LU 4 - Leaching Pt 1 - Lecture notes 1 PDF

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introduction of leaching, multistage operations...

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Block 2 Schedule 1. LU4: Solid-Liquid Extraction (Leaching) 2. LU5: Crystallization 3. LU6: Adsorption

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Block 2 Schedule 12-Apr-21 14-Apr-21

Monday Wednesday

Leaching Leaching

19-Apr-21 21-Apr-21 26-Apr-21 28-Apr-21 03-May-21

Monday Wednesday Monday Wednesday Monday

Leaching Tut Crystallization Crystallization Crystallization Crystallization

05-May-21 10-May-21 12-May-21 17-May-21 19-May-21

Wednesday Monday Wednesday Monday Wednesday

Crystallization Tut Adsorption Adsorption Assignment Adsorption

24-May-21 26-May-21

Monday Wednesday

Adsorption Adsorption Tut 2

LEARNING UNIT 4 Solid-Liquid Extraction (Leaching)

MSOCHA3: Multistage Operations 3A

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LU4 Schedule

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Learning Outcomes for LU5  Learn general principles  Distinguish between solid-liquid and liquid-liquid extraction processes.  Apply principles of leaching.  Analyze equipment for Leaching (Self study section)  Assess counter-current washing of solids  Calculate material balances and number of ideal stages for leaching using: o Analytical method o Graphical method 5

Introduction What is extraction? •

Extraction is a separation process based on differences in solubility.

•

Extraction differs from distillation in that disti distillation llation includes the purification of a substance that is in a liquid mixture whereas extr xtraction action includes the purification of a substance that is either in liquid phase or solid phase

• There are two types of extraction: • Solid liquid extraction • Liquid-liquid extraction

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Introduction Extraction processes (based on the principle of solubility)

Liquid-liquid extraction

Covered in LU 3

Liquid-solid extraction

Main focus in LU4

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Introduction •Soli Soli Solid-liquid d-liquid (SS-L) L) extra extraction ction is the pr proc oc ocess ess of separ separating ating soluble constituents of a so solid lid material usin sing g a ssuitab uitab uitable le solvent. Oth ther er erwise wise kn known own as leaching : it is se separation paration of a solut solute e from th the e solid mixtu mixture re by dis dissolving solving it in a liquid phase. Th The e pr pro ocess of S-L e exxtraction involves four steps:  In Intimat timat timate e contact bet betw ween the solid wit with h the sol olvent vent  Sep Separation aration of the so solution lution (extract) from e exxhaust hausted ed sol olid id  Sep Separation aration of the so solvent lvent (an (and d ent ntrained rained solids if any) from the e exxtrac tractt follo followed wed b byy product pu purific rific rification ation or re recover cover coveryy  Reco Recover ver veryy of solvent from moist sol solid id (i. (i.e. e. pre pressing ssing and dr drying ying to get dr dryy ca cak ke)

It is important to note that S-L extraction (leaching) is different to the process of dissolution. 8

Introduction On a molec olecular ular level the e exxtr traction action pr pro ocesses in comprised of stages/step stages/steps: s:  Con Contacting tacting th the e solvent an and d the solid to ef efffect a tr transf ansf ansfer er of a solute (le (leaching). aching).  Th The e change of phase of the sol olute ute as it dis dissolves solves in the so solvent lvent  Di Difffus fusion ion of the solute through the solvent in the pores of the soli solid d to the out outside side of the par arti ti ticle cle  Th The e transf transfer er of the solute from the solution in con contact tact with th the e par particl ticl ticles es to the mai main n bul ulk k of the solution (washing). Factors influencing the rate of extraction: There are four important factors to be considered: 1. Particle size. 2. Solvent. 3. Temperature. 4. Agitation of the fluid.

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Introduction Types of solids that are extracted

Biological

Organic

Inorganic

Typical industries where leaching is used: • the metals industry for removing mineral from ores (acid solvents) • the sugar industry for removing sugar from beets (water is solvent) • the oilseeds industry for removing oil from soybeans, etc. (hexane or similar organic solvents) The basic concepts of leaching also apply in the environment, where materials can be leached out by rainwater and carried into the groundwater supply. A simple, everyday example of a leaching process is making your morning coffee. 10

Introduction Prin Principl cipl ciples es of Leachi Leaching: ng: • Leaching can be batch, semibatch, or continuous. It usually operates at an elevated temperature to increase the solubility of the solute in the solvent. Calculations involve three component (solid, solvent, solute) systems. 1. Feed to a leaching system typically is solid, consisting of basically insoluble carrier material and a (usually desirable) soluble compound. 2. The feed usually must be prepared by grinding or chopping. It is then mixed with a liquid solvent. 3. The desired material dissolves (to some extent) and so leaves when the liquid is drawn off as overflow. The solids are then removed as underflow. The underflow is wet, and so some of the solvent/solute mixture is carried out here as well. 4. Flow through a leaching system may be crosscurrent or countercurrent.

