3_Leaching.pdf

LEACHING Solid- Liquid Extraction  Solid-Liquid Extraction Solid-liquid extraction is the process of separation of soluble constituents of a solid material using a Suitable solvent. Involves four steps: (i) intimate contact between the solid feed with the solvent (ii) separati...

LEACHING Solid- Liquid Extraction  Solid-Liquid Extraction Solid-liquid extraction is the process of separation of soluble constituents of a solid material using a Suitable solvent. Involves four steps: (i) intimate contact between the solid feed with the solvent (ii) separation of the solution (or extract) from the exhausted solid (iii) separation of the solvent from the extract followed by purification of the product (iv) recovery of the solvent from the moist solid CLASSIFICATION OF SOLID-LIQUID EXTRACTION SYSTEMS ¢ Diffusional extraction :Almost the entire mass transfer resistance lies in the solid phase ¢ Washing extraction: If the solid particle size is pretty small, the solid-phase diffusional resistance becomes small. Extraction becomes a process of a washing the solid with the solvent. ¢ Leaching: Dissolution of one or more substances from solid particles accompanied by chemical reaction(s). An acid, alkali or the solution of a complexing chemical is used for solubilizing the target materials.  Introduction Leaching is the preferential solution of one or more constituents of a solid mixture by contact with a liquid solvent. This unit operation, one of the oldest in the chemical industries, has been given many names, depending upon the technique used for carrying it out. Leaching originally referred to percolation of the liquid through a fixed bed of the solid The term extraction is also widely used to describe this operation in particular, although it is applied to all the separation operations as well Decoction refers specifically to the use of the solvent at its boiling temperature. When the soluble material is largely on the surface of an insoluble solid and is merely washed off by the solvent, the operation is sometimes called elutriation or elution. The metallurgical industries are the largest users of the leaching operation. Most useful minerals occur in mixtures with large proportions of undesirable constituents, and leaching of the valuable material is a separation method which is frequently applied.  Examples Copper minerals are preferentially dissolved from certain of their ores by leaching with sulfuric acid or ammoniacal solutions Gold is separated from its ores with the aid of sodium cyanide solutions. Leaching similarly plays an important part in the metallurgical processing of aluminum, cobalt, manganese, nickel, and zinc. Many naturally occurring organic products are separated from their original structure by leaching. For example, v sugar is leached from sugar beets with hot water, Y Vegetable oils are recovered from seeds such as soybeans and cottonseed by leaching with organic solvents, Y Tannin is dissolved out of various tree barks by leaching with water, Many pharmaceutical products are similarly recovered from plant roots and leaves. Tea and coffee are prepared both domestically and industrially by leaching operations.  Preparation of the Solid: Crushing and grinding of solids For the successful leaching , prior treatment be given to the solids. In some instances, small particles of the soluble material are completely surrounded by a matrix of insoluble matter. The solvent must then diffuse into the mass, and the resulting solution must diffuse out, before a separation can result. This is the situation with many metallurgical materials. Crushing and grinding of such solids will greatly accelerate the leaching action, since then the soluble portions are made more accessible to the solvent. A certain copper ore, for example, can be leached effectively by sulfuric acid solutions within 4 to 8 h if ground to pass through a 60-mesh screen, in 5 days if crushed to 6-mm granules, and only in 4 to 6 years if 150-mm lumps are used. Since grinding is expensive, the quality of the ore will have much to do with the choice of size to be leached. For certain gold ores, on the other hand, the tiny metallic particles are scattered throughout a matrix of quartzite which is so impervious to the leaching solvent that it is essential to grind the rock to pass through a 100- mesh screen if leaching is to occur at all. Vegetable and animal bodies are cellular in structure, and the natural products to be leached from these materials are usually found inside the cells. If the cell walls remain intact upon exposure to a suitable solvent, the leaching action involves osmotic passage of the solute through the cell walls. This may be slow, but it is impractical and sometimes undesirable to grind the material small enough to release the contents of individual cells. Sugar beets are cut into thin, wedge-shaped slices called cossettes before