866
T H E J O U R N A L O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y . Vol. 9 , No. 9
LABORATORY AND PLANT DISCONTINUOUS EXTRACTION PROCESSES B y L. F. H A W L E Y ~
a n d designated b y a.
single washing will remove
Received M a y 8, 1917
T h e operation b y which a soluble substance is removed from a n accompanying insoluble material b y washing with a suitable solvent is a common a n d imp o r t a n t operation i n chemical industrial processes; t h e terms lixiviation, leaching, percolation, extraction, diffusion, filtration, decoction, decantation, washing a n d others are applied in different industries t o t h e same general operation a n d on account of this varied nomenclature there seems t o be a lack of appreciation of t h e fact t h a t t h e fundamental principle is t h e same i n each of these operations, whatever name is applied t o it. Most of us have a hazy idea t h a t i n such a n operation several separate washings with fresh solvent will remove more soluble material t h a n a single washing with t h e same t o t a l a m o u n t of solvent, or t h a t a multiple extraction, i n which t h e same solvent is used in proper order on several charges of material, is more efficient t h a n repeated washings with fresh solvent, b u t when we are asked why, i n what way, or especially, how much, t h e answer is not forthcoming. I n certain single cases, as a result of long experience, we may be able t o give satisfactory answers, b u t t h e general case has not been developed, so far as is known t o t h e writer. T h e following discussion is for t h e purpose of developing t h e general case a n d giving a quantitative t r e a t m e n t of t h e subject. T H E SIMPLEST cAsE-In order t o simplify t h e conditions t o a point where a mathematical t r e a t m e n t is readily possible we will first assume a n ideal case i n which t h e original material t o be treated consists of small particles of a n impermeable, insoluble substance, wet with a strong solution of t h e substance t o be extracted; these conditions would be well met by a mass of small glass beads wet with a strong salt solution, i. e., containing only as much of t h e solution as would remain behind after draining. If a mass of this kind is shaken with a certain amount of \\-ater a n d as much as possible of t h e water is t h e n drained off, how much of t h e original salt remains with t h e beads? I n this particular example i t is easy t o conceive t h a t after shaking with t h e mater a weaker solution is formed a n d t h a t t h e concentration of t h e part drained off is t h e same as t h e concentration of t h e p a r t remaining; therefore. t h e portion of t h e salt remaining on t h e beads after such a t r e a t m e n t depends only on t h e relation between t h e volume of solution drained off a n d t h e volume of t h e solution retained. S O L V E X T RaTro-If, for instance, j volumes of solution are drained off a n d one volume is retained, t h e n 6,ls of t h e salt is removed a n d l,/6 remains n-ith t h e beads. This ratio, v o l u m e removed : v o l u m e r e t a i u e d . is of fundamental importance in considering any extraction problem a n d will hereafter be called t h e sole'ent r a t i o ,
.
1 Chemist in Forest Products, Forest Products Laboratory, Madison, Wisconsin
With a n y solvent ratio a, a
a of a
+
t h e original solu-
~
I
ble substance present, leaving behind
a + I'
If t h e
amount of solvent retained is known, t h e n t h e a m o u n t of soluble material removed can be computed from t h e amount of solvent removed. SUCCESSIVE T R E A T M E N T S WITH FRESH SOLVENT
By carrying this same conception a little farther, t h e amount of soluble material remaining after t w o or more successive treatments with t h e same a m o u n t of solvent can be computed.
If
2is left behind a
+
~
I
( a + 1piS I after 3, etc.'
after one t r e a t m e n t with solvent ratio a t h e n ___ left after two such treatments,
(a
+
113
Table I is computed from these formulas a n d gives t h e percentage of soluble material removed after different numbers of treatments with varying solvent ratios. T h e total volumes of t h e resulting solutions are also shown for comparison; these volumes are expressed in terms of t h e amount of solvent retained. EXTRACTION W I T H DIFFERENT S O L V E N T RATIOS DIFFERENT NUMBERS OF TREATMENTS: REPEATED WASHINGS WITH FRESH SOLVENT*
TABLEI - P E R C E N T A G E AND
TREATMENTS: ----I-
-23 ---4Per Volume of Per Vol. Per Vol. Per Vol. cent of cent of ExSolu- cent cent of Solvent Sol. Extr. Sol. Extr. tracted tion Extr. Sol. Ratio 1 75.00 2 87.50 3 4 93.75 1. .. .. . . . 98.76 2 6 8 88,89 4 96.30 2.,.,.. . . 9 99.61 12 3 93,75 6 98.44 3. . .... . . 12 4 16 99.84 96.00 8 99.20 4....... . 10 99.54 5 15 20 99.92 97.22 5 .. ..... . 18 99.96 99.71 24 6 . .. .. .. . 6 99 78..9464 12 28 14 21 99.98 99.81 I . . .. . . . 32 16 24 99.98 98.76 99.86 8 . . . . .. . . 9 27 36 18 99.99 99.00 99.90 9,. ...,,. 30 10 40 20 99.99 99.17 99.92 10.. ., . . . . * Just previous t o t h e publication of this paper t h e writer's attention was called t o a similar table in "Cyaniding Gold and Silver Ores," Julian and Smart, Chapter 31.
