US 3502575 A
Description (OCR text may contain errors)
March 24, 1970 P. s. HEPP ET AL 3,502,575
TREATING HOT WATER PROCESS DISCHARGE WATER BY FLOCCULATION AND VACUUM PRECOAT FILTRATION Filed June 20, 1968 DILUENT CENTRIFUGE 22 ZONE 24 BITUMEN TAR SANDS PRODUCT COMBINED FROTH\ 23 WATER SETTLED SCAVENGER FROTH FROTH CONDITIONING PRIMARY DRUM FROTH SETTLER 2o SCAVENGER FROTH SCREEN s \SOVERSIZE WATER |6\ m SEPARATION w ZON --l5 L J4 A|R FLOTATION SCAVENGER 4 ZONE TAILINGS 26 2? 26 SAND 4| PUMP 29 SAND DISTRIBUTION ZONE ZONE P|P|NG POND ZONE 2 INVENTOR.
PETER S. HEPP FREDERICK W. CAMP BY EMLM ATTORNE United States Fatent O ABSTRACT OF THE DISCLOSURE The specification discloses a process for clarifying water discharged from a hot water process for separating bitumen from tar sands. This Water is subjected to flocculation of its clay component followed by vacuum precoat filtration to produce a clarified Water suitable for recycle as at least a portion of the feed water to the hot water process.
This invention relates to a process for clarifying water discharged from a hot water process for separating bitumen from tar sands. It has been found that flocculation and vacuum precoat filtration can be applied to clay and silt containing discharge to produce a clarified water suitable for recycle as at least a portion of the feed water to the hot water process.
Numerous deposits of bituminous tar sands exist throughout the world. The most extensive deposits are found in northern Alberta, Canada. The sands are composed of a siliceous material, generally having a size greater than that passing a 325 mesh screen, saturated with a relatively heavy, viscous bitumen in quantities of from 5 to 21 Weight percent of the total composition. More typically the bitumen content of the sand-s is between about 8 to 15 percent. This bitumen is quite viscous and contains typically 4.5 percent sulfur and 38 percent aromatics. Its specific gravity at 60 F. ranges typically from about 1.00 to about 1.06. The tar sands also contain clay and silt. Silt is defined as mineral which will pass a 325 mesh screen but which is larger than 2 microns. Clay is mineral smaller than 2 microns including some siliceous material of that size.
There are several well-known processes for effecting separation of bitumen from the tar sands. In the hot water method, the bituminous sands are jetted with steam and mulled witha minor amount of hot water at temperatures in the range of 140 to 210 F. The resulting pulp is conducted to a sump where it is diluted with additional hot water and carried to a separation cell maintained at a temperature of about 150 to 200 F. In the separation cell,
sand settles to the bottom as tailings and bitumen rises to the top in the form of an oil froth. An aqueous middlings layer containing some mineral and bitumen is formed between these layers. A scavengers step may be conducted on the middlings layer from the primary separation step to recover additional amounts of bitumen therefrom. This step usually comprises aerating the middlings as taught by K. A. Clark, The Hot Water Washing Method, Canadian Oil and Gas Industries 3, 46 (1950). These froths can be combined, diluted with naphtha and centrifuged to remove more water and residual mineral. The naphtha is then distilled off and the bitumen is coked to a high quality crude suitable for further processing.
The hot water process is described in detail in Floyd et al., United States application Ser. No. 509,589. Floyd et al. disclose that froth formation in the process separation cell is dependent upon the viscosity of the middlings layer, and that the viscosity is dependent upon the middlings clay 3,502,575 Patented Mar. 24, 1970 content. It is thought that increasing viscosity retards the upward settling of bitumen flecks. When this occurs, the smaller bitumen flecks and those that are heavily laden with mineral matter stay suspended in the water of the cell and are removed with the middlings layer, or are lost altogether With the sand failings.
