GB2064972A - Filtration - Google Patents

Abstract

Filtration of a slurry and washing of the filtered solids are carried out by filtering the slurry in a first filtration zone, re-slurrying the solids in a re- slurrying zone, and then re-filtering in a second filtration zone. This procedure can be repeated using up to 6 or more successive filtration zones with a re- slurrying zone between each succeeding pair of filtration zones. Washing can be effected by re-slurrying the solids in a suitable washing liquid in the or a re-slurrying zone or by washing the filter cake in the final filtration zone. Liquid filtrate fractions from the second and any subsequent filtration zones can be recycled to upstream reslurrying zones. Each filtration zone comprises a separate filter belt moving over one or more suction boxes, or a tilting pan type filter. <IMAGE>

Description

SPECIFICATION Filtration This invention relates to filtration.

In many processes it is necessary to separate the solids content of a slurry from the liquid phase thereof. One method of achieving this is to filter the slurry through an appropriate filter medium such as a a filter paper or cloth. Although filtration can be achieved simply underthe influence of gravity, it is often expedient to apply vacuum to the underside of the filter paper or cloth in order to enhance the rate of filtration. In such a filtration process the solids are deposited on the filter paper or cloth in the form of a cake. Usually this cake must then be washed in order to remove traces of components of the liquid medium of the feed slurry still present in the filter cake. if the solids of the feed slurry constitute the desired product this washing step improves the purity of the product.If the liquid medium of the feed slurry is the desired product of the process then this washing step helps to ensure maximum recovery of the product. In conventional filtering machines the initial filtration and subsequent filter cake washing steps are normally effected on the same filter cloth, both with vacuum assistance. Since two operations are being performed on the one piece of filter cloth the machine must be of appropriately large dimensions in order to achieve a given throughput of slurry.

In operation of a conventional filtering machine conditions are usually selected such that the filter cake builds up to an appreciable thickness prior to washing in order to improve the efficiency of washing and to minimise the volume of washing liquid. However the rate of filtration tends to diminish as the thickness of the filter cake builds up. Thus a compromise must usually be struck between these conflicting requirements when selecting operating conditions for the machine.

In order that washing may be as efficient as possible it is necessary that the filter cake must be built up uniformly and care must be taken to prevent "channelling" in the filter cake during washing because in this case large parts of the interior of the filter cake may not in fact come into contact with the washing liquid.

In the washing step care must be taken to distribute the washing liquid as evenly as possible over the top surface of the filter cake in order that washing shall be as uniform as possible. This is in practice hard to achieve.

It would be desirable to reduce the size of the filtration equipment required for filtering solids from a slurry and for subsequent washing of the filter cake and also to reduce the vacuum requirements of the equipment and hence the running costs thereof. It would also be desirable to improve the efficiency of filtration equipment and to improve the efficiency of washing of the filter cake.

The present invention accordingly seeks to provide an improved method of effecting separation of solids from a slurry by filtration and an improved apparatus suitable for use therewith. It also seeks to provide a method of effecting separation of solids from a slurry in which the efficiency of washing is improved.

According to the invention there is provided a method of separating solids from a feed slurry which comprises: (a) providing n filtration zones arranged in sequence, where n is an integer of 2 or more, including a first filtration zone and a final filtration zone, and (n-1) re-slurrying zones, one between each successive pair of filtration zones; (b) filtering the feed slurry in the first filtration zone; (c) recovering from the first filtration zone a first filter cake of solids and at least one first liquid filtrate fraction; (d) re-slurrying in the or each re-slurrying zone in a liquid vehicle solids filtered in the preceding filtration zone so as to form a reconstituted slurry; (e) filtering resulting reconstituted slurry in the next succeeding filtration zone; and (f) recovering a final filter cake of solids and at least one final liquid filrate fraction from the final filtration zone.

Although in a preferred method n is 2, the value of n can range from 3 to 6 or upwards. In such a preferred method, when n is 2, the method for separating solids from a feed slurry simplifies to steps comprising: (i) providing first and second filtration zones, and a re-slurrying zone intermediate the first and second filtration zones; (ii) filtering the feed slurry in the first filtration zone; (iii) recovering from the first filtration zone a first filter cake and at least one first liquid filtrate fraction; (iv) re-slurrying solids of the first filter cake in the re-slurrying zone in a liquid vehicle to form a reconstituted slurry; (v) filtering the reconstituted slurry in the second filtration zone; and (vi) recovering from the second filtration zone a second filter cake and at least one second liquid filrate fraction.

