thus be passed on in the form of hydrogen fluoride to downstream processing. 
         [0012]    In the regenerating step, the heating of the spent bauxite, which may be by means of hot gas, electrical heating or infra-red heating, may be supplemented and accompanied by water removal by filtration, for example vacuum filtration by means of a belt filter. 
         [0013]    The downstream processing of the hydrogen fluoride driven off during the regenerating step is, when the method is associated with aluminium production from bauxite as mentioned above, conveniently by using the hydrogen fluoride to supplement the hydrogen fluoride used for the aluminium production by reacting the bauxite with hydrogen fluoride. Instead, however, the downstream processing may include or involve reacting the hydrogen fluoride with sodium hydroxide, for example in a scrubber, followed by addition of calcium hydroxide to the resultant sodium fluoride solution, to precipitate the fluoride as a calcium fluoride product or by-product. This will take place by way of Reactions (3) and (4): 
         [0000]      HF+NaOH→NaF+H 2 O   (3) 
         [0000]      2NaF+Ca(OH) 2 →CaF 2 +2NaOH   (4) 
         [0014]    The overall reaction can be expressed by Reaction (5): 
         [0000]      2HF+Ca(OH) 2 →CaF 2 +2H 2 O   (5) 
         [0000]    and indeed the same result can be achieved by scrubbing the hydrogen fluoride-containing gas or off-gas from the regenerating step with a calcium hydroxide solution in the form of a milk of lime solution. 
         [0015]    It is expected that the raw water will typically originate as surface water in the vicinity of an aluminium smelter which produces aluminium fluoride. Such surface water can contain suspended solids, and the method thus contemplates the preliminary step of filtering the raw water to remove suspended solids therefrom, prior to the contacting of the raw water with the bauxite. This filtration will typically be by means of one or more sand filters. Any bauxite which is carried over from the contacting step can similarly be filtered out, again typically by means of one or more sand filters; and back-wash from said sand filters can have any bauxite contained therein recycled to the process, for example by being fed into the bauxite regenerating step together with the spent bauxite from the contacting step. 
         [0016]    The raw water, which typically has a fluoride ion content in the range 1-50 mg/l, may furthermore, if desired, be subjected to flocculation, for example by means of aluminium sulphate as flocculant, to flocculate the suspended solids prior to removal of the suspended solids by filtration. 
         [0017]    It is expected that bauxite make-up will be intermittently fed to the contacting step, as required, to compensate for any losses of bauxite in the treated water. 
         [0018]    The invention will now be described, by way of example, with reference to the accompanying drawing, in which the single figure schematically shows a flow diagram of an installation for carrying out the method of the present invention. 
         [0019]    In the  FIG. 1 , the installation is generally designated by reference numeral  10 . A raw water feed line  12  leading from the environment is pumped by a feed pump and a back-up or booster pump (both not shown) upwardly into the bottom of a fluidized-bed reactor  14  from which a treated water discharge line  16  issues and leads to a sand filter  18 . A product water flow line  20  leads from the sand filter  18  back into the environment from which the raw water is pumped along line  12 . 
         [0020]    In the reactor  14 , a fluidized bed of particulate bauxite is shown at  22 , above a flow distributor  24 . A bauxite feed line  26  is shown feeding into the reactor  14  at the top of the bed  22 , and a bauxite withdrawal line  28  is shown leaving the reactor  14  at the bottom of the bed  22 , above the distributor  24 . The line  28  leads to a dryer  30  and is joined by a sludge backwash line  32  from the filter  18 . Flow line  26  leads from the dryer  30  to the reactor  14 . 
         [0021]    A hydrogen chloride gas/water vapour flow line  34  leads from the dryer  30  to a lime contact reactor  36 , provided with an air outlet line  38 , with a calcium hydroxide feed line  40 , and with a calcium fluoride outlet line  42 . 
         [0022]    In terms of the method, fluoride-rich raw surface water is pumped from the environment (ponds, dams, streams, rivers, lakes or the like in the vicinity of an aluminium smelter of the type mentioned above producing aluminium from bauxite) upwardly into the reactor  14  at a rate selected to fluidize the bed  22 , but to limit any carry-over of bauxite, along line  16 , to acceptably low levels. The booster or back-up pump is used at start-up as the bauxite tends to settle as a solid cake when fluidizing is discontinued. A high pressure water lance or the like fluidizing aid can also be used to assist start-up. 
