Патент USA US2107919код для вставки
l Feb/[8, 1938. c. o. TURNER Er AL CONTINUOUS DIGESTION PROCESS Filed Feb. l, 1935 7 2,107,919 Planear-gb. s, l l 2,107,919 - . I 2,107,919 ' . - . Helvlnl’owcllwm stipula, Mo.. to Aluminum Company of America, Pittsburgh', Pa., a corporation of Pennsylvania , y Application February 1, ms, sci-uu N0. 4.552 150mm». This invention relates to the extraction of a solid with a solvent in a process of continuous ^ pressure digestion. It relates particularly to, and _has as its chief object, the provision of an im 5 proved method of continuously extracting theA alumina from> alumina-bearing materials by> pres~ sure digestion with caustic liquor, and an im proved system adapted to carry out such a process. ` . 10 .,'I'he'copending application of -Weigel,v Serial No. 561,081, discloses a continuous method of digesting bauxite with caustic liquor. 'I'his con tinuous processl consists broadly in heating the mixture of bauxite and caustic liquor insuccessive -15 stages'of increasing temperature and pressure until the desired digestion temperature and pres sure are attained, holding until the digestion is complete, and then cooling in successive stages of decreasing temperature and` pressure. In this ' 20. process, steam released in the cooling stages is in Jected in some of the preliminary heating stages, thus materially increasing the utilization Aof the heat supplied to the system. Difference in pres sure throughout the system is secured as'a- rule lrvdrostatically by positioning -the component parts of the system in a plurality of diñerent elevations. 'I‘his invention allowed materially increased -thermal and material eillciencies to be secured and was a great advance over the older batch system. ’ , The present invention is similar to the above described process of Weigel in that it is capable of continuous operation through holding and coolingvstages and in that heat released from the «~- cooling stages may be utilized in the digestion. Our invention contemplates an essentially simpli iled process with provision for carrying it out in a system constructed substantially on one level with few moving parts and requiring a minimum of capital expense and operating labor. Provision is made for emciently heating the digestion with steam without direct contact so that dilution due to Vthe condensation of the steam is avoided. ' Owing to the elimination of the direct injection of steam as a heating means it has been found possible to usefully employ very low pressure >steam for heating, as there is no direct mechani cal relationship between the pressure in the steam system 4and the pressure in the digesting system. This gives a greater ñexibillty of control than would otherwise be possible. With independent control over the temperatures and pressures and working with a substantially uniform flow of un Si diluted digest mixture it has been found practical to make a separation of the caustic digest liquor (Cl. 23-52) from the solid residue after dlscstion at an ele vated temperature equal to or greater than the normal boiling point of the liquor under atmos pheric pressure, as will be further described here inbelow. . ~ o It has long been a desideratum to apply steam heat indirectly to a batch pressure digestion to avoid the dilution arising from heating by direct injection of steam.. This has proved diillcult, as the digestion mixture precipitates solidson the 10 heated surfaces, which seriously interfere with the transfer of heat. In a batch process this dim culty may be overcome to a certain extent by employing high pressure steam and carefully cleaning the heating surfaces between batches. l5 In a continuous .heat regenerative- digestion process this problem has presented a serious dim cuity because. no' opportunity is afforded for cleaning the- heating surfaces and a layer of solid material of increasing thickness is continually 20 formed. Furthermore, lin a heat regenerative system the dißerence in temperature between the material to be heated and the heating medium is small, and a material resistance to heat ilow'or transfer between-the two substantially negatives 25 the advantages of such a regenerative process. v In our solution of-this problem it was found that the caustic liquor, which at times contains small amounts of alumina and other substances in solu tion from prior treatments, may be satisfactorily superheated in respect to the desired digestion temperatures in known types of heaters, such as a straight tube heat exchanger, without the for mation of scale on the heater` walls. vIt was also found that a substantially complete extraction of 35 all available alumina from a1umina~bearing material might be made by treating such mate rial with superheated caustic liquor to produce a digestion mixture at a'predetermined digestion temperature. In actual practice it has been 40 found desirable to wet the alumina-bearing mate-_ rial with sufllcient caustic liquor to form a thick slurry which can be conveniently introduced into the digestion process under pressure, as by pump ing. » ’ In the preferred practice of our invention the 