The method of the invention can be utilised for any feed slurry, whether the liquid phase thereof is aqueous or organic. For example, the invention can be used in filtering precipitated calcium sulphate (gypsum) solids from the dilute phosphoric acid slurry produced by digesting phosphate rock with sulphuric acid. This is an example of a process in which the liquid phase (i.e. dilute phosphoric acid) is the desired product. An example of a process in which the solid is the desired product is the formation of a disperse dye by addition of a diazonium salt solution to a solution of a suitable coupling component.

Conveniently, the filtration zones each comprise a belt filter; however, other form of filtration equipment, such as tilting pan filters or table filters, can be used for one or more of the filtration zones.

One or more of the first liquid filtrate fractions may be passed to the succeeding re-slurrying zone to form part of the liquid vehicle used to form the reconstituted slurry, whilst at least one other first liquid filtrate fraction is passed on for disposal or further treatment. The liquid filtrate fraction or fractions from at least one of the second and any subsequent filtration zone or zones may be recycled to the re-slurrying zone or to one of the re-slurrying zones as the case may be, e.g. to the immediately preceding re-slurrying zone, or may be discarded or recycled to the process in which the feed slurry is formed.

Re-slurrying may be effected, for example, in a stirred tank reactor, in a reactor fitted with a recirculating pump, or in any other convenient form of mixer, such as an inclined trough provided with a mixing device such as a ribbon mixer.

in practising the method of the invention washing of the solids can be effected by re-slurrying the filter cake from the preceding filtration zone in a suitable washing liquid and passing the resulting reconstituted slurry to the next filtration zone. Such washing can be carried out in the final re-slurrying zone (if there is more than one such zone) or can be effected in several stages in different re-slurrying zones terminating with the final re-slurrying zone.

Alternatively washing can be carried out on the filter cake of the final filtration zone (i.e. the second filtration zone when n is 2).

Particularly in cases where it is desirable to limit the quantity of washing liquid used, it may be desirable to recycle all or part of the washings to an upstream re-slurrying zone to form at least a portion of the liquid vehicle supplied thereto.

The invention further provided apparatus for effecting separation of solids from a feed slurry which comprises: a a first filtration means and at least one other filtration means, including a final filtration means, each filtration means comprising a filtration zone, means for recovering a filter cake of solids from the respective filtration zone, and means for recovering at least one liquid filtrate fraction from the respective filtration zone; a re-slurrying means disposed between each successive pair of filtration means and providing a re-slurrying zone wherein filtered solids from the immediately preceding filtration means can be reslurried in a liquid vehicle to form a reconstituted slurry; means for feeding feed slurry to the first filtration zone;; means for feeding reconstituted slurry from the or each re-slurrying zone to the succeeding filtration zone; and means for supplying solids of the filter cake from the preceding filtration zone to the or each reslurrying means.

In order that the invention may be clearly understood and readily carried into effect some preferred forms of filtration plant and the methods of operation thereof, according to the invention, will now be described, by way of example only, with reference to Figures 1 to 3 of the accompanying diagrammatic drawings, each of which is a flow sheet of a filtration plant.

Referring to Figure 1 of the drawings, a dilute phosphoric acid slurry of calcium sulphate crystals resulting from the digestion of phosphate rock with sulphuric acid is supplied through line 1 to a first belt filter 2 comprising an endless belt 3 of filter cloth passing around end rollers 4, 5. Arrow 6 indicates the direction of movement of endless belt 3. A vacuum box 7 is disposed below the upper run of belt 3. A cloudy acid filtrate is removed from the upstream end of vacuum box 7 through line 8 and is passed to a separator 9. Line 10 indicates a connection to a vacuum pump (not shown). Cloudy acid from separator 9 passes on through line 11 by means of pump 12to line 13.Further acid filtrate is recovered from the downstream end of vacuum box 7 through line 14, is separated in separator 15 and passes on by way of line 16 to pump 17 which pumps the dilute phosphoric acid product on for further processing via line 18. Reference numeral 19 indicates a vacuum connection line to separator 15.

