EVAPORATIVE CONCENTRATION OF CLAY SLURRIES

Abstract: A method for concentrating a clay slurry by subjecting the slurry to indirect heat exchange within a heat exchanger to elevate the temperature of the slurry whilst specifically suppressing boiling of the liquid within the heat exchanger. The heated slurry is subsequently subjected to a rapid pressure reduction cause flash evaporation of part of the liquid. The rapid pressure reduction can occur in a separation vessel or a flash vessel.

Description: TITLE: EVAPORATIVE CONCENTRATION OF CLAY SLURRIES Field of the Invention This invention relates to the evaporative concentration of clay slurries, and more particularly to an improved method and apparatus for concentrating beneficiated clay slurries by an evaporation process. Background of the Invention Clay slurries, such as Kaolin, are beneficiated in an aqueous slurry having a low solids content. The most widely used method of dewatering such slurries after beneficiation involves initial processing in a vacuum or pressure filter to remove a first portion of the water from the slurry, typically to a level of about 50% to 70% solids. The resultant filter cake is then subjected to direct contact evaporation, usually by means of a spray dryer, to increase the solids content to about 95% to 99%. As has already been recognised in the disclosure of United States Patent 4,687,546 Willis, assigned to Georgia Kaolin Company Inc., the concentration of beneficiated clay slurries by spray drying is an inefficient method of evaporating the water from the slurry which requires the use of relatively clean hot gases to contact the slurry spray. This patent describes the use of non-contact or indirect evaporative heat exchangers to remove the water from the slurry. The specification of this patent claims that this method of processing the slurry avoids the problems of agglomeration which are prevalent with spray drying and results in a Kaolin product of improved brightness. As a result of test work conducted by the present applicant to evaluate the concentration of Kaolin slurries by evaporation using standard techniques of free boiling and natural circulation, it is believed that the use of evaporative heat exchange techniques in the concentration of Kaolin slurries will result in the formation of a thick skin on the surface of the Kaolin slurry and a build up or accretion of high solids slurry on the heat exchange surfaces even at relatively low solids levels (40% to 45%) . When this technique of free evaporation was tested in a long tube vertical evaporator,, it was quickly found that plugging of the tubes rapidly occurred and concentration could not effectively be achieved as a continuous process even when low solids slurries were processed at high specific circulation rates. Summary of Invention and Object It is an object of the first aspect of the present invention to provide an improved method and apparatus for the concentration of clay slurries in which the problems associated with the use of indirect evaporative heat exchange are substantially overcome or ameliorated. In a first aspect, the invention provides a method for the concentration of clay slurries comprising the steps of subjecting the slurry to indirect heat exchange in a heat exchanger to elevate the temperature of the liquid in the slurry whilst specifically suppressing boiling of the liquid within the heat exchanger, and subjecting the heated slurry to rapid pressure reduction to flash-evaporate part of the liquid from the slurry. By suppressing boiling in the heat exchanger, the build up of high solids slurry at the heat exchange surfaces is significantly reduced thereby allowing continuous processing on a more efficient basis. The use of indirect heat exchange to heat the slurry allows the use of waste heat which is often available at processing plants, although not generally suitable for use in spray drying techniques. Thus, the combination of indirect heat exchange with the suppression of boiling to avoid fouling of the heat exchanger provides a particularly efficient and relatively inexpensive method of concentrating the clay slurr . The rapid pressure reduction of the heated slurry is preferably achieved at the entrance to a separator vessel positioned downstream of the heat exchanger. The necessary concentration of a clay slurry, such as Kaolin, to the necessary solid level (for example, from about 34% solids to about 69% solids) is most suitably achieved by the use of multiple-effect evaporation, with each effect including an indirect heat exchanger, such as a plate heat exchanger, and a separator connected in series with similar effects. In such an arrangement, the vapour from each separator is directed to the following effect to achieve more evaporation at a correspondingly lower temperature and pressure. The temperatures and pressures in each effect are progressively lower until the practical limits of vacuum in the separator have been reached. In another aspect, the invention provides an apparatus for concentrating a clay slurry comprising an indirect heat exchanger adapted to elevate the temperature of the liquid in the slurry, means for specifically suppressing boiling of the liquid in the heat exchanger, and means connected to the heat exchanger for rapidly reducing the pressure of the heated slurry to cause evaporation of part of the water from the slurry. In a preferred embodiment, a multiplicity of heat exchangers and pressure reduction means are connected in series to provide a multiple-effect evaporation system, and means for passing the vapour from the pressure reduction means of the first to last but-one effects in the system to the heat exchanger of the subsequent effect to provide heat exchange medium for that heat exchanger. In considering the design of a multiple effect heat exchange and evaporation