45 caustic liquor is divided. This liquor may be at room temperature or at a somewhat elevated tem- ' perature, depending upon the general operation -of which this process may form a part. One por- 50 tion is passed under pressure through 'a step of indirect heating, which may consist of several stages of increasing temperature until the liquor has been given a superheat with respect to the contemplated digestion temperature. The other 55 aimais . portion, preferably the smaller portion, is added I4 which may be a tubular heater, as indicated, to the alumina-bearing material to form a ,thick slurry which is subsequently maintained under pressure. -The two portions are then combined under pressure and the superheat of the liquor through which it passes and is partially brought to a superheated condition by heat supplied by portion is absorbed by the slurry portion to pro duce a digestion mixture having a temperature predetermined by the degree of lsuperheat in the liquor. The digestion mixture'is passed through 10 a plurality of holding stages under these condi tions of `temperature and pressure until substan ` tially all of the available alumina is dissolved by the caustic liquor. Under kthese conditions the digest solution is rendered stable and there is no 15' precipitation of alumina, which assures a. maxi mum yield. The mixture is then passed through` a plurality of cooling stages. Throughout the ' entire cycle of operation a constant flow of diges tion mixture from step to step and from stage to During each cooling stage heat is absorbed in the evaporation of water/from the liquor under reduced pressure. This heat may be applied to the liquor-heating stages and, owing to the use of indirect heating, steam generated atsubstantially any pressure in 20 stage is preferably maintained. the cooling stages may be efficiently utilized in supplying heatto the heating steps. Even very low pressure exhaust steam may be used 4in the initial stages of liquor heating. 30 i » For the purpose of explanation, reference will be had to the drawing which forms a part of this specification and which represents, in schematic outline, a flow sheet of a process vembodying .this invention in the extraction of alumina from 35 alumina-bearing material. Referring. to the drawing, the apparatus which may be used in the practice of our inven tion comprises proportioning feed hopper I for the alumina-bearing material and caustic liquor 40 proportioning tank 2, slurry mixer 4, surge tank 6, tubular liquor heaters I4, I5, I6 and I1, diges ters II, 2I, 22, 23, 24 and 25, cooling chambers 26, 21 and 28, thickener 44, and tank-connecting and mixture-transferring means hereinafter re 45 ferred to. The slurry mixer andthe digesters are equipped with revolving stirring arms 52 to prevent the solids settling from the liquor. It will be understood that while the process will be described as having specifically _four heating 50 stages and three cooling stages and six digesting vessels, this is merely by way of example, and fewer or more stages may be used to best satisfy any particular condition. „ ‘ The reagents, that is, alumina-bearing mate Irial, which may be bauxite, and caustic liquor, by which is meant solutions 'of potassium or l sodium hydroxide which may contain in addi tion small amounts of other substances, such as carbonates and aluminates, are drawn continu 60 ously at a constant rate or in batches from the tanks I and 2 respectively. The bauxite passes low pressure steam through the tube walls. In practice it is desirable to use oversize pumps and reduce the flow by valves- in the pump discharge line, such as 9a and I2a. From the heater I4 the liquor passes through similar heaters I5 and I6 which are heated progressively by somewhat higher pressures of steam to higher tempera 10 tures, after whichA the liquor finally passes through a heater 'I1 which is heated with still higher pressure steam, which lraises the tempera- , ture of the liquor to a superheat with respect to the other temperatures employed in the process. 15v The superheated liquor from l1 passes through. pipe line I8 into the digester II. The superheat of the liquor is given up to the slurryl from mixer 4 and may be adjusted to give the desired diges-tion temperature by means of the degree of__su 20 perheat, which in turn is regulated by the ther mostatic steam valve I9 on the steam supply l line 20. From the digestion vessel I I the mixture vpasses through similar digesters 2|, 22, 23, 24 and 25 25 under substantially the conditions of tempera ture and pressure maintained in digester II. During this passage the alumina is in a large part extracted in digesters II and 2|.> Digesters 2|, 22, 23, 24 and 25 serve aslholding digesters to 30 allow sufficient time to elapse to substantially complete the digestion or solution of all of the available alumina andl to stabilize the digestion mixture. v _ ` The digestion mixture, which now consists of a caustic aluminate solution and the insoluble residue from the bauxite, passes from digester 25 to the cooling tanks or blow-off- tanks 26, 21 and 28. The digestion mixture is released con tinually from digester 25 through the pipe line 40 29 and the control or blow-off valve 30 to tank 26. 