Upon discharge from the separation zone the Water from the middlings must eventually be stored, disposed of, or recycled back into the process. Because this water contains bitumen emulsions, finely dispersed clay With poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the water into rivers, lakes or other natural bodies of water. It has been proposed that the water be stored in evaporation ponds but this proposal would involve large space requirements and the construction of expensive enclosure dikes. It has also been suggested that the Water in the efiluent discharge be recycled back into the process as an economic measure to conserve both heat and water. Floyd et al. teach that some of this Water can be so recycled but that the amount of recycle is limited by the dispersed silt and clay content of the water which can reduce froth yield by increasing the viscosity of the middlings layer and retarding the upward settling of bitumen flecks as pointed out supra. A proportion of water in the diluted tar sands pulp fed into the separation cell must therefore be fresh water-water which is substantially free of the clay and silt found in middlings water. In fact, with some high. clay content tar sands feeds, all of the water in the diluted pulp must be added as fresh water.
It has been found that silt and clay containing water discharged from the process can be made suitable for recycle as at least a portion of the hot water process water feed by treating the discharge according to a method of flocculation and precoat vacuum filtration described herein. It is known that flocculation, even with extended settling, cannot be used to clarify water from a hot water process. Furthermore, it has been found that precoat vacuum filtration alone cannot be used to clarify the water. It has been found by the present invention that precoat vacuum filtration of a water discharge in which the clay component has been flocculated does produce a clear filtrate suitable for further use or discard. Rotary precoat filtration procedures typically comprise rotating a cylindrical filter drum or chamber to continuously submerge a segment or portion of the peripheral screen filtering surface, carrying a vacuumor pressure-held filter medium consisting essentially of a permeable bed or cake of filter aid, into the suspension or slurry of liquid and solids to be filtered, the pressure differential inducing passage of the liquid filtrate component of said suspension or slurry through the filtering medium, thereby separating the liquid from the solids, the solids being retained on the surface of, or entrapped within, the interstices of the precoat cake adjacent to the surface. Collection and retention of thesolids on the surface and within the interstices of the filter medium adjacent the surface, however, fill and thus block the pores or interstices, inhibiting further passage and separation of filtrate which necessitates the removal of the surface portion of the precoat containing the retained and entrapped solids to permit further filtration. In rotary precoat filtration procedures removal of retained solids is typically effected by means of a continuously advancing doctor knife or blade which penetrates into the precoat cake to a depth approximately equal to that reached by the entrapped solids. Thus, uninterrupted filtration through rotation of the cylindrical filter drum is permitted by the continuous cutting away and removing of the retained solids with their entrapping precoat cake and in turn exposing a substantially uncontaminated new surface for further filtration.
The process of the present invention is an improvement to the hot water process for separating bitumen from tar sands. The improvement comprises subjecting at least a portion of the effiuent discharge from the hot water process to clarification by flocculation followed by vacuum precoat filtering. It is quite surprising that flocculation and vacuum precoat filtering substantially clarify the water portion of this effluent in view of its unique composition, i.e., water containing about 0.1 to 1.5 weight percent bitumen and up to about 20 weight percent mineral between 80 and 100 percent of which is fine clay of a size smaller than 2 microns. A substantial portion (about 50 percent by weight) of this mineral is fines of a size smaller than 0.2 micron. This fine clay has extremely poor settling characteristics. It is this material in combination with other components of the water which makes this water portion diflicult to clarify to a degree such that the water is suitable for reuse or discard.