leaching in order to reduce the time required for the solvent water to reach the individual plant cells. For many pharmaceutical products recovered from plant roots, stems, and leaves, the plant material is frequently dried before treatment, and this does much toward rupturing the cell walls and releasing the solute for direct action by the solvent. Vegetable seeds and beans, such as soybeans, are usually rolled or flaked to give particles in the size range 0.15 to 0.5 mm. The cells are smaller than this, but they are largely ruptured by the flaking process, and the oils are then more readily contacted by the solvent. When the solute is adsorbed upon the surface of solid particles or merely dissolved in adhering solution, no grinding or crushing is necessary and the particles can be washed directly.  Temperature of Leaching It is usually desirable to leach at as high a temperature as possible. Since higher temperatures result in higher solubility of the solute in the solvent, higher ultimate concentrations in the leach liquor are possible. The viscosity of the liquid is lower and the diffusivities larger at higher temperatures, leading to increased rates of leaching. With some natural products such as sugar beets, however, temperatures which are too high may lead to leaching of excessive amounts of undesirable solutes or chemical deterioration of the solid.  Methods of Operation and Equipment Leaching operations are carried out under batch and semibatch (unsteady-state) as well as under completely continuous (steady-state) conditions. stage-wise and continuous-contact In each category, both types of equipment are to be found. Two major handling techniques used are : - Spraying or trickling the liquid over the solid, - immersing the solid completely in the liquid. The choice of equipment to be used in any case depends greatly upon the physical form of the solids and the difficulties and cost of handling them. This has led in many instances to the use of very specialized types of equipment in certain industries.  UNSTEADY-STATE OPERATION The unsteady-state operations include those where the solids and liquids are contacted in purely batch-wise fashion and the batch of solid is contacted with a continually flowing stream of the liquid (semi-batch method). Coarse solid particles are usually treated in fixed beds by percolation methods, whereas finely divided solids, which can be kept in suspension more readily, can be dispersed throughout the liquid with the help of some sort of agitator. In-Place (in Situ) Leaching This operation, also called solution mining, refers to the percolation leaching of minerals in place at the mine, by circulation of the solvent over and through the ore body. In the solution mining of uranium, the ore must be oxidized in place in order to solubilize it in carbonate solutions. The reagent may be injected continuously through one set of pipes drilled down to the ore and the resulting liquor removed through different set. a  Heap Leaching e Low-grade ores whose mineral values do not warrant the expense of crushing or grinding can be leached in the form of run-of-mine lumps built into huge piles upon impervious ground. ¢ The leach liquor is pumped over the ore and collected as it drains from the heap. ¢ Copper has been leached from pyritic ores in this manner in heaps containing as much as 2.2 x 10' tones of ore, using over 20 000 m? (5 x 10° gal) of leach liquor per day. It may require up to 7 or more years to reduce the copper content of such heaps from 2 to 0.3 percent.  Percolation Tanks Solids of intermediate size can conveniently be leached by percolation methods in open tanks. The construction of these tanks varies greatly, depending upon the nature of the solid and liquid to be handled and the size of the operation, but they are relatively inexpensive. Small tanks are frequently made of wood if it is not chemically attacked by the leach liquid. Small tanks may also be made entirely of metal, with perforated false bottoms upon which filter cloth is placed, as in the leaching of a pharmaceutical products from plants. Very large percolation tanks (45 by 34 by 5.5 m deep) for leaching copper ores have been made of reinforced concrete and lined with lead or bituminous mastic. Small tanks may be provided with side doors near the bottom for sluicing away the leached solid, while very large tanks are usually emptied by excavating from the top. ¢ Tanks should be filled with solid of as uniform particle size as practical, since then the percentage of voids will be largest and the pressure drop required for flow of the leaching liquid least. ¢ This also leads to uniformity of the extent of leaching individual solid particles and less difficulty with channeling of the liquid through a limited number of passageways through the solid bed. ¢ The operation of such a tank as follows: - After the tank is filled with solid, batch