'I’he pressure'inV tank 26 is lower than the digestion pressure and steam isv released. The steam passes through pipe 3I'and control valve 32 to the tubular heater I6. The pressure in tank 26 is largely controlled or set by valve 32 and is higher than the pressure in tank, 21. The mixture therefore flows from tank 26 to tank 21 through the loop pipe seal 33 and the valve 34. Steam is again released at lower pressures and passes through the pipe line 35 and valve 36 to heater I5. The cooling mixture is .still under substantial pressure and ñows through loop seal pipe line 31 to tank 28. Steam is re leased in this tank and is led to heater I4 through the pipe 38 and the control valve 39. The mix ture is drawn from tank 28 by a pump 48 through pipe 4I and delivered to ñlters for separating the residue from the liquor, not shown, which is a step preliminary to treating the caustic liquor 60 through the proportioning apparatus 3 directly for the recovery of alumina therefrom in known manner, which treatment forms no part` of this into a slurry mixer 4. The caustic liquor passes ’ invention. through the pipe line 5 which has two branches, l65 onelleading to the slurry mixer 4 and the other to the clear liquor surge tank 6. The two branch lines are equipped with proportioning orificesr 1 ‘and 8 which permit a measured flow of both streams. Orifice 1 preferably discharges about 70 one-tenth of the liquor and orifice 8 about nine-tenths. A pump 9 draws thev slurry from .the mixer 4 and forces it through the pipe line I0 into a pressure digesting vessel Il. Pump I2 forces clear caustic liquor- from the surge 75 tank s through the une la to the heater' - ' In this process the mixture of bauxite and liquor is digested without dilutionfrom the con 65 densate of heating steamand,V in fact, a certain not unsubstantial amount of concentration is ef fected by the amount of steam released in the blow-off tanks. It is sometimes desirable, for instance, in order to reduce capital: expense, to 70 heat the -mixture of caustic liquor from the re generative heaters I4, I5 and I6 and slurry from mixer 4 by the direct injection of high pressure steam in the digester II` and thereby eliminate the 'superheater I1. In this case the digestion u 2,107,919 temperature is controlled by the thermostatic valve 42 in the steam supply line 26. In this modiñcation of our invention all -of the advan 3 through the discharge pipe 49 by any suitable form of sludge pump, not shown. The clear liq ucr overflows into the tank 50 and flows through are preserved unaffected, and because of the fact - the valve 46 into the loop seal pipe line 33 to tank that the digestion mixture is partially heated 21. The pipes 5I equalize the pressure through out the thickening steps and maintain a pres indirectly and to a degree concentrated, satis sure substantially equal to the pressure of tank factoryoperation may be secured by the injec tion of high pressure steam to effect the last 26, and the thickener is of course thus main stage ofheating.- If desired, a combination of tained under approximately the temperature of tank 26. _The conditions of thickening above set 10 both means of heating may be used. 1 Primarily,l the flow of materials through the forth have been found to materially increase the process is controlled by the pumps 9 and I2 and settling rate of ,the solid material from the mix tages of the various steps of heat regeneration is valves 9a and I2a, which maintain a pressure at least Aequal to the pressure in the digesters, and the blow-off valve 3II. Secondarily, the ñow is `controlled by the valves 32, 36 and 39 and the pumplw. `If desired, the elevation of the blow-off tanks 26, 21 and 23 may be adjusted to secure the desired action through the loop seals 33 and 31. The temperature of the process is controlled primarily by valves I9 and 42 which regulate the primary source of heat; Secondar ily, the temperature is controlled through the ture, which serves to decrease the size of the thickener or settling equipment. For example, the settling rate may be increased ñfty per cent or more by elevating the tempera-ture. 'I'he tem perature is preferably increased above the nor- ' mal boiling point of the liquid at atmospheric pressure.' This remarkable change in settling rate is thought to be due principally to a change in viscosity of the liquor with temperature, but " it is not desired to limit the fact in any way by this explanation. , _ . valves 32, 36 and 39 and the orifices 1 and 3. It will be noted that in the operation of this the process. The digesters are kept filled with material at all times by the pumps 9 andl I2 and the blow-off valve 30. The range of possible ad to apply heat at one point, heater I1 or digester` II, which is done with steam pressurel as from the steam supply line 23, which in turn may be connected to any source of steam >oi' sumcient They ñow of materials is continuous throughout '_ process now being described it is necessary only justmentsof valves 32, 36 and 39 is very wide, as the steam entering the heaters I4, I5 vand I6 has no mechanical relation to the pressure main tained on the digestion solution, and the heat can therefore be supplied by steam at -any de sired pressure having the desired temperature within the range of the pressure of the released steam. This gives the utmost flexibility in con trolling the conditions of material flow, temper atur'e and pressure. 