The present invention achieves clarification of this water by flocculation and application of a vacuum precoat filtration technique whereby at least a portion of the water discharged from the hot water process is subjected to a flocculation process and is passed to a vacuum precoat filter zone. This zone comprises a rotary filter drum the periphery of which is covered with a filtering medium, means for maintaining a vacuum in the drum, a feed vessel for containing the water portion through which the filtering surface of the drum moves as the drum rotates, a blade located in shaving relationship to the drum outer surface for maintaining the filtering medium layer on the drum at a decreasing depth, and a receiving vessel for receiving under vacuum the clear filtrate recovered from the drum. The drum is rotated so that its filtering surface moves through the water discharge portion held in the feed vessel. A substantially clarified water is withdrawn by vacuum from the discharge portion into the drum and thence into the filtrate receiver, and the filtering medium is coated with mineral from the discharge. The coating mineral is shaved from the filtering medium by the blade. The substantially clarified water which is recovered can be recycled as at least a portion of the water used to form the mixture of bituminous tar sands and water which is passed to the separation zone in the hot water process.
The invention can be described in more detail with reference to the figure which is a schematic representation of the present improvement to a hot water process.
In the figure,-bituminous tar sands are fed into the system through line 1 where they first pass to a conditioning drum or muller 3. Water and steam are introduced from 2 and mixed with the sands. The total water so introduced is a minor amount based on the weight of the tar sands processed and generally is in the range of 10 to 45 percent by weight of the total mixture. Enough steam is introduced to raise the temperature in the conditioning drum to within the range of 130 to 210 F. and preferably to above 170 F. Monovalent alkaline reagents can also be added to the conditioning drum, usually in amount of from 0.1 to 3.0 pounds per ton of tar sand. The amount of such alkaline reagent preferably is regulated to maintain the pH of the middlings layer in separator zone 12 within the range of 7.5 to 9.0. Best results are obtained at a pH value of 8.0 to 8.5. The amount of the alkaline reagent that needs to be added to maintain a pH value in the range of 7.5 to 9.0 can vary from time to time as the composition of the tar sands as obtained from the mine site varies. The best alkaline reagents to use for this purpose are caustic soda, sodium carbonate or sodium silicate, although any of the other monovalent alkaline reagents can be used if desired.
Mulling of the tar sands produces a pulp which then passes from the conditioning drum as indicated by line 4 to a screen indicated at 5. The purpose of screen 5 is to remove from the tar sand pulp any debris, rocks or oversized lumps as indicated generally at 6. The pulp then passes from screen 5 as indicated by 7 to a sump 8 where it is diluted with additional water from 9 and a middlings recycle stream 10 In the event the clay content of the tar sands is high, a relatively high rate of fresh or treated feed water introduction through 9 can be employed to compensate for the high clay introduc tion while a correspondingly high rate of transfer of middlings layer through line 15 as hereinafter described can be maintained. Under these circumstances recycling of the other stream of middlings through line 10 to the sump is not required.
Modifications that may be made in the process as above described include sending a minor portion of the middlings recycle stream from line 10 through a suitable line (not shown) to the conditioning drum 3 to supply all or a part of the water needed therein other than that supplied through condensation of the stream which is consumed. Also, if desired, a stream of the middlings recycle can be introduced onto the screen 5 to flush the pulp therethrough and into the sump. As a general rule the total amount of water added to the natural bituminous sands as liquid water and as steam prior to the separation step should be in the range of 0.2 to 3.0 tons per ton of the bituminous sands. The amount of water needed within this range increases as the slit and clay content of the bituminous sand in creases. The amount of water added should be regulated along with the amount of middlings withdrawn via line 15 so as to maintain the viscosity of the separation zone middlings between about 0.4 to 5.7 centipoises, preferably about 1 to 2 centipoises. As a general rule the rate of withdrawal of bitumen-rich middlings to scavenger zone 16 will be 10 to 75 gallons per ton of tar sands processed when 15 percent by weight of the mineral matter is below 44 microns and to 250 gallons per ton when from 25 to 30 percent of the mineral is of this fine particle size. The amount of water added will generally be about 0.3 to 0.5 ton per ton of tar sands when 15 percent by weight of mineral matter of the bituminous sands has a particle size below 44 microns. On the other hand, when 30 percent of the mineral matter is below 44 microns diameter, generally 0.7 to 1.0 ton of water should be used per ton of tar sand.