.
h
Table I shows t h e correctness of t h e common notion t h a t in washing with a limited amount of solvent it is better t o wash several times with small portions t h a n once n-ith t h e whole a m o u n t ; besides this t h e results which can be obtained with different amounts of solvent a n d different methods of application are shown quantitatively. For instance, 6 volumes of solvent, if used in one t r e a t m e n t , may give only 8 j.7 per cent extraction (solvent ratio 6), b u t if used in two equal parts separately will give 9 3 . i j per cent (solvent ratio 3 ) , or i n three parts, 96. 29 (solvent ratio 2 ) . 1 See Ostwald's "Foundations of Analytical Chemistry" sections o n t h e Theory of \Tashing Precipitates and on Adsorption Phenomena. Ostwald says t h a t his similar formulas "do not agree a t all with observed facts" on account of t h e adsorption phenomena which were not considered in their calculation b u t he is speaking of analytical results where 00.01 per cent might be a large error, I n commercial extraction where 00.1 per cent might be a negligible discrepancy between calculated results a n d observed facts t h e effect of adsorption would not seriously interfere with t h e accuracy of t h e formulas. Finely divided analytical precipitates would also show adsorption phenomena t o a much greater degree t h a n would t h e coarser granular or fibrous materials commonly used in extraction processes.
Sept., 1917
T H E J O U R N A L OF I X D C S T R I A L A N D ENGINEERING CHEMISTRY
Even more striking examples are t h a t 4 volumes of solvent in 4 equal parts give a better extraction ( 9 3 . 7 5 per cent, solvent ratio I ) t h a n I O volumes used all a t once ( 9 0 . 9 per cent, solvent ratio I O ) , and t h a t 8 volumes in 4 parts give t h e same results ( 9 8 . 7 6 per cent, solvent ratio 2 ) as 16 volumes in two parts (solvent ratio 4).’
is shown in t h e following table, which traces t h e course of t h e operation from t h e time of starting with fresh materials until the results become practically constant when a solvent ratio of 2 is used in a series of two extractions: TABLE 11-TABLE SHOWING DETAILEDCOURSEOF EXTRACTIOX I N SERIES. Two TREATMENTS. SOLVENT RATIO 2 Cell
BEFORE Beads
NO.
11U L T I P L E E X T R A C T I 0 N
These simple cases have served t o clarify our views of t h e terms “retained solvent” and “solvent ratio” a n d we can noR- proceed t o t h e more efficient a n d more complex process of multiple extraction. I t can be readily seen from t h e consideration of t h e results of a n y series of repeated washings with fresh solvent, as shown in Table I , t h a t as t h e number of washings increase t h e solvent is used less and less efficiently, i. e., t h e same amount of solvent removes less and less salt. For instance, when using enough solvent each time so t h a t t h e solvent ratio is 4, washing S o . I removes 80 per cent; washing No. 2 , 16 per cent; No. 3, 3 . 2 per cent; N o . 4 , o 64 per cent. After these last washings t h e solution contains so little of t h e salt t h a t it could very evidently be used t o better advantage as solvent on a fresh charge of beads t h a n as a finished solution. By means of a procedure whereby t h e weaker solutions are used over again, either on new charges of t h e original material or on charges which have been already extracted b y stronger solutions, t h e final solution can be obtained in much more concentrated form t h a n b y using t h e fresh solution only once; i. e., t h e solvent is used more efficiently. T h e most rational method of applying the solvent t o obtain this effect is according t o t h e countercurrent principle, in which each lot of solvent is used on charges of beads containing successively more and more salt and in which each charge of beads is treated with solvents containing successively less and less salt. Another may of considering t h e counter-current principle is t h a t each lot of solvent progresses in one direction, meeting charges of beads which have been treated fewer and fewer times until i t finally treats a fresh charge, is fully concentrated, and is removed f r o m t h e system. while each charge of beads progresses in t h e other direction, meeting solvents which have been used fewer and fewer times until finally it is treated with fresh solvent, is completely washed. and removed f r o m t h e system. We lvill now endeavor t o develop t h e formulas which mill express t h e amount of soluble material removed b y varying solvent ratios and varying numbers of treatments. according t o this counter-current principle. D E ~ E L O P U E S T O F FowruLhs-These formulas are more difficult t o work out on account of the fact t h a t on t h e s t a r t with fresh solvent and fresh material the results are different from those obtained after the process has been in operation for some time. This I t rz-ill not be necessary t o consider the case in which unequal portions of solvent are used because t h e mathematicians tell us t h a t if x Y z = a constant, ny; is a maximum when x = y = 3 and, therefore, with the same total amount of solvent equal portions give better results t h a n t h e same number of unequal portions.