40 - « ,. In such a process having a continuous flow of materials at predetermined controllable pres sures and temperatures as described, it has been found practical to insert a step of thickening or separation of the solid residue dispersed in the pressure. . ' ~ Although we have described our invention with reference to .a particular embodiment thereof, it will be apparent that it is by no means limited to the details of operation, temperature, pres sure, composition or substances described` but may be> utilized to advantage in any and all similar methods of pressure digesting aluminous material or other solids with a liquid. We claim: 1. A method of continuously pressure digest ing solids with a solvent, comprising forming a slurry of the solids with some of the solvent, and indirectly heating another portion of. the solvent a plurality .of successive stages of increasing caustic aluminate liquor after the digestion is in temperature to> a predetermined superheat with complete. The residue may be, therefore, in this respect to the slurry, combining the slurry and case, discharged separately from the process as ' a thick sludge containing a small amount of liq uor and the extract liquor discharged substan .50 tially clear an'd containing very little residue. . the solvent under pressure to obtain a predeter mined digestion temperature, digesting the mix ture in a plurality of stages, then cooling the mixture in a plurality of separate stages of de The technical prerequisites for such a separation are that the ñow of material through the thick ener be constant, or'nearly so, and that- the creasing. temperature and pressure, controlling treated in the thickener be small, and`v that the settling rate be rapid; otherwise, the size of thick por- into heat exchangers which prevent contact the decrease in pressure between successive cool ing stages so' as to release vapor from the sol - temperature and the c'oncentration of dissolved vvent in such- quantity that it is adaptedA tov sup 55 material be such that no precipitation occurs in A vply heat to several. of the solvent-heating stages. 1. the thickener. The practical prerequisite is that heating of the solvent in several such stages in addition the amount (i. e., the volume) to be the being accomplished by passing said released va 65 between said vapor and said solvent, and main ener necessary may become larger than can be taining a flow of materialsthroughout all the economically operated under the» conditions of. steps of the process. . ~ temperature and pressure, or the time required 2. A method of continuously extracting alu may be so long as to negative any advantage mina from aluminous material, which comprises in its use. In -the case of vextracting alumina forming a slurry of said aluminous material and with caustic liquor as described, the.thickening step may .be placed, for example, Vbetween the ñrst two stages of cooling, that is, between blow olf tanks 26 and 21. This is shown in the draw ` ing. The enclosed thickener 44 may be connected 70 in the system in the-„loop seal pipe line 33 by caustic liquor, indirectly heating `a further amount of caustic liquor in a plurality of suc 65 cessive lseparate stages of increasing tempera tures to a temperaturey such that when com bined with the slurry a predetermined tempera closing valve 34 and opening valves 45, 46 and 41. ture of the mixture is attained, combining the 70 and the heated caustic liquor under pres This -mixture discharged from tank 26 ñows slurry sure, and digesting under suitable conditions of through valve 45 andV pipe 43 into the thickener temperature and pressure until substantially all » below the liquid level where the residue settles 76 in the cone bottom and is drawn oil.' as a sludgeA of the alumina is in solution, then cooling the mixture in a plurality of» separate stages of de 76 4 2,107,919 creasing temperature and pressure, predetermin ing the decrease in pressure between successive cooling stages so as to release steam from the caustic liquor in such quantity that it is adapted to supply heat for several of the caustic liquor heating stages by passing said released- steam into heat exchangers which prevent contact be tween said steam and said caustic liquor, and maintaining a ñow of materials- throughout all 10 the steps of the process. 