Batch Leaching of NaCl-Sand Mixture with Water as Solvent V.D.C. Aguilar, A.C.G Cabotaje, S.Y Naval, G.R Olvido Chemical Engineering Department, Adamson University
Ab A bstr str act: ct: The different modes of extraction; single stage, cross current and countercurrent were done in this experiment using the beaker scale extraction operation. This experiment uses mainly of the sand, NaCl and water mixture together with the series of laboratory beakers. The extractions were carried out at different values of solvents for each case. The solutions are withdrawn off after extraction process for the determination of density using westphal balance. The density-concentration plot aids in the calculation of the amount of solute in the final extract and raffinat for different cases. The efficiencies were computed using the formula for extraction efficiency. The obtained efficiencies showed that a large amount of solute was removed from the solution. The cross current operation also showed a decreasing pattern for the values of the efficiencies. solid, the porous structure may break down almost immediately to give a fine deposit of insoluble residue, and access of solvent to the solute will not be impeded. Generally, the process can be considered in three parts: first the change of phase of the solute as it dissolves in the solvent, secondly its diffusion through the solvent in the pores of the solid to the outside of the particle, and thirdly the transfer of the solute from the solution in contact with the particles to the main bulk of the solution. Any one of these three processes may be responsible for limiting the extraction rate, though the first process usually occurs so rapidly that it has a negligible effect on the overall rate.
Introduction
Leaching is concerned with the extraction of a soluble constituent from a solid by means of a solvent. The process may be used either for the production of a concentrated solution of a valuable solid material, or in order to remove an insoluble solid, such as a pigment, from a soluble material with which it is contaminated. The method used for the extraction is determined by the proportion of soluble constituent present, its distribution throughout the solid, the nature of the solid and the particle size. If the solute is uniformly dispersed in the solid, the material near the surface will be dissolved first, leaving a porous structure in the solid residue. The solvent will then have to penetrate this outer layer before it can reach further solute, and the process will become progressively more difficult and the extraction rate will fall. If the solute forms a very high proportion of the
Factors influencing the rate of extraction The selection of the equipment for an extraction process is influenced by the factors which are responsible for limiting the extraction rate. Thus, if the diffusion of the 1
solute through the porous structure of the residual solids is the controlling factor, the material should be of small size so that the distance the solute has to travel is small. On the other hand, if diffusion of the solute from the surface of the particles to the bulk of the solution is the controlling factor, a high degree of agitation of the fluid is required.
secondly because the solution will generally become more viscous. Temperature. In most cases, the solubility of the material which is being extracted will increase with temperature to give a higher rate of extraction. Further, the diffusion coefficient will be expected to increase with rise in temperature and this will also improve the rate of extraction. In some cases, the upper limit of temperature is determined by secondary considerations, such as, for example, the necessity to avoid enzyme action during the extraction of sugar. Agitation of the fluid . Agitation of the solvent is important because this increases the eddy diffusion and therefore the transfer of material from the surface of the particles to the bulk of the solution, as discussed in the following section. Further, agitation of suspensions of fine particles prevents sedimentation and more effective use is made of the interfacial surface.
There are four important factors to be considered:
Particle size. Particle size influences the extraction rate in a number of ways. The smaller the size, the greater is the interfacial area between the solid and liquid, and therefore the higher is the rate of transfer of material and the smaller is the distance the solute must diffuse within the solid as already indicated. On the other hand, the surface ma y not be so effectively used with a very fine material if circulation of the liquid is impeded, and separation of the particles from the liquid and drainage of the solid residue are made more difficult. It is generally desirable that the range of particle size should be small so that each particle requires approximately the same time for extraction and, in particular, the production of a large amount of fine material should be avoided as this may wedge in the interstices of the larger particles and impede the flow of the solvent. Solvent . The liquid chosen should be a good selective solvent and its viscosity should be sufficiently low for it to circulate freely. Generally, a relatively pure solvent will be used initially, although as the extraction proceeds the concentration of solute will increase and the rate of extraction will pr ogressively decrease, first because the concentration gradient will be reduced, and
Processes Involved in Leaching
Three distinct processes are usually involved in leaching operations: (a) Dissolving the soluble constituent. (b) Separating the solution, so formed, from the insoluble solid residue. (c) Washing the solid residue in order to free it of unwanted soluble matter or to obtain as much of the soluble material as possible as the product. Leaching has in the past been carried out mainly as a batch process although many continuous plants have also been developed. The type of equipment employed depends on the nature of the solid — whether it is granular or cellular and whether it is coarse or fine.