Further following the process, the pulped and diluted tar sands are pumped from the sump through line 11 into the separation zone 12. This zone comprises a cell which contains a relatively quiescent body of hot water. In the cell, the diluted pulp forms into a bitumen froth layer which rises to the cell top and is withdrawn via line 13, and a sand tailings layer which settles to the bottom to be withdrawn through line 14. An aqueous middlings layer between the froth and tailings layer contains silt and clay and some bitumen which failed to form froth. In order to prevent the buildup of clay in the system and maintain the middlings viscosity within the desired range, it is necessary to continually remove some of the middlings layer and supply enough water in the conditioning operations to compensate for that so removed. The rate at which the middlings need to be removed from the system depends upon the content of clay and silt present in the tar sands feed and this will vary from time to time as the content of these fines varies. If the clay and silt content is allowed to build up in the system, the viscosity of the middlings layer will increase. Concurrently with such increase, an increase in the proportions of both the bitumen and the sand retained by the middlings will occur. If the clay and silt content is allowed to build up too high in the system, effective separation no longer will occur and the process will become inoperative. This can be avoided by regulating the recycling and withdrawal of middlings and input of fresh water per the invention disclosed and claimed in the Floyd et al. application. However, even when the separation step is operating properly the middlings layer withdrawn through line 15 will contain a substantial amount of bitumen which did not separate. Hence the middlings layer withdrawn through line is, for purpose of description, herein referred to as oil-rich or bitumen-rich middlings.
The oil-rich middlings stream withdrawn from separator 12 through line 15 is sent to a scavenger zone 16 wherein an air flotation operation is conducted to cause the formation of additional bitumen froth. The processing conducted in the scavenger zone 16 involves air flotation by any of the air flotation procedures conventionally utilized in processing of ores. This involves providing a controlled zone of aeration in the flotation cell at a locus where agitation of the middlings is being effected so that air becomes dispersed in the middlings in the form of small bubbles. Aeration causes the formation of additional bitumen froth which passes from the scavenger zone 16 through line 17 to a froth settler zone 18. A bitumen-lean middlings stream is removed from the bottom of the scavenger zone 16 via line 19.
In the settler zone 18, the scavenger froth forms into a lower layer of settler tailings which is withdrawn and recycled via line 20 to be mixed with bitumen-rich middlings for feed to the scavenger zone 16 via line 15. In the settler zone an upper layer of upgraded bitumen froth forms above the tailings and is withdrawn through line 21 and is mixed with primary froth in line 13. The combined froths are at a temperature of about 160 F. They are heated with steam and diluted with sufficient naphtha or other diluent from 22 to reduce the viscosity of the bitumen for centrifuging in zone 23 to produce a bitumen product 24 suitable for further processing.
The oil-lean middlings in line 19 and the sand tailings in line 14 are combined to form an efiiuent discharge which is delivered via line 25 to a sand pile zone 26 via distribution piping 27. The effluent contains between 25 and 50 weight percent sand and silt material which is larger than about two microns. The distribution piping provides for continuous and uniform delivery of the efliuent to the sand pile zones where the sand and silt material is deposited. The water in the eflluent discharge percolates down through and over the sand pile zone 26 to the pond zone 28 where it collects as pondwater containing up to about 20 weight percent suspended solids, between 80 and 100 percent of which is fine clay of a size smaller than two microns. The pondwater also contains between about 0.1 and 1.5 weight percent bitumen. Because of the particular composition of this pondwater, and especially because of the extreme fineness of the suspended clay material which has extremely poor settling characteristics, the water cannot be discarded or to any great extent recycled back into the hot water system.