3. A method of continuously extracting alu mina from. aluminous material, which comprises forming a slurry of said aluminous material and caustic liquor, indirectly heating a further to a temperature higher than that of the slurry such that~ when combined with said slurry the temperature of the resultant mixture is increased to a predetermined digestion temperature, com bining said slurry and heated caustic liquor under pressure, digesting the mixture under controlled conditions of temperature and pressure until sub stantially all of the alumina is in solution, and ` maintaining a ilow of materials throughout all 10 the steps of the process. '7. vIn apparatus for extracting solids with sol vents by continuous pressure digestion, the com bination comprising a mixing unit adapted to form a pumpable slurry from a solid to be ex tracted and a solvent, a plurality of digesting tanks, means for continuously supplying said ' sive separate stages of increasing temperature slurry tov said digesting tanks, means forl continu to a suiilcient temperature so that when com ously supplying additional solvent to said digest bined with the _slurry a predetermined tempera ture of the mixture is attained, combining the- ing tanks in an amount having a predetermined ratio to the amount supplied to said mixing unit, 20 slurry fand the heated caustic liquor and digest ing under suitable conditions. of temperature and a plurality of> heat exchange units adapted to ' pressure until substantially all of the alumina raise the temperature of said additional solvent in progressive steps, a plurality of cooling cham is in solution, then cooling the mixture in a plu amount of caustic liquor in a plurality of succes rality of separate stages of decreasing tempera bers connected in series, transfer means adapted steam is released from the caustic liquor in such means for regulating the pressure in the digest to deliver solution and undissolved solids from 25 25 ture and pressure, the decrease in pressure be-> » said digesting tanks to said 'cooling chambers, tween successive cooling stages being such that quantity that it is adapted to supply heat for' several of the caustic liquor heating stages, the '30 heating of the caustic liquor in several such >stages being accomplished 'by passing said re leased steam and caustic liquor into heat ex changers which prevent contact between said steam and said caustic liquor, separating by sed 35 imentation the undissolved portion from the liq uor solution at a temperature higher than the ing tanks, control means between adjacent cool ` ing chambers in said series adapted to maintain progressively decreasing pressures in said _cham 30 bers, and means for conveying the vapor released in atleast Ione of said cooling chambers to at least one of said heat exchange units. ' 8. In apparatus for extracting solids with sol vents by continuous pressure digestion, the com 35 bination comprising a surge tank for solvent, a boiling point of said liquor at atmospheric pres mixing unit adapted to form a pumpable slurry minous material, which comprises, in combina unit, means for supplying solvent to said surge sure„ and maintaining a ì ilow of materials , from a solid to be extracted and a solvent, means for supplying said solid to said mixing unit, throughout all the steps of the process. ' 4. Apparatus for extracting alumina from alu- g means for supplying said solvent to said mixing tion, means for forming a slurry of aluminous material and caustic liquor, means for indirectly 45 heating caustic liquor in a plurality of separate stages of increasing temperature, means for com bining and digesting the slurry and caustic liquor under controlled conditions of temperature and pressure, means for cooling the digestion product in a plurality of successive stages of decreasing 50 temperatures and pressures, means >for indirect heat exchange between said heating and cooling means, and means for maintaining a constant ilow of material throughout. 5. Apparatus for extracting alumina from alu 55 minous material, which comprises, in combina tank, a plurality of digesting tanks connected in series, means for continuously supplying said slurry to the ñrst of said series of digesting tanks from said mixingunit, means for continuously supplying additional solvent to the first of said series of digesting tanks from said surge tank in an amount- Ihaving a predetermined ratio to the amount supplied to said mixing unit, a plurality of heat exchange units- interposed in the supply means for additional solvent and adapted to raise the temperature of said additional solvent in progressive steps, a plurality of cooling chambers connected in series, transfer means adapted to deliver solutionand undissolved solids from the tion, means for forming a slurry of aluminous last of said series of digesting tanks to the iìrst material and caustic liquor, means for indirectly~ ‘of said series of cooling chambers and including -heating caustic liquor in a plurality of separate- means for regulating pressure in the digesting tanks, control _means between adjacent cooling -stages of increasing temperature, means for con chambers in said series adapted to maintain pres 60 and for digesting the mixture under controlled sures in said chambers decreasing progressively conditions of temperature and pressure,V means _in steps from the digestion pressure, and means for cooling the digestion product in at least two for conveying vapor released by the decrease of stages of decreasing temperatures and pressures, pressure in at least one of said cooling chambers 65 means for indirect heat exchange between said to at least one of said heat exchange units. 