The filter cake of calcium sulphate crystals formed on belt 3 falls (as indicated by arrows 20) into an intermediate hopper 21 and thence into a tank 22 which is provided with a stirrer 23 mounted on shaft 24 and driven by motor 25. In tank 22 the filter cake from belt filter 2 is re-slurried in a mixture of washing liquid, which is supplied from a downstream part of the process through line 26, and of cloudy acid supplied through line 13. The resulting slurry is removed from tank 22 through line 27 and is pumped by means of pump 28 through line 29 to a second belt filter 30. This is similar to belt filter 2 and comprises an endless belt 31 of filter cloth passing around rollers 32, 33 in the direction of arrow 34 with a vacuum box 35 under the upper run of endless belt 31.Filter cake on belt 31 falls off its downstream end (as shown by arrows 36) into hopper 37 and is carried away for disposal on belt conveyor 39. An acid stream is removed from vacuum box 35 through line 40 which leads to a separator 41.

Reference numeral 42 indicates a connection line to a vacuum pump (not shown). From separator 41 the dilute acid is returned by means of line 43, pump 44 and line 45 for recycle to the phosphate rock digesters.

Water is supplied via line 46 to belt filter 30, the downstream end of which constitutes a washing zone for washing the filter cake on belt 31. The washings are removed from vacuum box 35 via line 47 and pass to separator 48 from which they flow on through line 49 to pump 50. Pump 50 is used to recycle the washings through line 26 to re-slurrying tank 22. Reference numeral 51 indicates a connection line from separator 48 to a vacuum pump (not shown).

The apparatus of Figure 1 has two belt filters 2 and 30. In this arrangement belt filter 30 provides both a filtration zone at its upstream end for filtration of the slurry from re-slurrying tank 22 and also a washing zone at its downstream end for washing the gypsum filter cake. In the modification illustrated in Figure 2 of the drawings separate vacuum boxes 52 and 53 connected to lines 40 and 47 respectively replace the single vacuum box 35 and thus provide separate filtration and washing zones. Additionally tank 22 has no stirrer 23, shaft 24 or motor 25; instead pump 28 is replaced by a recirculation pump 54 (which is generally of larger capacity than pump 28) which serves to recirculate slurry withdrawn from tank 2 through lines 55 and 56 back to the tank 22, thereby keeping its contents agitated.A valve 57 is provided by means of which a portion of the recirculating slurry in line 55 can be passed on through line 29 to belt filter 30. Apart from these modifications the plant of Figure 2 is otherwise identical to that of Figure 1 and identical reference numerals are utilised to designate identical items of equipment.

The plant of Figure 3 comprises three belt filters 101, 102, 103 each having an endless belt 104 which passes around end rollers 105, 106 and each having a vacuum box 107 connected by means of a line 108 to a separator 109. Line 110 leads from each separator 109 to a suitable vacuum pump (not shown); each line 110 can be connected to a separate vacuum pump or two of them, or all three of them, can be connected to a common vacuum pump. Filtrate can be withdrawn from each separator 109 through line 111.

Each endless belt 104 moves in the direction of arrow 112. Slurry to be filtered (e.g. a calcium sulphate in dilute phosphoric acid slurry resulting from phosphate rock digestion in sulphuric acid) is supplied through line 113 to first belt filter 101.

Solids fall off the end of belt 104 thereof (as shown by arrows 114) into intermediate hopper 115 and thence pass into first re-slurrying tank 116 which is fed with re-slurrying liquid through line 17. Motor ll8drivesimpellerll9mountedonshaftl20to provide the agitation necessary for re-slu rrying.

Pump 121 serves to pump slurry from tank 116 through lines 122 and 123 to second belt filter 102.

Filter cake on belt 104 of filter 102 falls off (as shown by arrows 124) into intermediate hopper 125 of second re-slurrying tank 126 which is supplied with re-slurrying liquid through line 127 and which is fitted with a motor 118, stirrer 119 and shaft 120.

Reconstituted slurry is pumped forward to belt filter 103through lines 128 and 129 by means of pump 130.

As indicated by arrows 131 in Figure 3, filter cake falls offthird belt filter 103 into intermediate hopper 132 of final re-slurrying tank 133. This is supplied in turn with re-slurrying liquid via line 134. As before motor 118, impeller 119 and shaft 120 provide sufficient agitation to assist re-slurrying. The resulting reconstituted slurry can be pumped forward for further treatment or to disposal through lines 135 and 136 by means of pump 137. Thus if, for example, the filter cake consists essentially of calcium sulphate (gypsum) crystals waste water or sea water can be supplied via line 134 and the resulting slurry passed to disposal (e.g. pumped out to sea) through line 136.