of solvent sufficient to a immerse the solid completely may be pumped into the tank and the entire mass allowed to steep or soak for a prescribed period of time. - During this period the batch of liquid may or may not be circulated over the solid by pumping. The liquid is then drained from the solid by withdrawing it through the false bottom of the tank. - This entire operation represents a single stage. Repetition of this process will eventually dissolve all the solute. An alternative method is continuously to admit liquid into the tank and continuously to withdraw the resulting solution, with or without recirculation of a portion of the total flow. Such an operation may be equivalent to many stages. Since the solution which results is usually denser than the solvent, convective mixing is reduced by percolation in the downward direction. Upward flow is sometimes used, in order to avoid clogging the bed or the filter with fines, but this may result in excessive entrainment of the fines in the overflow liquid. Still a further modification, less frequently used, is to spray the liquid continuously over the top and allow it to trickle downward through the solid without fully immersing the solid at any time.  Countercurrent Multiple Contact ¢ The Leaching and washing of the leached solute from the percolation tanks by the cross-current methods described above will result in weak solutions of the solute. ¢ The strongest solution will result if countercurrent a scheme is used, wherein the final withdrawn solution is taken from contact with the freshest solid and the fresh solvent is added to solid from which most of the solute has already been leached or washed. ¢ In order to avoid moving the solids physically from tank to tank in such a process, the arrangement shown schematically in the fig. for a system of six tanks, is used. This is called as Shanks system  Shanks system A Spent solid Liquid Frash OSS? _ flow solid 4 ~ ~~.6 2 2 3 3 Concentrated solution (ga) (d) Tank 6 is empty, tanks 1 to are filled with solid, tank e5 most recently and tank 5 1 for the longest time. Tanks 1 to are also filled with leach liquid, and the 5 most concentrated is in tank 3 since it is in contact with the freshest solid. 5 Fresh solvent has just been added to tank I. Withdraw the concentrated solution from tank 5, transfer the liquid from tank 4 to tank 5, from 3 to 4, from 2 to 3, and from 1 to 2. Add fresh solid to tank 6.  Spent solid Liquid Fresh Y//7 flow solid 4 2 2 3 Concentrated solution ) 1b) Refer fig. b: Discard the spent solid from tank 1. Transfer the liquid from tank 5 to tank 6, from 4 to 5, from 3 to 4, and from 2 to 3. Add fresh solvent to tank 2. The circumstances are now the same as they were at the start in Fig. 13.2a, except that the tank numbers are each advanced by one. Continue the operation in the same manner as before. After several cycles have been run through in this manner, the concentrations of solution and in the solid in each tank approach very closely the values obtaining in a truly continuous counter-current multistage leaching. The system can, of course, be operated with any number of tanks, and anywhere from 6 to 16 are common. The tanks may be placed at progressively decreasing levels, so that liquid can flow from one to the other by gravity with a minimum of pumping.  Percolation in Closed Vessels e When the pressure drop for flow of liquid is too high for gravity flow, closed vessels must be used and the liquid is pumped through the bed of solid. Such vessels are sometimes called diffusers. e Closed tanks are also necessary to prevent evaporation losses when the solvent is very volatile or when temperatures above the normal boiling point of the solvent are desired. Filter-Press Leaching Finely divided solids, too fine for treatment by percolation in relatively deep percolation tanks, can be filtered and leached in the filter press by pumping the solvent through the press cake. This is, of course, common practice in washing mother liquor from precipitates which have been filtered.  Agitated Vessels Solid and Channeling of the solvent in percolation or filter-press solvent | leaching of fixed beds, with its consequent slow and icomplete leaching, can be avoided by stirring the liquid and solid in leaching vessels. In such cases, closed cylindrical vessels are arranged vertically and are fitted with power-driven paddles, or stirrers on vertical shafts, as in Fig. a, as well as false bottoms for drainage of the leach solution at the end of the -_ Solid operation. In others, the vessels are horizontal, as in Fig. b, with the Solution | stirrer arranged on a horizontal shaft. (a) In some cases, a horizontal Sotid and Solid and } solvent drum is the extraction solvent vessel, and the solid and liquid are tumbled about { Solid inside by rotation of the drum on rollers, as in Fig. c. I I aE _- songd | Solution (0) Solution (c)  