9. In apparatus for extracting solidsA with sol heating and cooling means, means for separating vents by continuous pressure digestion, the com insoluble portions by sedimentation from the di bination comprising a mixing unit adapted to gested liquor solution, and means for maintain form a pumpable slurry from a solid to be ex ing a constant ilow of material throughout. tracted and a solvent, a plurality of digesting 70 6. A method of continuouslyextracting alu mina from aluminous materials with caustic tanks, means for continuously supplying slurry - liquor, which comprises dividing the' caustic to said'digesting tanks from said mixing unit, liquor into major and minor portions, forming a means for continuously supplying additional sol yslurry of the aluminous material with the minor vent to said digesting tanks, a plurality of cooling chambers connected in series, transfer means 'ItA 76 portion and heating the major portion indirectly 60 trollably combining the slurryand caustic liquor 2,107,919 adapted to deliver _solution and undissolved solids fromsaid ldigesting tanks tothe ñrst of said series of cooling chambers, means for regulating the pressure in the digesting tanks, control means between adjacent cooling chambers in said series adaptedvto maintain-pressures in said chambers tity oi! caustic liquor by indirect contact with steam, digesting said slurry and heated caustic liquor in a closed vessel maintained at the desired superatmospheric pressure and elevated temper ature by direct contact with steam to dissolve alu mina, reducing the pressure of the resulting so~ decreasing in steps from the digestion pressure, lution and withdrawing steam liberated thereby, and returning the steam to indirectly heat'a fur and a closed sedimentation tank positioned be ther quantity of caustic liquor. tween and operatively connected with two adja 13. ’I'he method of extracting alumina from cent cooling chambers in said series and adapted » aluminous material, comprising digesting alumi to separate solids from solution under substan nous material' with preheated caustic liquor in a tially the conditions of superatmospheric pres sure and elevated temperature maintained in the closed vessel, maintaining the pressure above at mospheric and the temperature above the boiling ilrst of said connected cooling chambers. 10. In apparatus for extracting solids with a l point at iatmospheric pressure by injecting steam 15 solvent by continuous pressure digestion, the into said vessel, separating the resulting solution ‘ combination comprising a mixing tank adapted from undissolved residues by sedimentation at vto Iorm a slurry of solid and solvent, at least one elevated temperature and superatmospheric pres heat exchange unit, at least one digesting unit, ' sure, reducing the pressure on said solution by at least one cooling chamber, controlled means withdrawing vapor liberated thereby, and em 20 for delivering slurry from said mixing tank to ploying said vapor to preheat a further quantity 'said digesting unit, means for passing a controlled , of said caustic liquor.4 quantity of ,additional solvent through saidv heat exchange unit and into said digesting unit at a 25 controlled temperature and pressure, and means for delivering material from said digesting unit to said cooling chamber- while maintaining-the desiredvpressure in the digesting unit. 11. The method of extracting alumina from aluminous material >with caustic liquor, compris ing mixing part of the caustic liquor with alumi "' nous material to form a slurry, heating a further quantity of the caustic liquor, mixing the heated liquicr with the slurry, digesting the mixture at 35 an elevated temperature and superatmospheric pressure, and separating >the resulting solution from undissolved residues by sedimentation at a temperature -above the boiling point of tion at atmospheric pressure. 40 ' 12. The method of extracting alumina from aluminous material with causticliquor, compris ing mixing solid aluminous material with caustic ' liquor to form a slurry, `heating a further quan ` 14. The methodvof extracting alumina from aluminous material, comprising digesting alumi nous material with caustic liquor atsuperat 25 mospheric pressure and at a temperature higher than the boiling point of the liquor at atmos pheric pressure, land. separating the resulting so lution from undissolved residues by sedimenta tion at elevated temperature and superatmos 30 pheric pressure. ` , . 15. The method of extracting alumina from aluminous material, comprising digesting alum! nous material with caustic liquor at superat» mcspheric pressure and at a temperature higher 35 than the boiling point of the liquor at atmos pheric pressure, and maintainingv the resulting solution and undissolved residues at superatmos pheric pressure and elevated temperature until said residues are separated'from the solutionby sedimentation. CARRELL ORISON TURNER.“ MELVIN POWELL WEIGEL.