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Test your K Knowledge nowledge Exercise: Try identify the solute, the feed and the solvent in each of the following cases:

CL CLASS ASS ASSIFIC IFIC IFICA ATION OF S-L EXT EXTRA RA RACTION CTION S SY YSTE TEMS MS 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Ma Making king cof cofffee Fis Fish h oils (extracting fish oils, omega etc.) Sug Sugar ar e exxtraction from sug ugar ar beets or sug sugar ar cane Dri Dried ed fru fruits its Vegetable oils Per Perffumes/fla umes/flavourants vourants Vanilla extract Gelatin Lign ignin in Ph Phos os osphoric phoric A Accid Gol Gold d

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Test your K Knowledge nowledge Exercise: Try identify the solute, the feed and the solvent in each of the following cases:

An Answers: swers:

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Test your K Knowledge nowledge Exercise: Try identify the solute, the feed and the solvent in each of the following cases:

An Answers swers (ct (ctd): d):

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Dif Diffferen erentt type pess of Extra Extracti cti ction on Diffusional extraction Example: the extraction of sugar from sugar beet. 1. Sugar beets are cut into small pieces which are treated with warm water (70-75°C). 2. Diffusion within the solid occurs through denatured cell walls and through the interstitial liquid. 3. The solvent (water) also diffuses in the opposite direction, thus diluting the solute within the cells. 4. (a) (b) (c)

The rate and time of extraction depend on: thickness of the pieces the ‘effective diffusivity’ of the solute in the solid the concentration of sugar in the extracting liquor 15

Dif Diffferen erentt type pess of Extra Extracti cti ction on Washing extraction 1. If the solid particle size is small, the solid-phase diffusional resistance becomes negligibly small. In such a case, extraction virtually becomes a process of washing the solid with the solvent. 2. An example is extraction of oil from ‘flakes’ of oil seeds. 3. Flakes are pretty thin. During the flaking process, much of the cells are ruptured and the diffusional resistance for transport of the solute within the solid becomes small. 4. In washing extraction, a substantial amount of solvent is retained in the slurry after washing

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Dif Diffferen erentt type pess of Extra Extracti cti ction on Leaching 1. This involves dissolution of one or more substances from solid particles accompanied by chemical reaction(s). 2. An acid, alkali or the solution of a complexing chemical is commonly used for solubilizing the target materials 3. Leaching of ores (oxides, carbonates, sulphides) is a major step in hydrometallurgy. 4. Over Overflo flo flow w – concentration of the solute in the clear liquid 5. Under Underflow flow – fraction of the liquid in the slurry

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Dif Diffferen erentt type pess of Extra Extracti cti ction on Chemical Extraction 1. Functionally similar to leaching but usually refers to recovering solutes from solids of organic nature. 2. Example, recovery of gelatin from animal bones using an alkaline solution is an example of chemical extraction.

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FACT ACTORS ORS AFFEC AFFECTTING SS-L LE EXTR XTR XTRA ACT CTION ION 1. Ph Phyysical cha haract ract racte eristi ristics cs of tthe he solid • E. E.g. g. type of sol solid id mat matrix rix i.i.e. e. porous or non-porous • Par Parti ti ticle cle ssiz iz ize e • Type i.e. biolo biological gical material or met metal al ore oress 2. Solvent • Hi High gh solu solubili bili bility ty ty,, selec electi ti tivve, low visc iscos os osity ity and low boiling poin pointt 3. TTemperature emperature • Di Diff fusivi fusivity ty of ssolu olu olutte and solu olubility bility in inccrease wi witth temperature • Uppe Upperr limi imitt of temperature is de depen pen pendent dent on secon secondar dar daryy conside considerations rations suc uch h as prev ction during the ext preventin entin enting g en enzym zym zyme e action extraction raction of sug sugars. ars. 4. Ag Agitation itation • In Increases creases solid solid-liquid -liquid mas asss tr transfer ansfer coef oeffficient 19