system, authoritative texts indicated that a backward fed system with a low solids feed entering the low temperature last effect, and being progressively heated on its way to the high temperature, high solids first effect, would give the best economy in the balance of fuel and power costs against capital expenditure. In this regard, the disclosure contained in the United States Patent 4,687,546 referred to above describes a backward fed system as its preferred system. Thus, in the initial design work associated with the present invention a backward fed system was considered-. However, it was subsequently determined that a modified form of forward fed system provided the best balance of operating costs against capital expenditure, bearing in mind the greater simplicity of a forward fed system. In this aspect of the invention, there is provided a multiple effect system for the concentration of clay slurries, comprising a multiplicity of evaporative effects connected in series, with each effect comprising indirect heat exchange means for heating the slurry and means for causing evaporation of part of the water from the slurry, means for forward feeding the clay slurry from the first effect to the last effect, supplementary heat exchange means for preheating the slurry prior to reaching the first effect, said supplementary heat exchangers receiving heat exchange fluid from each of the heat exchangers in the second to last effects. In a preferred form of the above system, each effect comprises an indirect heat exchanger with means to suppress boiling of the water contained in the slurry, and means to rapidly reduce the pressure of the heated slurry to cause evaporation of part of the water from the slurry. Brief Description of the Drawings In order that the invention may be more readily understood, one presently preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a five effect forward fed evaporator with vapour-heated feed, and Figures 2a and 2b are more detailed schematic diagrams of the evaporator show in Figure 1 of the drawings. Table 1 shows a set of process flow details for the production of about 150,000 tonnes of product per annum using the evaporator shown in Figure 2. Description of Preferred Embodiment Referring to Figure 1, the presently preferred embodiment of the present invention comprises a forward- feed, multiple effect evaporation having five effects 10, 20, 30, 40, 50. Each effect is arranged to heat incoming slurry by indirect heat exchange, whilst specifically suppressing boiling in the heat exchanger. Each effect also includes means to reduce the pressure of the heated slurry to cause evaporation of at least part of the liquid from the slurry. The heated slurry 11 from effect 10 passes to effect 20. Process vapour 12 from effect 10, which comprises vapour formed by evaporation of part of the liquid from the slurry, is also passed to effect 20. Process vapour 12 acts as the heating medium for effect 20. As shown in Figure 1, the heated slurry and process vapour from effects 20, 30 and 40 are also passed to the subsequent downstream effects. Slurry 51 is the product slurry of the desired solids content. Effects 20, 30, 40, 50 also include a heat exchange section to pre-heat the feed slurry 8. In the embodiment shown in Figure 1, effect 10 is operated at 110°C, effect 20 at 99°C, effect 30 at 88°C, effect 40 at 77°C and effect 50 at 60°C. Feed slurry at 30°C is supplied to feed heater 70 and thereafter through the pre-heating sections of effects 50, 40, 30 and 20. As shown in Figure 1, process condensate 13, 23, 33, 43 is also used as a heat exchange medium in the respective subsequent downstream effects. Referring to Figures 2a and 2b of the drawings, the presently preferred embodiments comprises a forward fed evaporator having five separate effects connected in series, with each effect including an indirect heat exch¬ anger 110, 120, 130, 140 and 150, in the present embodiment a plate heat exchanger and a separator 111, 121, 131, 141 and 151, and control valve means 112, 122, 132, 142 and 152 for controlling the pressure of the slurry in the assoc- heat exchanger 110 to 150 to specifically suppress boiling of the feed slurry within each heat exchanger 110 to 150. Following the fifth effect, a direct condenser 160 is provided to condense the final vapour. The heat exchanger 110 of the first effect is fed with steam from a boiler (not shown) while the vapour from each separator 111 to 141 is fed to the downstream heat exchanger 120 to 150 to provide heat exchange medium for those effects. The feed slurry is fed to the heat exchanger 110 of the first effect via supplementary heat exchangers 163, 153, 143, 133 and 123, with the first supplementary heat exchanger 163 receiving heating vapour from the separator of the last effect, while the supplementary heat exchangers 153, 143, 133 and 123 receive heating vapour from the principal heat exchangers 150, 140, 130 and 120, respectively. These supplementary heat exchangers preheat the feed slurry before it reaches the heat exchanger 110 of the first effect, which is at the highest temperature, as indicated in the process flow data in Table 1, and the feed slurry is at a suitable temperature for heating to the desired temperature in the heat exchanger 110. This arrangement enables a forward fed arrangement to be efficiently utilised to benefit from the lower capital cost of such a system. The supplementary heat exchangers 123 to 153 may be provided by constructing the heat exchangers 120 to 150 as double units, with heat exchange from vapour to recirculating slurry occurring on one side of a divider plate, and feed slurry heating occurring on the other side of the divider plate, from a common vapour supply and condensate drain. Slurry is recirculated in each effect by means of a slurry pump 114, 124, 134, 144 and 154 connected between