In the apparatus of Figure 3 filtrates may be extracted via lines 111 in each case by means of a pump (not shown) or by means of a barometric leg (not shown). One or more of the filtrates can be collected in this manner in a suitable sump.

In each of the re-slurrying stages the re-slurrying liquid can be any suitable liquid, for example a filtrate from one of the belt filters.

One or more further belt filters and re-slurrying tanks can be added downstream from tank 133 if the process so requires.

It will be appreciated by those skilled in the art that, since Figures 1 to 3 are diagrammatic, some conventional items of equipment such as valves, temperature sensing devices, level control switches, and automatic cut out devices have been omitted for the sake of clarity. It will readily be apparent to the skilled reader that such additional items of equipment may be necessary for the successful operation of the illustrated forms of plant and the provision and positioning of such ancillary items of equipment form no part of the present invention and will be in accordance with standard chemical engineering practice.

In each of Figures 1 and 2, a common vacuum pump (not shown) can be connected to lines 10, 19, 42 and 51. Alternatively separate vacuum pumps can be provided for each line or two or more pumps can be used one of which is connected to at least two of the vacuum lines 10,19,42 and 51.

In a modification of the apparatus shown in Figure 1 belt filter 2 is arranged at a higher elevation than belt filter 30 and discharges filter cake into the upper end of an inclined trough provided with a ribbon mixer. Lines 13 and 26 also lead to this inclined trough so that the ribbon mixer serves to re-slurry the filter cake from belt filter 2. The resulting slurry is discharged from the lower end of the inclined trough onto belt filter 30. Intermediate hopper 21, tank 22, pump 28 and lines 27 and 29 are omitted.

Although the plants of Figures 1 to 3 utilise belt filters, one or both of the belt filters 2 and 30, or any one or more of belt filters 101, 102 and/or 103, can be replaced by another form of filtration equipment, such as tilting pan filters or table filters.

In operation of the plants of Figures 1 and 2 it will usually be necessary to utilise a slower belt speed for belt filter 30, since washing takes place thereon, than for belt filter 2 which is being used solely for filtration. Thus it will often be possible, and in many cases desirable, to run belt filter 2 at a faster belt speed than belt filter 30. In a plant in which belt filter 2 can be run faster than belt filter 30 it may be possible to make a corresponding reduction in the size of belt filter 2 compared with that of belt filter 30, despite the fact that both belt filters are handling essentially the same throughput of filter cake. Thus, in appropriate circumstances, the combined effective working area of vacuum boxes 7 and 35 can be less than the corresponding area of the single vacuum box that would be required in a conventional filtration plant handling the same slurry throughput in which filtration and washing are effected consecutively on the same belt filter.

If desired the plant of Figure 2 can be modified by use of a third belt filter (not shown), like filter 103 of the plant of Figure 3, arranged downstream from belt filter 30 with a further re-slu rrying tank, corresponding to tank 126 of Figure 3, arranged between this further belt filter and filter 30. In this case lines 46,47 and 49, separator 48 and pump 50 are omitted and a calculated limited quantity of fresh water is supplied to the further re-slurrying tank to effect the washing step. In this way each of the three belt filters can be operated at the same speed since washing is not performed on the filter cake but in the further re-slurrying vessel.

Although the illustrated embodiments have been described for use in filtering gypsumidilute acid slurries they can be utilised for filtering other slurries, for example a burkeite slurry of the type encountered in the production of soda ash from Searles Lake brine (see page 41 of "Introduction to the Chemical Process Industries" by Richard M.

Stephenson, published by Reinhold Publishing Corporation (1966)).

Claims (20)

1. A method of separating solids from a feed slurry which comprises: (a) providing n filtration zones arranged in sequence, where n is an integer of 2 or more, including a first filtration zone and a final filtration zone, and (n- 1) re-slurrying zones, one between each successive pair of filtration zones; (b) filtering the feed slurry in the first filtration zone; (c) recovering from the first filtration zone a first filter cake of solids and at least one first liquid filtrate fraction; (d) re-slurrying in the or each re-slurrying zone in a liquid vehicle solids filtered in the preceding filtration zone so as to form a reconstituted slurry; (e) filtering resulting reconstituted slurry in the next succeeding filtration zone; and (f) recovering a final filter cake of solids and at least one final liquid filtrate fraction from the final filtration zone.