Detlector Finely divided solids can be suspended in leaching solvents by agitation, and for batch operation a variety of agitated vessels are Hoops used. 3 Wood stoves HW _ The simplest is the Pachuca tank (Fig.), which is employed extensively in the metallurgical Pp. Air-lift tube industries. These tanks are constructed of wood, metal, Concrete or concrete and may be lined with inert metal a g Z such as lead, depending upon the nature of hir tor =k ; itt Z Air for loosening f\ settled the leaching liquid. solids vA Agitation is accomplished by an air lift: the ad Sond bubbles of air rising through the central tube cause the upward flow of liquid and suspended solid in the tube and consequently vertical circulation of the tank contents Slot tor gate L bn Section 4-4  Percolation vs. Agitation of large lumps is to be leached, a decision must If a solid in the form frequently be made whether to crush it to coarse lumps and leach by percolation or whether to grind it fine and leach by agitation and settling. No general answer can be given to this problem because of the diverse leaching characteristics of the various solids and the values of the solute Fine grinding is more costly but provides more rapid and possibly more thorough leaching. It suffers the disadvantages that the weight of liquid associated with the settled solid may be as great as the weight of the solid, or more, so that considerable solvent is used in washing the leached solute free of solute and the resulting solution is dilute. Coarsely ground particles, on the other hand, leach more slowly and possibly less thoroughly but on draining may retain relatively little solution, require less washing, and thus provide a more concentrated final solution. For more fibrous solids, such as sugar cane, which is leached with water to remove the sugar, the leaching is generally more efficient in a thoroughly agitated vessel than by percolation.  Assignment Dorr Agitator Dorr Thickener Dorr Balanced tray Thickener Hydro cyclones Rotocel Kennedy Extractor Bollman Extractor Continuous Horizontal Extractor  METHODS OF CALCULATION It may be necessary to compute the number of washings, or number of stages, required to reduce the solute content of the solid to some specified value, knowing the amount and solute concentration of the leaching solvent. The methods of calculation are very similar to those used for liquid extraction. Stage Efficiency Consider a simple batch leaching operation, where the solid is leached with solvent to dissolve all the soluble solute and there is no preferential adsorption of either solvent or solute by the solid. If adequate time of contact of solid and solvent is permitted, all the solute will be dissolved, and the mixture is then a slurry of insoluble solid immersed in a solution of solute in the solvent. The insoluble phases are then separated physically by settling, filtration, or drainage, and the entire operation constitutes one stage. If the mechanical separation of liquid and solid were perfect, there would be no solute associated with the solid leaving the operation and complete separation of solute and insoluble solid would be accomplished with a single stage. This would be an equilibrium stage, of 100 percent stage efficiency. In practice, stage efficiencies are usually much less than this: (1) the solute may be incompletely dissolved because of inadequate contact time; (2) most certainly it will be impractical to make the liquid-solid mechanical separation perfect, and the solids leaving the stage will always retain some liquid and its associated dissolved solute. When solute is adsorbed by the solid, even though equilibrium between the liquid and solid phases is obtained, imperfect settling or draining will result in lowered stage efficiency. SOLID-LIQUID EXTRACTION EQUILIBRIUM solid A 'inert' (or "carrier') containing a soluble substance C distributed is extracted with the solvent B for a sufficient time Physicochemical phenomena (a) Ifthe solid is in the form of fine particles or flakes, the solid-phase diffusional resistance is small. Given sufficient time, the system will reach 'equilibrium'. Here equilibrium means that the concentration of the solute in the clear bulk solution will be equal to that in the liquid retained in the slurry. The amount of liquid or solution retained in the slurry depends upon the characteristics of the solid and the density and viscosity of the solution. (b) If the solid is in the form of lumps or slices, the concentration of the bulk solution and that of the intersticial or retained solution will again be equal at 'equilibrium'. However, it may take a substantially longer time to attain equilibrium. (c) If the solute reacts with a reagent present in the solution, leaching occurs till either the solute or the reagent is exhausted. "equilibrium data' for design calculations are reported in terms of the concentration of the