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(B ) Preparation of the F resh F eed
The normal distinction between coarse and fine solids is that the former have sufficiently large settling velocities for them to be readily separable from the liquid, whereas the latter can be maintained in suspension with the aid of only a small amount of agitation.
1. Weight 12 dry 1L beakers and label as follows: I, II, III, IV, V, VI, VII, VIII, IX, X, XI, XII. 2. Place into each beaker the sand-salt mixture pack. The pack contains 150g sand; the amount salt is unknown. 3. Pour 150 mL of water into each beaker and mix thoroughly. 4. Measure the liquor density of beaker XII. This is the fresh feed liquor density. 5. Determine the weight percent composition of the fresh feed.
The objectives of the experiment are to compare single-stage, crosscurrent and countercurrent beaker-scale extraction operation and to calculate single-stage, crosscurrent and countercurrent extraction efficiencies.
Materials
(C ) Single-Stage E xtraction Operation
7 pcs – 100mL beakers NaCl Water Clean Dry Sand 12 pcs – 1L beakers Westphal Balance
1. Pour another 150 mL of water to beaker I, 300mL to beaker II, and 600mL to beaker III. 2. Stir well and allow settling. 3. Decant the supernatant liquid from each beaker with a volume equal to that added in step C-1 into separate containers. 4. Determine the densities of the liquors in each beaker. 5. Convert densities to weight percent salt using the density-concentration plot. 6. Calculate the single-stage salt extraction efficiencies.
Experimental Procedure
(A ) Construction Concentration solution
of Plot
Densityfor NaCl
1. Using seven (7) 100 mL beakers, weigh corresponding amout of NaCl to make 0, 4, 8, 12, 16, 20 and 25% (by weight) salt dissolved in water. 2. Add clean dry sand equivalent to the amount of water added. 3. Stir properly and measure the liquor density of each using Westphal Balance 4. Plot density (y-axis) against the weight percent NaCl (x-axis).
(D ) Multistage Cr osscurrent E xtraction Operation 1. Prepare 600mL of water for each of beakers IV, V, and VI. Add into beaker IV the water into two 300mL portions, in three 200mL portions to
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beaker V, and in four 150mL portions to beaker VI (e.g. Pour 300mL of water into beaker IV, stir well and allow to settle; then, draw off 300mL of the clear liquor into a separate container. Add the next portion of water. Mix well, and then draw off 300mL into another container.) 2. Determine the density and percent weight of NaCl of the clear liquors.
7. Add Decant IX into beaker X, mix well, and then decant 600mL of solution. Label this solution Decant XI again. 8. Add 600mL of water into Beaker X, mix well, and then decant 600mL of solution. The measuring mixture is the raffinate. 9. Measure the density of the raffinate solution and determine the wt% composition. 10. Add Decant X into beaker XI, mix well, and then decant 600mL of solution. This solution is called the extract. 11. Measure the density of the extract and determine the weight percent composition. 12. Calculate the salt extraction efficiency using two different formulas.
(E) Steady-state Multistage Countercurrent Extraction Operation
1. Add 600mL of water into beaker VII, mix well, and then decant 600mL of solution. Label this solution Decant VII. 2. Add 600 mL of water into beaker VII, mix well, and then decant 600mL of solution. Label this solution Decant VIII. 3. Add Decant VII into beaker VIII, mix well, and then decant 600mL of solution. Label this solution Decant VII again. 4. Add 600mL of water into beaker VIII, mix well, and then decant 600mL of solution. Label this solution DecantXI. 5. Add Decant VIII into beaker IX, mix well, and then decant 600mL of solution. Label this solution Decant VIII again. 6. Add 600mL of water into beaker IX, mix well, and then decant 600mL of solution. Label this solution Decant X.
Results and Discussion
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Density
Weight % NaCl
0.10 mg/L
0%
0.25 mg/L
4%
0.35 mg/L
8%
0.39 mg/L
12%
0.3925 mg/L
16%
0.435 mg/L
20%
0.65 mg/L
25%
A. Construction Concentration Solution
of Plot
Densityfor NaCl
Weight % Composition vs. Density 0.7
0.6
0.5
Y 0.4 T I S N E D0.3
0.2
0.1
0 0%
5%
10%
15%
20%
25%
30%
WEIGHT % COMPOSITION
hook holding the plummet, and the screw on the foot is adjusted until the index pointer on the end of the beam lines up with the point on the frame. The plummet is then completely immersed in the unknown liquid, and the system is rebalanced, using a series of riders on the nine equally paced notches on the beam, thus giving the value of the added mass for each decimal place. This gives the buoyant force of the liquid relative to water, and hence the specific gravity, which may be obtained to four decimal places.