By the improvement of the present invention, the pondwater is withdrawn from pond zone 28 by means of pump 29 and fed via 30 into zone 31 Where suspended solids in the water are flocculated. For the purpose of this invention, pondwater is effluent discharge from a hot water process which elfluent has been settled to give a composition comprising water containing up to about 20 percent solids, between 80 percent and 100 percent of which is fine clay of a size smaller than 2 microns. The eflluent discharge from a hot water process comprises middlings material of depleted bitumen content which has undergone final treatment, the sand tailings layer from the process and other discharged water-containing fractions which are not the primary products of the hot water process. The discharge is removed from the process area as a slurry of about 25 to 60, typically 45, percent solids by weight. The efiluent contains virtually all of the clay material which was present in the feed. Typically the amount is 2 to 10 weight percent of the feed. This material is smaller than two microns and has extremely poor settling characteristics.
The flocculation step on the pondwater can be carried out by adding a conventional flocculating reagent via line 44 to the Water with gentle agitation. Among the various reagents useful for flocculating clay are aluminum sulfate (alum), polyalkylene oxides such as polyethylene oxide, compounds of calcium such as calcium hydroxide, calcium oxide, calcium chloride, calcium nitrate, calcium acid phosphate, calcium sulfate, calcium tartrate, calcium citrate, calcium sulfonate, calcium lactate, the calcium salt of ethylene diamine tetraacetate and similar organic sequestering agents. Also suitable are guar flour or a high molecular weight acrylamide polymer such as polyacrylamide or a copolymer of acrylamide and a copolymerizable carboxylic acid such as acrylic acid. Additional flocculants include the polymers of acrylic or methacrylic acid derivatives, for example, acrylic acid, methacrylic acid, the alkali metal and ammonium salts of acrylic acid or methacrylic acid, acrylamide, methacrylamide, the aminoalkyl acrylates, the aminoalkyl acrylamides, the aminoalkyl methac rylamides and the N-alkyl substituted aminoalkyl esters of either acrylic or methacrylic acids.
Preferably flocculation is accomplished by changing the pH of the water. Classically clay is flocculated by catins such as iron and aluminum by the mechanism of compression of the double layer charge on the suspended particles. Variation of pH causes a change in the charge on the edges of the clay particles, thus allowing flocculation. The effiuent discharge from the hot water process has a pH ranging from about 7.5 to 9.0, typically about 8.3. In this range, the contained mineral material does not fiocculate; however, raising the pH above about 9.0 or lowering it below about 7.5 does cause flocculation. More preferably flocculation in zone 31 is accomplished by adding sulfuric acid (to the pondwater via line 44 or the middlings or eflluent discharge portion as the case may be) to about pH 5.
If desired a filter aid can be added in zone 31 via line 45 -to aid in the subsequent vacuum precoat flltration step.
Such aids are solids added to filter feeds for filtering out with the feed solids. The purpose of the aid is to increase filter cake porosity thereby increasing filtration rate. Commonly the same material used as precoat is also used as a filter aid; however, only filter aids of fine particles size are effective in the present invention. An example of such an effective aid is a fine diatomaceous silica with an average particle diameter of about 4 microns. It has been found that conventional aids of average particle diameters greater than about 6 microns are ineffective for increasing filtration rate in the process of the present invention.
Further with reference to the drawing, water containing flocculated clay solids and with or without a filter aid is passed from zone 31 via 32 to a feed vessel 33 in a precoat vacuum filtration zone which is indicated in the drawing as the combination of the vessel 33, a rotary filter drum 34, a shaving blade 35 and a filtration receiver 38. The filter drum comprises a rotating, perforated cylinder carrying a coat of uniform thickness of a porous material such as diatomaceous earth. A fine diatomaceous silica with an average particle diameter of about 4 microns is the preferred porous material for forming the precoat on the filter. Materials of coarser particle size are suitable but not preferred. A one to two inch precoat should be used when fine precoat is used while a precoat of about four inches in thickness should be applied when the precoat material has an average particle size of about 6 microns or greater. A recycle line 41 can be provided from the feed vessel 33 back to the flocculated feed in line 32 whenever a filter aid is utilized in the process. This recirculation loop serves to maintain the filter aid suspended in the water. In the filtering zone, the filter drum 34 is rotated so that its filtering surface covered with the filter coat moves through the pondwater contained in the feed vessel 33.