solute in the clear liquid (called overflow) and the fraction of the liquid in the slurry (called underflow) Overfiow(100 kg), solution Underflowf (100 kg), slurry Wa (kg) Wa (kg) We (kg) Wa (kg) wa(kg) w(kg) 0.3 99.7 0.0 67.2 28 0.0 0.45 90.6 8.95 67.1 29.94 2.96 0.54 84.54 14,92 66.93 2.811 4.96 0.70 74.47 24.83 66.58 25.06 8.36 0.77 69.46 29.77 66.26 23.62 10.12 0.91 60.44 38.65 65.75 20.9 13.35 0.99 54.45 4,455 65.33 19.07 15.6 1.19 44.46 54.35 64.39 16.02 19.59 1.28 38.50 60.22 63.77 14.13 22.10 1.28 34.55 64.17 63.23 12.87 23.90 1.48 24.63 73.89 61.54 9.61 28.85 Triangular Diagram: As in the case of presentation of liquid-liquid equilibrium data, the three vertices of a right-angled triangle stand for 100% A, 100% B and 100% C respectively. Mass fractions of the solute inthe corresponding streams (x, and y,-) are plotted against the solvent concentrations (x, and y,). 100%B (solvent) 1.0 Tie lines 0.8 Overflow YBYVSYc 0.6 Xp Yp 0.4 un XBderflo VS tc 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 100% A XoYec 100% C (inert) (solute) Ponchon-Savarit type Underflow: Xoo X¢ ze -7A *B Xe tare diagram: The mass ratio Z of the inert or Overflow: Me Ye, Z aires 5 Bt Yc Yet + Ye carrier A to that of B Band C together is plotted against the 2.0 mass fraction of the solute on A-free basis in both under 1.6 Z flow and overflow Underflow, X¢ vs 1.2 - Tie lines Zz 0.8 0.4 Overflow, YCVs Z 0.0 0.0 0.2 0.4 0.6 0.8 Xo Vc  Practical Equilibrium To make calculations graphically, this will require graphical representation of equilibrium conditions We must deal with three-component systems containing pure solvent (A), insoluble carrier solid (B), and soluble solute (C). Computations and graphical representation can be made on triangular coordinates for any ternary system of this sort Because of frequent crowding of the construction into one corner of such a diagram, it is preferable to use a rectangular coordinate system The concentration of insoluble solid B in any mixture or slurry will be expressed as N (mass B/mass (A + C)), whether the solid is wet with liquid solution or not. Solute C compositions will be expressed as weight fractions on a B-free basis: x = wt fraction C in the effluent solution from a stage (B-free basis), and y = wt fraction C in the solid or slurry (B-free basis). The value of y must include all solute C associated with the mixture, including that dissolved in adhering solution as well as undissolved or adsorbed solute.  ¢ If the solid is dry, as it may be before leaching operations begin, N is the ratio of weights of insoluble to soluble substance, and y = 1.0. For pure solvent A, N = 0, x = 0. The coordinate system then appears as ¢ Consider first a simple case of a mixture of a insoluble solid from which all the solute has been leached, suspended in aa solution of Me the solute in a solvent, as represented by _ + e S point M, on the figure. * The concentration of the clear solution is x, Fe g and the insoluble solid/solution ratio is Nv): Ma Ld e Let the insoluble solid be non-adsorbent. If e this mixture is allowed to settle, as ina 2 batch-settling tank, the clear liquid which Mz can be drawn off will be represented by 0 ta point R1 and the remaining sludge will consist of the insoluble solid suspended in a x, y"= weight fraction " C,B ~ free basis small amount of the solution. ¢ The composition of the solution in the sludge will be the same as that of the clear liquid withdrawn, so that y* = x. The concentration of solid B in the sludge N,, will depend upon the length of time 6, allowed for settling, so that point E, then represents the slurry. Line E,R, is a vertical tie line joining the points representing the two effluent streams, clear liquid and slurry. If the circumstances described are maintained in an actual leaching, points E, and R, can be taken as the practical conditions of equilibrium for that leaching. Clearly if less time is allowed for settling, say 6,, the sludge will be less concentrated in insoluble solids and may be represented by point E,'. There will be some maximum value of N for the sludge, corresponding to its ultimate settled height, but usually in practice insufficient time is allowed for this to be attained. Since the concentration of insoluble solid in a sludge settled for a fixed time depends upon the initial concentration in the slurry, a mixture M, settled for time 6, might result in a sludge corresponding to point E,. If the solid does not settle to give an absolutely clear solution, if too much solution is withdrawn from the settled sludge so that a small amount of solid is carried with it, or if solid B dissolves to a small extent in the solution, the withdrawn solution will be represented by some point such as R,, somewhat above the lower axis of the graph. Similar interpretations can be made for compositions obtained when wet solids are filtered or drained of solution rather