+ +
86 7
1 . . . . . . . . . . . . . . . . . 100.00 2 . . . . . . . . . . . . . . . . . 100.00 55.56 2. . . . . . . . . . . . . . . . . 3 . . . . . . . . . . . . . . . . . 100.00 3 . . . . . . . . . . . . . . . . . 45.68 4 . . . . . . . . . . . . . . . . . 100.00 4 . . . . . . . . . . . . . . . . . 43.48 5 . . . . . . . . . . . . . . . . . 100.00 43.00 5................. 6 . . . . . . . . . . . . . . . . . 100.00 6 . . . . . . . . . . . . . . . . . 42.89 7 . . . . . . . . . . . . . . . . . 100.00
( a ) = n. ( b ) = ax. ( c ) =
Added Solvent
0 66.67 0 37.04 0 30.45
AFTER Beads Solvent 66 67 33.33 (iii,ii) 55.56 37.04 18.52
45.68
(91.36) 30.45 (86.97)
15.23
43.48
0
14.49
28.99
43,00
0
14.33
28.66 0
42.89
28,99
(85.99) 28.66 (85.77) 28.59(b) (85.,2)(d)
14,30(a) 42.86(c)
28.59
s.c- + (d) =
1)a a f l
ax
- 1-x
I n this table t h e figures represent percentages of salt, I O O per cent being t h e amount in a fresh charge of beads. For instance, IOO per cent in t h e column headed “Beads” signifies a fresh charge of beads, while 0 . 0 per cent in t h e “Sol\-ent” column indicates fresh solvent. After t h e first treatment of fresh beads with fresh solvent 33 33 per cent of rhe original salt remains on t h e beads and 6 6 . 6 6 per cent are removed with t h e solvent. This solvent containing 6 6 . 6 6 per cent of t h e salt on a charge is added t o a fresh charge of beads (100 per cent salt) and b y mixing and draining off 1 , 3 (jj.j per cent) of t h e total 1 6 6 . 6 6 per cent remains behind on t h e beads while 2 ’ 3 ( 1 1 1 . I per cent) is removed in t h e solvent drained off. T h e beads containing jj. j per cent salt are now treated with fresh solvent which removes 3 7 . 0 4 per cent, leaving 1 8 . j z per cent. I n this way t h e extraction is continued, t h e figures in italics representing t h e amount of salt left on t h e beads after two washings and t h e figures in parentheses representing t h e amount removed from t h e system in t h e solvent after t h e second washing. I t mill be noted t h a t these two figures gradually become smaller until they are practically constant a t about 1 4 . 3 0 per cent and 8 j. ; 2 per cent, respectively, and since together they represent the final disposition of the 1 0 0 . 0 0 per cent of salt added with each fresh charge of beads t h e y will finally reach a limit a t which the s u m of t h e two will be IOO 00. I t is now possible t o determine t h e value of both these figures in terms of t h e solvent ratio a. If t h e amount of salt finally left in t h e beads is called x, then t h e salt in the solution drained off after t h e treatment is ax. since t h e ratio between these figures is t h e solvent ratio a. The solution drained off containing a n is added t o a fresh charge of beads and the total salt in t h e mixture is then ax I ; by t h e treatment this amount is divided in two parts, t h a t remaining on the beads and t h a t drained off, which are t o each other as I : a ; t h a t is, t h e p a r t remaining on
+
the beads is
a + 1
(ax off is a
(ax
+ I).