Water is drawn from the flocculated middlings through the filter medium by vacuum into the drum 34 while the flocculated solids contained in the middlings are retained on the precoat. As the solids are deposited, they are shaved off together with a thin layer of precoat by means of the shaving blade 35 and are discarded 36. A clarified water is recovered from the drum interior and is passed via line 37 to the filtrate receiver 38 where the water (entrained in the vapor stream) is disengaged. Vapor is discaarded via line 39 by a vacuum pump 42. The treated water can be introduced via line 40 by a filtrate pump 43 into the system as all or a portion of the water in line 9 to the sump as shown or can be alternatively introduced into the system via lines 2 or 10 or as a screen wash or at any desired point of introduction into the process.
Although the invention has been described supra with reference to the treatment of pondwater from the hot water process effluent discharge it should be pointed out that the invention can be practiced on any water stream from the separation cell. For example, referring again to the drawing, the bitumen-lean middlings line 19 from the flotation scavenger zone 16 can be directly treated by the invention to make these middlings suitable fer recycle back into the process. Also the middlings in line 10 can be treated before recycle into sump 8 for dilution of the tar sands pulp.
EXAMPLES The following runs were made on a continuous rotary drum vacuum filter. Pondwater was first introduced to a flocculating tank where it was gently agitated and flocculated by the addition of H 50 to pH In runs using a filter aid the aid was added to the pondwater in the flocculating tank. The pondwater was then pumped by means of a slurry pump to the feed tank of the filter.
Filter drum size was 20 inches diameter by 12 inches (5.124 square feet). Submerged drum area was varied by controlling the slurry level in the feed tank. Submergence time of a given point on the drum was controlled by the slurry level and drum revolutions per minute. Rate of advance of the filter doctor blade was controlled independent of drum revolutions per minute by a separate drive. Precoat depth was about one inch for all runs. Vacuum was 20 inches mercury for all runs. A coarse weave polypropylene fabric was used for all runs to support the precoat iayer. Feed for all runs was hot water process pondwater.
Table I gives the conditions for each run and Table IE shows the results. The runs in the tabies generally illustrate that vacuum precoate filtration can be used to produce a clarified water useful for recycle back into the hot water process for separating bitumen from tar sands. The runs aiso show that a precoat with a particle diameter of about 4 microns or less is effective in increasing filter cake porosity thereby increasing filter rate. Runs to show that drainage time has little significant effect on cake percent water and within tested limits does not limit filter performance. All runs generally show that increased submergence time decreases average filtrate rate. For instance, the run series 31, 32, 27, 28, l3, 14, 25, 26, 29, 30 and the series 46, 47, 50, 51, 35 to 40, 48, 49, 52, 53, 83 to 86, 56, 57, 54, 55, 66 to 69, 62 to 65, 61, 58, 59 illustrate this point. Runs 5, 6, 9, 10, 33, 34 and 70 to 82 show that diatomaceous earth with an average particle diameter of less than about 6 microns as a filter aid does give some effective increase in filtrate rate= TABLE L-FILTRATION TEST RESULTS Operating conditions Filter Aid Pi'ecoat Drum Feed, weight percent mineral Pounds Type per gallon Type Filter- Col 1 do D. D. 0. '0. l. 1. D. 2. 3. 3.
None Filteir-Cel l Knife advance, inches per revolution Fraction of area submerged Drainage time, seconds Revolutions per minute water; passing said mixture into a separation zone to form an upper bitumen froth layer, a middlings layer comprising water, finely divided mineral and bitumen, and a sand tailings layer, and separately removing said bitumen froth layer, middlings layer and tailings layer, and said process for clarifying water discharged from said hot water process comprises:
(a) flocculating finely divided mineral in at least a portion of said middlings; and
(b) vacuum precoat filtering said portion to remove flocculated mineral to produce a water filtrate substantially reduced in mineral content and suitable for recycle back into the hot water process as at least a portion of the water utilized to form said mixture of tar sands and water.