than settled, or when continuously thickened. In washing operations where the solute is already dissolved, uniform concentration throughout all the solution is rapidly attained, and reduced stage efficiency is most likely to be entirely the result of incomplete drainage or settling. In this case it is possible (but not necessary) to distinguish experimentally between the two effects by making measurements of the amount and composition of liquid retained on the solid after short and long contact time and to use the latter to establish the equilibrium conditions. Few of the types of equilibrium curves Figure (a) represents data obtained for cases where solute C is infinitely soluble in solvent A, so that x and y may have a0 P values over the entire range from 0 to 1.0. Nvs.y* This would occur in the case of the system soybean oil (C)- soybean meal (B)-hexane (A), where the oil and hexane are Nv E infinitely soluble. The curve DFE represents the separated solid under NS. conditions actually to be expected in practice. 4 J Curve GNJ, the composition of the withdrawn solution, lies t G H# L_-+- above the N = 0 axis, and in this case, therefore, either solid 0 ay" 10 B is partly soluble in the solvent or an incompletely settled liquid has been withdrawn. 10 The tie lines such as line FH are not vertical, and this will result (1) if insufficient time of contact with leaching solvent to e dissolve all solute is permitted, yx (2) if preferential adsorption of the solute occurs, or (3) if the solute is soluble in the solid B and distributes unequally between liquid and solid phases at equilibrium. The data may be projected upon aa plot of x vs. y, as in the 0 ; 10 manner of adsorption or liquid-extraction equilibria. (a)  K Avy" ¥ t Figure (b) represents a case where no adsorption of solute occurs, so that withdrawn solution and solution associated with the solid have the same wv composition and the tie lines are vertical. This results in an xy curve in the lower figure identical with the 45° line, and a distribution coefficient m, defined as y* / x, equals unity. f" vS. 7 1.0 Line KL is horizontal, indicating that the solids are settled or drained to the same extent at all solute 10 concentrations. It is possible to regulate the operation of continuous thickeners so that this will occur, and the conditions y are known as constant underflow. The solution in this case contains no substance B, either dissolved or suspended. a 10 to) Figure (c) represents a case where solute C has a NVvsoy limited solubility x, in solvent A. No clear solution stronger than x, can be obtained, P so that the tie lines joining slurry and saturated a U solution must converge, as shown. af In this case any mixture M to the right of line PS will Nvs.x tT settle to give a clear saturated solution S and a slurry Ss U whose composition depends on the position of M. oO ",y 145 q Point T represents the composition of pure solid 10 solute after drainage or settling of saturated solution. Since the tie lines to the left of PS are shown vertical, no adsorption occurs, and flow liquids are clear. It will be appreciated that combinations of these 10 x various characteristics may appear in a diagram of te) an actual case.  Single-Stage Leaching SOLID TO BE LEACHED LEACHED SOLID B mass insolubles 8 moss insolub! e F mass (A+C) E, mass (A + C) Ne mass B/mass (A + C) N, moss 8/mass{A + C) Ye moss C/moss {A + C) ¥, moss C/mass (A + C) LEACHING SOLVENT LEACH SOLUTION Fg moss solution (A + C) fF, mass solution (A + C} Xq mass C/mass (A + C) x4 mass C/mass (A + C) Consider the single real leaching or washing stage of Fig. The circle represents the entire operation, including mixing of solid and leaching solvent and mechanical separation of the resulting insoluble phases Weights of the various streams are expressed as mass for batch operation or as mass/time [or mass/(area)(time)] for continuous flow. Since for most purposes the solid is insoluble in the solvent and a clear liquid B a leach solution is obtained, the B discharged in the leached solids will be taken as the same as that in the solids to be leached. By definitionofN, B= NF = EN, A\ solute (C) balance gives Fy, + RoXp = Ey y, + Rix) A solvent (A) balance gives (1 y;) + Rl - - x) = E(l-y) + R01 - x,) A "solution" (solute+ solvent) balance gives F+R 0 = E, +. R, - M, Mixing the solids to be leached and leaching solvent produces a mixture of B-free mass M, such that B B NMI = F+R, M, Vek + Roxy om F+ R, Point F represents the solids to be leached and R, the leaching solvent. fF Point M,, representing the Nvsy overall mixture, must fall on Es fe the straight line joining Ro M and F / Points E, and R,, representing / Vib 8/IbtA+C)} / the effluent streams, are / located at opposite ends of / the tie line through M1, and Nyy My their compositions can be / / read from the diagram. / Modification to allow for the presence of in the liquid B / / withdrawn, necessitating an / equilibrium diagram of the type shown in Fig.a, is readily made by analogy with the 0 0 Ry +0 Ry / / yh fe YF 1.0 4, y= weight fraction C, é8- tree bosis corresponding problem in liquid extraction.  