2. A method according to claim 1, wherein n is from 2 to 6 and the filtration zones each comprise a belt filter.

3. A method according to claim 1 or claim 2, wherein one or more of the first liquid filtrate fractions is or are passed to the succeeding reslurrying zone to form part of the liquid vehicle used to form the reconstituted slurry, whilst at least one other first liquid filtrate fraction is passed on for disposal or further treatment.

4. A method according to any one of claims 1 to 3, wherein the liquid filtrate fraction or fractions from at least one of the second and subsequent filtration zone or zones is or are recycled to one of the re-slurrying zones.

5. A method according to claim 4, wherein the said liquid fraction or fractions is or are recycled to the immediately preceding re-slurrying zone.

6. A method according to any one of claims 1 to 5, wherein washing of the solids is effected by re-slurrying the filter cake from the preceding filtration zone in a suitable washing liquid and passing the resulting reconstituted slurry to the next filtration zone.

7. A method according to claim 6, wherein at least a part of the washings is recycled to an upstream re-slurrying zone to form at least a portion of the liquid vehicle supplied thereto.

8. A method according to any one of claims 1 to 5, wherein washing of the solids is carried out on the filter cake of the final filtration zone.

9. A method according to claim 8, wherein at least a part of the washings is recycled to an upstream re-slurrying zone to form at least a portion of the liquid vehicle supplied thereto.

10. A method for separating solids from a feed slurry comprising: (i) providing first and second filtration zones, and a re-slurrying zone intermediate the first and second filtration zones; (ii) filtering the feed slurry in the first filtration zone; (iii) recovering from the first filtration zone a first filter cake and at least one first liquid filtrate fraction; (iv) re-slurrying solids of the first filter cake in the re-slurrying zone in a liquid vehicle to form a reconstituted slurry; (v) filtering the reconstituted slurry in the second filtration zone; and (vi) recovering from the second filtration zone a second filter cake and at least one second liquid filtrate fraction.

11. A method according to claim 10, wherein the filtration zones each comprise a belt filter.

12. A method according to claim 10 or claim 11, wherein one or more of the first liquid filtrate fractions is or are passed to the succeeding reslurrying zone to form part of the liquid vehicle used to form the reconstituted slurry, whilst at least one other first liquid filtrate fraction is passed on for disposal or further treatment.

13. A method according to any one of claims 10 to 12, wherein the second liquid filtrate fraction or fractions is or are recycled to the re-slurrying zone.

14. A method according to any one of claims 10 to 13, wherein washing of the solids is effected by re-slurrying the filter cake from the first filtration zone in a suitable washing liquid and passing the resulting reconstituted slurry to the second filtration zone.

15. A method according to claim 14, wherein all or a part of the washings are recycled to the re-slurrying zone to form at least a portion of the liquid vehicle supplied thereto.

16. A method according to any one of claims 10 to 13, wherein washing of the solids is carried out on the filter cake of the second filtration zone.

17. A method according to claim 16, wherein all or a part of the washings are recycled to the re-slurrying zone to form at least a portion of the liquid vehicle supplied thereto.

18. Apparatus for effecting separation of solids from a feed slurry which comprises: a first filtration means and at least one other filtration means, including a final filtration means, each filtration means comprising a filtration zone, means for recovering a filter cake of solids from the respective filtration zone, and means for recovering at least one liquid filtrate fraction from the respective filtration zone; a re-slurrying means disposed between each successive pair of filtration means and providing a re-slurrying zone wherein filtered solids from the immediately preceding filtration means can be reslurried in a liquid vehicle to form a reconstituted slurry; means for feeding feed slurry to the first filtration zone; means for feeding reconstituted slurry from the or each re-slurrying zone to the succeeding filtration zone; and means for supplying solids of the filter cake from the preceding filtration zone to the or each reslurrying means.

19. A method of separating solids from a feed slurry conducted substantially as herein described.

20. Apparatus for effecting separation of solids from a feed slurry constructed and arranged substantially as herein described with particular reference to the accompanying drawings.