Mass tr trans ans ansffer gov gove ern rning ing the leachi leaching ng process Mass transf transfe er rat rates es within por porous ous residues are dif difficult ficult to assess because it is impossible to define the shape of the channels thr through ough which transf must ta plac It is possible, ho to obtain an transfer er take ke place. e. howe we wever ver ver,, appro approxximat imate e indic indication ation of the ra rate te of tra transf nsf nsfer er from the par artic tic ticles les to the bulk of the liqu liquid. id. Using the concept of a thi thin n film as pro providing viding the resistance to transf ransfer er er,, the eq equation uation for mas masss trans transffer ma mayy be writt written en as as:: Leac Leaching: hing:



.  . 

where: DAL= diffusion coefficient in the liquid phase A = the area of the solid-liquid interphase b = the effective thickness of the liquid film surrounding the particles c = the concentration of the solute in the bulk of the solution at time t 𝑐 = the concentration of the saturated solution in contact with the particles M = mass of solute transferred in time t k = diffusion constant V = volume of solution

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Mass tr trans ans ansffer gov gove ern rning ing the leachi leaching ng process

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Mass tr trans ans ansffer gov gove ern rning ing the leachi leaching ng process 𝑑𝑀 𝑘𝐴 𝑐 − 𝑐 = 𝑏 𝑑𝑡

For a batch proces process, s, th the e total vo volume lume of the solution, V V,, stay stayss cons onstant: tant: And 

By integration, the tim ime, e, t, required ffor or an initi initial al conc oncentration entration (co) to ris rise e tto o c pro rovi vi vided ded A and b are const constant ant is de defi fi fined ned as: 

 





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Mass tr trans ans ansffer gov gove ern rning ing the leachi leaching ng process Hence  

If the sol olvent vent is ini initially tially pure hen ence ce



 𝟎= 0 :    

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Mass tr trans ans ansffer gov gove ern rning ing the leachi leaching ng process Number of wa wash sh shing: ing:

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Exa Example mple 1 In a pilot plant scale test using a vessel 2 m3 in volume, a solute was leached from an inert solid and the water was 65% saturated in 200 s. If, in a full scale unit, 600 kg of the inert containing, as before, 25% by mass of the water soluble component, is agitated with 100 m3 of water, how long will it take for all the solute to dissolve, assuming conditions are equivalent to those in the pilot scale plant? Water is saturated with the solute at a concentration of 2.5 kg/m3. Solution For pilot plant: 𝑐 = 2.5 ×

65 = 1.625 kg/m3 100

Given: Cs= 2.5 kg/m3 and V = 2.0 m3 and t= 200 s

      1 − 𝑒 (.)

𝑐 = 𝑐 1 − 𝑒



1.625 = 2.5 × 𝑘𝐴 = 0.0105 𝑚 /𝑠 𝑏

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Solu Solution tion co cont.. nt..

For full scale plant: kg/m3 Given: Cs= 2.5 kg/m3 and V = 100 m3 .  () 

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Exa Example mple 2 500 kg of an inert solid containing 28 percent by mass of the water-soluble component (A), is agitated with 100 m3 of water for 600 seconds. After each decanting, 25% of the solution produced remain in the residue. Water is saturated with the solute at a concentration of 2.5 kg/m3 .Find the concentration of the solute (A) in the solution after the leaching and number of washing such that the concentration of A in the solid residue is 0.01% by mass. In a pilot scale test using a vessel 1 m3 in volume, a solute was leached from an inert solid and the water was 75 percent saturated in 10 s. Assuming conditions are equivalent to those in the pilot scale vessel. Solution

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Solu Solution tion co cont.. nt..

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Self study Equipment for leaching Prescribed text book: “Chemical Engineering: Particle technology and Separation Processes”. Coulson and Richardson, 5th Edition, Vol 2, Butterworth-Heinemann. Chapter 10 – Leaching (page 506-515)

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Co Count unt unter er ccurrent urrent w was as ash hing of soli solids ds

Figure 1: Agitator and washing systems

Figure 1 depicts the counter current washing of a solid. • After separation residual solid is mixed with solution • Battery of washers used for countercurrent flow of solids and solvent • Fresh liquid solvent - OVERflow • Residue + solution – UNDERflow • Overflow and underflow are bought into contact • Intimate mixing is achieved • Overflow leaving has same composition as underflow entering one unit • Each unit is an ideal stage

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Co Count unt unter er ccurrent urrent w was as ash hing of soli solids ds Test your Knowledge: Can you derive a series of mass balances in order to determine the compositions of all the stream in Figure 1? State all assumptions.

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