Multistage Crosscurrent Leaching by contacting the leached solids with a fresh batch of leaching solvent, additional solute can be dissolved or washed away from the insoluble material. The calculations for additional stages are merely repetitions of the procedure for a single stage, with the leached solids from any stage becoming the feed solids to the next. When the number of stages for reducing the solute content of a solute to some specified value must be determined, it must be based on the "practical" equilibrium data. This may require adjustment by trial of either the amount of solute to be leached or the amount and apportioning of solvent to the stages.  Multistage Countercurrent Leaching A general flow sheet for either leaching or washing is shown in Fig. Operation must necessarily be continuous for steady-state conditions to prevail, although leaching according to the Shanks system will approach the steady state after a large number of cycles have been worked through. In the flowsheet shown, it is assumed that solid B is insoluble and is not lost in the clear solution, but the procedure outlined below is readily modified to take care of cases where this may not be true. SOLIOS TO GE LEACHED LEACHED SOLIDS i 8 mass/time insoluble F moss (A + C}/time é, f A of mn We ria 3 Ne moss B/moss (A+ C) ye moss C/moss(A + C) yy LON V2 NT lf yen Pig Pig fj 3 ieee 2 LEACHING SOLVENT kR moss (A + C}/time Rs Ra MY) R, f3 Ruy Ff Tel lly x, mass 5/moss (A + C)  Jr + RyP +1 R, En? = M A\ solvent balance for the entire plantis as Solution" (A+C)balanceis Fy. + Ry,+ XN, 0, = Rx, + En? Yn, ld = Myny 'MW'represents the hypothetical B-free mixture obtained by mixing solids to be leached and leaching solvent. The coordinates of point M are = B N,,"OF +Ry Yu= yp + Ry pel XNPri F+ Ry a) The points E,,,, and R,, representing the effluents from the cascade, must lie on a line passing through M, and E,,,, will be on the "practical" equilibrium curve. F R, RNi+1 =A R F R, 3 R3 AR A, represents the constant difference in flow (E-R) (usually a negative quantity) between each stage. In Fig. it can be represented by the intersection of td &, & F lines FR, and EqpRyp., extended, in accordance é, Wesy® with the characteristics of these coordinates. Since the effluents from each stage are joined by Ww the practical tie line for the particular conditions which prevail, E, is found at the end of the tie line aM through R,. vse A line from E, to A, provides R,, and so forth. NW 4 Alternatively the stage constructions may be Rs Rez R a 1.0 made on the x, y coordinates in the lower part of the figure after first locating the operating line. This can be done by drawing random lines from 4, point A, and projecting their intersections with 10 'es the equilibrium diagram to the lower curve in the usual manner. + Operating line The usual staircase construction then establishes " the number of stages. Equilibrium curve The stages are real rather than equilibrium, the Pr yess practical equilibrium data having already taken 3 into account the stage efficiency, and hence there must be an integral number. Vy 'n,01 % £0 In the special case where constant underflow, or constant value of N for all sludges, pertains, the operating line on the xy diagram is straight and of constant slope R/E. In addition the practical equilibrium curve on this plot is straight, so that m = y* / x = const. the modification of Kremeser Equation provides, Ye- Jn, (R/mE)*! R/mE Yr -MXy, 4) (R/mE)* = 1 If the tie lines of the equilibrium diagram are vertical, m = 1.0. The form of the equation shown is that which is applicable when the value of F for the feed solids is the same as E, so that R/E is constant for all stages, including the first. It frequently may happen, especially when dry solids constitute the feed, that the ratio R,/E, for stage will be different from that pertaining to the rest of the | cascade. In this case above Eq. should be applied to that part of the cascade excluding the first stage, by substitution of y, for y, and Np for Np + 1. In general, the equation can be applied to any part of the cascade where operating line and equilibrium line are both straight, and this may be particularly useful for cases where the solute concentration in the leached solution is very small. Caustic soda is being made by treatment of slaked lime, Ca(OH),, with a solution of sodium carbonate. The resulting slurry consists of particles of calcium carbonate. CaCO,, suspended in a 10% solution of sodium hydroxide, NaOH, 0.125 kg suspended solid/kg solution. This is settled, the clear sodium hydroxide solution withdrawn and replaced by an equal weight of water, and the mixture