The graph showed that the weight % composition and the density of the liquid have a directly proportional relationship. The density of the liquid is obtained by using the westphal balance. Westphal Balance is used to measure the specific gravity (or density) of liquids. The arm is first balanced with the plummet totally immersed in water at 15.5 degrees C. The plummet has a built-in thermometer and has a known volume (equal to 5 grams of water at 15.5 degrees C) and mass (15 gm). A 5 gm mass is placed on the 5
B. Preparation of the F resh F eed
C. Single-Stage E xtraction Operation
(8)(5) + 5(50) + 6(500) = 150 = 0.2833
V2 L1
V1 Beaker I
L2
L1: 150g sand, 17.12 g NaCl, 150 g H2O Based on Density-Concentration plot, concentration of NaCl in feed is 0.054
V1: 150 mL H2O
= = 0.054 = 150 + 150 + = 17.12
= 0.3067
(8)(5) + 2(6)(500) 150
Baed on Density-Concentration concentration of NaCl in V2 is 0.062 The fresh feed is the contents of beaker XII. The density of the fresh feed was measured using the westphal balance. Since the density of the fresh feed is given, the concentration of NaCl in the feed can be read from the Density-Concentration plot in letter A. Using this weight fraction, the amount of NaCl in the feed can be obtained, which is the unknown. Therefore, the amount of NaCl in the fresh feed is 17.12 grams. This amount will be also used for the calculations of different extraction operations.
plot,
= 0.062 = 150+ = 9.91 =
9.91 17.12
100
= 57.89%
V2 L1
V1 Beaker II
L2
L1: 150 g sand, 17.12 g NaCl, 150 g H2O V1: 300 mL H2O
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= = 0.25
(8)(5) + (7)(5)
= 0.02 = 600+ = 12.24
300
= Based on Density-Concentration concentration of NaCl in V2 is 0.04
plot,
12.5 17.12
V2 L1
100
V1 Beaker III
L2
L1: 150 g sand, 17.12 g NaCl, 150 g H2O V1: 600 mL H2O
2(9)(5) + 2(4)(5) + 2(500) = 600 = 0.22
Based on Density-Concentration concentration of NaCl in V2 is 0.02
100
In beaker I, II and III single stage extraction operation, the feed consists of sand, NaCl, and the solvent which is water. This feed was contacted with 150 mL, 300mL and 600mL of water respectively for each beaker to extract the salt from the solution. After extraction, 150mL, 300mL, and 600mL of the solution were removed from each beaker respectively, and their liquor density were measured using the westphal balance. The liquor densities for each beaker were measured as 0.3067 g/mL, 0.25g/mL, and 0.22 g/mL. The densities are decreasing from beaker I to beaker III due to the variation of the amount of solvent added. These densities were used to obtain the weight percent of NaCl using the density-concentration plot provided earlier. The obtained concentrations were equated to the formula for getting the amount of NaCl in the extract. The amount of NaCl in the extract were used for the calculation of extraction efficiencies. The extraction efficiencies were determined by using the formula,
= 73.01%
150 g sand 17.12g NaCl 150 g H2O
17.12
= 71.50%
= 0.04 = 300+ = 12.5 =
12.24
ℎ = 100% ℎ
plot,
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The extraction efficiencies obtained were 57.89%, 73.015% and 71.50% for beaker I, II and III respectively.
() = 0.018 = 300+ = 5.50
D. Multistage Cr osscurr ent Extraction Operation
9.28 + 5.50 = 100 17.12 = 86.33%
Beaker IV 300 g H2O
300 g H2O
150 g sand 17.12g NaCl 150 g H2O E1 x g NaCl
Beaker V E2 y g NaCl
(8)(5) + (5)(5) + (3)(500) + (7)(500) = 300 = 0.2393 150 g sand
200 g H2O 200 g H2O 200 g H2O
17.12g NaCl 150 g H2O
E1 Based on Density-Concentration concentration of NaCl in E1 is 0.03
plot,
x g NaCl
=
() = 0.03 = 300+ = 9.28
= 0.205
y g NaCl
E3 z g NaCl
(8)(5) + (2)(500) 200
Based on Density-Concentration concentration of NaCl in E1 is 0.018
(7)(5) + (5)(5) + (2)(500) + (7)(500) = 300 = 0.215
Based on Density-Concentration concentration of NaCl in E2 is 0.018
E2
plot,
8
() 0.018 = 200+ = 3.67
plot,
= = 0.2025
(8)(5) + (1)(500)
Beaker VI
200 150mL
150 mL
150 mL
150mL
E3 y
E4 z
150 g sand 17.12g NaCl 150 g H2O
Based on Density-Concentration concentration of NaCl in E2 is 0.045
plot, E1 w
() 0.045 = 200+ = 9.42 = = 0.1925
E2 x
= = 0.333
(9)(5) + (1)(5) 150
(7)(5) + (7)(500) Based on Density-Concentration plot, concentration of NaCl (w) in E2 is 0.071
200
Based on Density-Concentration concentration of NaCl in E3 is 0.008
() 0.071 = 150+2 = 11.46
plot,
= () 0.008 = 200 + = 1.61
0.20
(5)(5) + (1)(5) 150
Based in Density-Concentration concentration of NaCl in E2 is 0.008
3.67 + 9.42 + 1.61 = 100 17.12 = 85.89%
plot,
() 0.008 = 150+ = 1.21 = 9
(5)(5) + 9(500) 150
= 0.1967
from the solution. The densities were then measured using the westphal balance. The corresponding amount of NaCl in the extract were calculated using the readings from the density-concentration plot. As you can see in the data, the values of efficiencies were 86.33%, 85.88% and 80.61%. These values showed a decreasing pattern, and it means that the solute extracted for the first two stages has a higher value than that in the last stage. This is simply because, as extraction proceeds, the solute being extracted decreases. The computation also showed that extraction of the solute is highest at the 2nd stage. This must be due to human and instrumental error, because usually, the first extract has the greatest value.