the outlet from each separator 111 to 151 to the slurry inlet of each heat exchanger 110 to 150, while valves 115 to 155 control the feed rate of the heated slurry from one effect to the next or to the next process. As mentioned previously, the forward-fed arrangement described above was selected in favour of the backward fed arrangement indicated by authoritative texts to be preferred. Analysis of small-scale test work suggested that the optimum values of specific mass flow rate (SMFR) would be in the range 0.9 to 1.0kg/sm 2 . However, early pilot test work demonstrated that the heat exchangers of some effects would tend to bog at such low rates. Practical lower limits of SMFR were defined and adoption of these minimum values, and vapour preheating of the feed slurry, changed the economic balance of the corresponding full scale evaporator. The invention ' described herein is suitable for concentrating clay slurries, especially Kaolin clay slurries, from an initial solids content at 30-45% to a final solids content of from 65 - 75%, by weight, more preferably 68-73% by weight. The claims form part of the invention as described in the present specification. TABLE 1 Process Conditions for the Flowsheet Shown in Figure 2

Claims: CLAIMS 1. A method for the concentration of clay slurries comprising the steps of subjecting the slurry to indirect heat exchange within a heat exchanger to elevate the temperature of the liquid in the slurry whilst specifically suppressing boiling of the liquid within the heat exchanger, and subjecting the heated slurry to rapid pressure reduction to flash-evaporate part of the liquid from the slurry. 2. A method as claimed in claim 1 wherein the heated slurry is passed from the heat exchanger to an associated separating vessel operated at a lower pressure than the heat exchanger to thereby cause said rapid pressure reduction. 3. A method as claimed in claim 2 wherein valve means is disposed between the heat exchanger and the separating vessel to suppress boiling in the heat exchanger. 4. A method as claimed in claim 1 .wherein the heat exchanger and associated separating vessel form one of a multiplicity of evaporative effects, said multiplicity of evaporative effects being connected in series to form a multiple effect evaporator. 5. A method as claimed in claim 4 wherein the slurry removed from one separating vessel is passed to the subsequent downstream heat exchanger and the vapour separated from the slurry in the one separating vessel is used as a heating medium for indirect heat exchange in the subsequent downstream heat exchanger. 6. A method as claimed in claim 5 wherein subsequent downstream effects are operated at a lower pressure than corresponding upstream effects. 7. A method as claimed in claim 6 wherein feed slurry fed to the first effect of the multiple effect evaporator is preheated by indirect heat exchange with the heating medium from one or more of the downstream heat exchangers. 8. A method as claimed in claim 1 wherein the clay slurry is concentrated from 30-45% solids content to 65-75% solids content, by weight. 9. A method as claimed in claim 1 wherein said clay slurry comprises a Kaolin clay slurry. 10. An apparatus for concentrating a clay slurry comprising an indirect heat exchanger adapted to elevate the temperature of the liquid in the slurry, means for specifically suppressing boiling of the liquid in the heat exchanger, and means connected to the heat exchanger for rapidly reducing the pressure of the heated slurry to cause evaporation of part of the liquid from the slurry. 11. An apparatus as claimed in claim 10 wherein a multiplicity of heat exchangers and pressure reduction means are connected in series to provide a multiple effect evaporation system. 12. An apparatus as claimed in claim 11 further comprising means for passing vapour generated by evaporation of part of the liquid in the slurry- in the first to last-but-one effects to the heat exchanger of the subsequent effect to provide heat exchange medium for the heat exchanger of the subsequent downstream effect. 13. A multiple effect system for the concentration of clay slurries, comprising a multiplicity of evaporative effects connected in series, with each effect comprising indirect heat exchange means for heating the slurry and means for causing evaporation of part of the water from the slurry, means for forward feeding the clay slurry from the first effect to the last effect, supplementary heat exchange means for preheating the slurry prior to reaching the first effect, said supplementary heat exchangers receiving heat exchange fluid from each of the heat exchangers in the second to last effects. 14. A multiple effect system as claimed in claim 13 wherein each effect comprises an indirect heat exchanger with means to suppress boiling of the water contained in the slurry, and means to rapidly reduce the pressure of the heated slurry to cause evaporation of part of the water from the slurry. 15. A multiple effect system as claimed in claim 14 wherein the means to rapidly reduce the pressure of the heated slurry comprises a separating vessel connected to the heat exchanger, said separating vessel being operated at a lower pressure than the heat exchanger of the effect. 16. A multiple effect system as claimed in claim 15 wherein valve means is disposed between the heat exchanger and the separating vessel. 17. A multiple effect system as claimed in claim 15 wherein the vapour from the separating vessel of one effect is used as the heat exchange medium for the heat exchanger of the subsequent downstream effect.

Inventors: GREIG CHRISTOPHER ROY, NOBLE PETER JOHN, TAIT PETER JAMES, GREIG, CHRISTOPHER, ROY, NOBLE, PETER, JOHN, TAIT, PETER, JAMES
Applicants: COMALCO ALU, COMALCO ALUMINIUM LIMITED
Application year: 1992
Main classification: B28C1
IPC codes: B01D1, B01D3, B01D43, B28C1