thoroughly agitated. After repetition of this procedure (a total of two freshwater washes), what fraction of the original NaOH in the slurry remains unrecovered and therefore lost in the sludge? x = wi fraction NaOH N = kg CaCO, /kg soln y® = wi fraction NaOH in in clear soln in settled sludge soln of the settled sludge 0.0900 0.495 0.0917 0.0700 0.525 0.0762 0.0473 0.568 0.0608 0.0330 0.600 0.0452 0.0208 0.620 0.0295 0.01187 0.650 0.0204 0.00710 0.659 0.01435 0.00450 0.666 0.01015 Basis: kg solution in the original mixture, containing 0.1 kg NaOH (C) and 0.9 kg | H,0(A). B = 0.125 kg CaCO,. The original mixture corresponds to M, with N,,, = 0.125 kg CaCO3/kg soln, y,,, = 0.10 kg NaOH/kg soln. Niwi is plotted on the figure, and the tie line through this point is drawn. At point E, representing the settled sludge, N, = 0.47, y, = 0.100..08 Es Nf vs.y E> 06 , € 2 2 al Ei 4 > wy cd & 04 / tJ) 7 Ww a = o2t- be 3 M> ¢ Af, MOG. 2 R, Ry 002 #2 O04 +4006 #4O08 O10 O42 Ao x,y = weight fraction NaOH in liquid  _ 8 0.125 = E\ 0.266 kg soln in sludge N, 047 | -0.266 = 0.734 kg clear soln withdrawn Stage 2: R, = 0.734 kg water added, x, = 0 kg NaOH/kg soln. M,= E, + Ro = Ey + Ry = 0.266 + 0.734 = 1.0 kg liquid B B 0.125 NAf2 0.125 E, tR 0 M, 0 M, is located on line R,E, at this value of N, and the tie line through M, is drawn. At E,, N, = 0.62, Y, = 0.035. 7 E, = Ny" O18 0.202 ke I -0.202 = 0.789 kg soln withdrawn 0.62 Stage 3: R, = 0.798 kg water added, x, = 4/3 = £3 + Ro = 0.202 + 0.798 = 1.0 B 0.125 = 0.125 M, =-- ™ Nua Tie line E,R, is located through M, and, at E;, N, = 0.662, y, = 0.012., E, = B/N, = 0.125/0.662 = 0.189 kg soln in final sludge. E,y, = 0.189(0.012) = 0.00227 kg NaOH in sludge, or (0.00227/0.1)100 = 2.27% of original. Flaked soybeans are to be leached with hexane to remove the soybean oil. A 0.3-m- thick layer of the flakes (0.25-mm flake thickness) will be fed onto a slowly moving perforated endless belt which passes under a series of continuously operating sprays. As the solid passes under each spray, it is showered with liquid which percolates through the bed, collects in a trough below the belt, and is recycled by a pump to the spray. The spacing of the sprays is such that the solid is permitted to drain 6 min before it reaches the next spray. The solvent also passes from trough to trough in a direction countercurrent to that of the moving belt, so that a truly continuous countercurrent stagewise operation is maintained, each spraying and draining constituting one stage. Experiments show that the flakes retain solution after 6 min drain time to an extent depending upon the oil content of the solution, as follows: Wt % oif in soln 0 20 3 kg soln retained/kg insoluble solid 0.58 0.66 0.70 It will be assumed that the retained solution contains the only oil in the drained flakes. The soybean flakes enter containing 20% oil and are to be leached to 0.5% oil (on a solvent-free basis). The net forward flow of solvent is to be 1.0 kg hexane introduced as fresh solvent per kilogram flakes, and the fresh solvent is free of oil. The solvent draining from the flakes is generally free of solid except in the first stage: the rich miscella contains 10% of the insoluble solid in the feed as a suspended solid, which falls through the perforations of the belt during loading. How many stages are required?  Percent oil in kg soln retained kg oil y* soln = 100y* N kg insoluble solid WM kg insoluble solid NV 0 0.58 1.725 0 20 0.66 1.515 0.132 30 0.70 1.429 0.210 20 ty The tie lines are vertical, x = y*. ! ba fy Wvsy 6 Rearrange the drainage data as insoluble solid/kg solution é, follows: 2 ¢ Basis: kg flakes introduced. | Soybean feed B = 0.8 kg / ¢ VA os insoluble; / = kg ° F=0.2kgoil;N,=0.8/0.2=4.0 = AN of J tox, =t9 © kg insoluble solid/kg oil; y,=1.0 mass fraction oil, solid- fom 008 oe 018020 ine 7024 ia aw ' 4+ 7" 078 sltion free basis. Gin solution &p  = Solvent Ry 4, 1.0 kg hexane; xy. = 0 mass fraction oil. Leached solids kg oil/kg insoluble solid = 0.005/0.995 = 0.00503, By interpolation in the equilibrium data, Ny, = 1.718 kg solid/kg soln. Insoluble solid lost to miscella = 0,8(0.1) = 0.08 kg Insoluble solid in leached solids = 0.8(0.9) = 0.72 kg O72 = = 0.420 kg soln retained Ey, 1718 Oil retained = 0,00503(0.72) = 0.00362 kg Hexane retained = 0.420 - 0,00362 = 0.416 kg 0.00362 * = 0.0086 mass fraction oil in retained liquid In, 9420 Miseella Hexane = 0.416 = 0.584 kg; | - oil = 0.2 0.00362 = 0.196 kg. - R, = 0.584 + 0.196 = 0.780 kg clear miscella; x, = 0.196/0.780 = 0.252 mass fraction oil in liquid. Np, = 0.08/0.780 = 0.1027 kg insoluble solid/kg soln. The operating diagram is shown in Fig. Point R, represents the cloudy miscella and is therefore displaced from the axis of the graph at N,,. Point A, is located as usual and the stages determined with the N = 0 axis for all stages but the first. Between four and five stages are necessary.