Based on Density-Concentration concentration of NaCl in E3 is 0.004
plot,
() 0.004 = 150+ = 0.60 =
(5)(5) + (8)(500) 150
= 0.1933
Based on Density-Concentration concentration of NaCl in E4 is 0.0035
plot, E. Steady-state Multistage Countercurrent Extraction Operation
() 0.0035 = 150+ = 0.53 11.46 + 1.21 + 0.60 + 0.53 = 100 17.12 = 80.61%
Extract (E) y g NaCl
600 g H2O
150 g sand 17.12g NaCl 150 g H2O
Raffinate (R) x g NaCl
Raffinate:
(6)(5) + (9)(500) = 150 = 0.23
In crosscurrent extraction operation, the feed containing the solute is contacted with the solvent for each extraction stage. The extracts are collected separately at each stage. In this case, in each stage, the feed was contacted with 150 mL of water.
Based on Density-Concentration concentration of NaCl in R is 0.024
With the aid of beaker IV, V and VI, the extracts from each stage were withdrawn 10
plot,
counter-currently. The extract phase contains the solute we desire to remove. In this case, series of decantation were performed to finally get the extract and raffinate solutions. The raffinate solution contains less of the solute. The raffinate and extract solution’s density were measured using the westphal balance. The density-concentration plot in the letter A was read to get the corresponding concentration of NaCl in both extract and raffinate phases. The amount of NaCl in extract and raffinate was then calculated using the concentration obtained.
( ) = 0.029 = 150+ = 4.489 (17.12 − 4.48) = 100 17.12 = 73.83%
Extract:
Using the formula for extraction efficiency, the extraction efficiency based on the extract and raffinate was calculated. The extraction efficiencies are 73.83% and 67.87% for the raffinate and extract respectively. These values of efficiencies showed that the solute was quite efficiently removed from the solution. This experiment may be prone to human and instrumental errors but still, we got good results by what the efficiencies showed.
(2)(9)(5) + (2)(4)(5) + (1)(500) = 600 = 0.2175
Based on Density-Concentration concentration of NaCl in E is 0.019
plot,
() 0.019 = 600+ = 11.62
Conclusion
This experiment was successfully done with the aid of laboratory beakers. In the industrial processes, three modes of extraction are employed, namely: Single-stage, cross current and countercurrent. The single-stage from the word itself consists of a first stage that mixes the phases together. Cross current extraction is done by contacting the feed with the solvent in each stage. The extracts are withdrawn off from each stage. Countercurrent extraction comprises of several stages wherein the final extract is
11.62 = 100 17.12 = 67.87%
In the steady state multistage counter-current operation,the feed is contacted with the solvent passing through series of stages 11
withdrawn off at the final stage. In this experiment we employed the use of beaker scale extraction operation. Series of beakers are used to separate the desired solute from the solution. One must utilize the use of beakers because they are used to transfer the solution, decant and withdraw the extract. The values of efficiencies computed were satisfactory. This only showed that even though this experiment is prone to human and instrumental error, the extraction was quite efficient. The efficiencies showed decreasing pattern in cross current extraction.
References
Coulson and Richardson’s Chemical Engineering, Fifth Edition – Particulate Teachnology and Separation Processes https://www.vanderbilt.edu/leaching/ leaching-process/ Separation Process Principles with Mass Transfer Analysis – Philip Wankat
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