3. The process of claim 2 in which the flocculating step comprises adding a flocculating agent to fiocculate said mineral component.
4. The process of claim 2 in which the flocculating step comprises adjusting the pH of said middlings portion below about 7.5.
5. The process of claim 2 in which the flocculating step comprises adjusting the pH of said middlings portion to above about 9.0.
6. The process of claim 2 in which said water substantially reduced in mineral content is recycled back into the hot water process as at least a portion of the water utilized to form said mixture of tar sands and water.
7. The process of claim 2 in which a filter aid comprising diatomaceous silica with an average particle diameter of less than about 6 microns is added prior to said vacuum precoat filtering step.
8. The process of claim 2 in which a diatomaceous silica is used as a precoat in said vacuum precoat filtration step.
9. The process of claim 7 in which a portion of said water filtrate is recycled to said middlings at the point of addition of said filter aid.
10. In a hot water process for separating bitumen from bituminous tar sands which comprises:
(a) forming a pulp of said bituminous sands with a minor amount of water in a pulping zone;
(b) removing said pulp therefrom and mixing the same with hot water and a hereinafter specified recycle stream in a dilution zone;
(c) flushing the mixture with water from the dilution zone into a separation zone;
(d) settling the mixture in said separation zone at a temperature in the range of 130210 F. to form an upper bitumen froth layer, a middlings layer comprising water, clay and bitumen and a sand tailings layer;
(e) separately removing said bitumen froth layer and said sand tailings layer;
(f) removing a stream of middlings layer from said separation zone and passing it to said dilution zone as the aforesaid recycle stream;
(g) passing a second stream of middlings layer to a separate recovery zone and therein subjecting it to air flotation to recover an additional amount of bitumen;
(h) regulating the amount of water incorporated with sair bituminous sands and the rate in Step (g) of passage of said second stream to said separate recovery zone so as to regulate and maintain the viscosity of said middlings layer within the range of 0.4 to 5.7 centipoises; and
(i) removing fro msaid separate recovery zone middlings material of depleted bitumen content comprising clay dispersed in water; the improvement which comprises:
(j) flocculating clay in at least a portion of said middlings of depleted bitumen content;
(k) vacuum precoat filtering said portion to remove fiocculated clay to produce a water filtrate substantially reduced in mineral content; and
(l) recycling said portion to Step (c) as at least a portion of the water used to flush said mixture to said separation zone.
11. The process of claim 10 in which Step (1) comprises recycling said portion to Step (a) as at least a portion of the water used to form said pulp.
12. The process of claim 10 in which the flocculating Step (j) comprises adding a flocculating agent to flocculate said clay component.
13. The process of claim 10 in which the flocculating Step (j) comprises the pH of said middlings portion to below about 7.5.
14. The process of claim 10 in which the flocculating Step (j) comprises adjusting the pH of said middlings portion to above about 9.5.
15. The process of claim 10 in which a filter aid comprises diatomaceous silica with an average particle diameter of less than about 6 microns is added prior to said vacuum precoat filtering step.
16. The process of claim 10 in which a diatomaceous silica is used as a precoat in said vacuum precoat filtra tion step.
17. The process of claim 15 in which a portion of said water filtrate is recycled to said middlings at the point of addition of said filter aid.
References Cited UNITED STATES PATENTS 1,830,962 11/1931 Read et al. 21075 X 2,692,229 10/1954 Heise et al. 210X 2,715,466 8/1955 Esposito 210-75 3,392,833 7/1968 Baillie 210- JAMES L. DECESARE, Primary Examiner US. Cl. X.R. 21060, 75, 196