         [0023]    In the reactor  14 , the bauxite in the bed  22  reacts with fluoride ions in the raw water according to Reaction (1): 
         [0000]      Al 2 O 3  (bauxite)+3H 2 O+6F − →2AlF 3 +6OH −   (1) 
         [0000]    If necessary, depending on the pH on the raw water, a suitable acid such as carbonic acid is fed (not shown) to the reactor  14  to keep the water pH in the bed  22  in the range 6.5-7.5. Fresh or regenerated bauxite is fed to the bed  22  along line  26  and spent bauxite is withdrawn therefrom along line  28 , continuously or intermittently, at a matched rate to ensure that there is no unwanted carry-over of bauxite into line  16 , and to ensure that the depth of the bed  22  remains substantially constant, the rate being selected to ensure that the water in line  16  has an acceptably low fluoride content. 
         [0024]    Water is allowed to drain from the bauxite withdrawn in line  28 , to obtain wet spent bauxite with a water content of about 55% by mass, which is fed into the dryer  30  where it is dried by a combination of warm air and electrical (radiant) heating. In the dryer, the bauxite is heated to a temperature in the range 100-250° C., selected to ensure that fluoride is driven off the AlF 3  in the spent bauxite as HF, while the bauxite is simultaneously reactivated to Al 2 O 3 , according to Reaction (2): 
         [0000]      2AlF 3 +3H 2 O→Al 2 O 3 +6HF   (2) 
         [0025]    The reactivated bauxite is fed along line  26  from the dryer  30  to the bed  22  in the reactor  14 . 
         [0026]    The HF gas released during the bauxite drying and reactivation in the dryer  30  passes, together with air and water vapour, along line  34  to reactor  36  which is fed with milk of lime along line  40 . The HF feed to the reactor  36  passes up reactor  36 , countercurrent to the downward feed of milk of lime from line  40  through the reactor  36 . In the reactor  36  the HF from line  34  reacts with the milk of lime from line  40  according to the Reaction (5): 
         [0000]      2HF+Ca(OH) 2 →CaF 2 +2H 2 O   (5) 
         [0027]    The CaF 2  produced issues from reactor  36  along line  42  and air and water vapour issue from the reactor  36  along line  38  to the atmosphere. 
         [0028]    In the sand filter  18 , the treated water from flow line  16  is filtered to remove bauxite particles therefrom, and the filtered water is released from the filter  18  into the environment along the flow line  20 . Filter  18  is periodically back-washed to remove accumulated bauxite fines therefrom, the removed bauxite in the back-wash being fed along line  32  from the filter  18  into line  28 , for drying and regeneration in the dryer  30  and subsequent recycling to the bed  22  in the reactor  14 . 
         [0029]    It is a feature of the invention as illustrated with reference to the drawing that it provides an effective and easily-applied method of reducing the fluoride content of surface waters in the environment, while producing calcium fluoride as a valuable by-product, useful, for example, for making toothpaste. 
         [0030]    Alternatively, the hydrogen fluoride gas from the dryer  30  can be used to treat bauxite to render it fluoride-rich, for use as a bauxite raw material or feed supplement in the smelting of bauxite to produce aluminium according to the process mentioned above. 
         [0031]    In a further variation of the illustrated method, back-wash from the filter  18  can be centrifuged prior to feeding the bauxite from the back-wash to the dryer  30 , water from the centrifuge (not shown) being fed to the reactor  14 , for example along line  12 . 
         [0032]    In a still further variation of the method, carried out on a batch-wise basis rather than the continuous basis described with reference to the drawing, a batch of fresh or regenerated bauxite is used in the reactor  14  to form the bed  22 , and raw water is fed through the bed  22  until an unacceptable break-through of fluoride is noted in line  16  and the bauxite is spent. The spent bauxite is then dumped from the reactor  14  and replaced by a further batch of fresh or regenerated bauxite. The spent bauxite is dumped into a completely-mixed reactor where it is kept in suspension, from which it is pumped to a filter and dryer, before it is passed on for downstream use, for example as smelter feed or for recycling in a regenerated state to the reactor  14 .