+
I)
and t h e p a r t drained
But in this latter figure, the
U + I
amount of salt in t h e final solution is also
I
- x,
SO
T H E J O U R N A L O F I N D U S T R I A L ALTD E N G I i V E E R I N G C H E M I S T R Y
868 a
-(ax
that a2x
a + 1
+a
= a-
+
I) = I
ax
+ - x,
- x;
I
clearing of fractions, I
or, x =
aZ+a+1 I n a multiple extraction with t w o treatments in series t h e a m o u n t of unextracted salt is t h e n
I
a2+a+I
and
Vol. 9, NO. 9
parison, Table I V is prepared which is simply a combination of Tables I 1 a n d 111. This table shows plainly t h e advantages of multiple extraction i n t h e way of removing a greater proportion of t h e soluble material per unit of solvent used. E X T R A C T I O S O F MATERIAL XOT WET WITH SOLVENT
T h e foregoing conclusions were all obtained on t h e assumptions: ( I ) t h a t t h e soluble substance was alU 2 + U + I ready in solution before t h e extraction process was a = t h e solvent ratio. I n t h e same way t h e a m o u n t of salt extracted b y started, a n d ( 2 ) t h a t t h e material t o be extracted was multiple extraction with 3, 4, or more treatments wet with t h e a m o u n t of solution normally retained i n series can be expressed in terms of t h e solvent after draining. There are several instances in comratio. T h e details of t h e development of t h e formulas mercial practice where both these assumptions would need n o t be repeated a n d t h e results only will be given. hold b u t there are m a n y instances where neither For a series of 3 t r e a t m e n t s t h e unextracted salt is would hold and we will now develop t h e formula for multiple extraction a n d for repeated washings with I I , for 4 treatments fresh solvent, similar t o t h e preceding, except t h a t . a3 a2 + a I a4 a3 u2 a I' t h e original material will be assumed d r y a n d with etc. Tabfe I11 shows t h e percentage extraction obtain- t h e soluble substance i n solid form. A concrete case would be a mass of glass beads whose surfaces were able with different solvent ratios a n d different number encrusted with a layer of salt or which were uniformly of treatments, as computed from these formulas. mixed with finely ground salt. I n this case another TABLE111-PERCENTAGE EXTRACTION WITH DIFFERENT SOLVENT RATIOS A N D DIFFERENT NUMBERS OF TREATMENTS. MULTIPLEEXTRACTION assumption must be made in order t o make t h e re-------Number of Treatments* sults capable of simple mathematical t r e a t m e n t , viz., Solvent t h a t all t h e salt is dissolved during t h e first t r e a t Ratio 1 2 3 4 5 6 1 ................... 50.00 66.67 75.00 80.00 83.33 85.71 ment with solvent. 2. .................. 66.67 85.71 93.33 96.77 98.42 99.21 3 ................... 75.00 92.31 97.50 99.17 99.73 99.91 S O L V E N T RATIOS-Extraction under this second 4 ................... 80.00 95.24 98.82 99.71 99.93 ... 5 ................... 83.33 96.77 99.36 99.87 99.97 ... set of conditions will give results t h e same as under 6 . . . . . .............. 85.71 97.67 99.61 99.94 ... 87.50 98.24 99.75 99.96 i ................... ... t h e first set of conditions, except t h a t when t h e beads ... ... 8 ................... 88.89 98.63 99.83 99.98 ... 9 . . . . . .............. 90.00 98.90 99.88 99.99 ... are treated with solvent for t h e first time p a r t of t h e .. ... ... 10 ................... 90.90 99.10 99.91 solvent added will b e retained a n d t h e solvent ratio *Number of treatments must of course be integral but solvent ratios will be less t h a n when t h e same a m o u n t of solvent is may be fractional. T h e results bbtainable' with fractional solvent ratios may he determined b y substitution in the formulas or by inspection and added t o wet beads. After t h e first t r e a t m e n t , howinterpolation in the tables. It should be noted that with solvent ratios less ever, t h e beads will be wet a n d just as much solvent as t h a n unity the value of 1 +,a + a 2 a 3 - + am = 1 - a: i . e.. howis added will be drained off. T h e solvent ratio for ever many treatments are given with a solvent ratio of, say, 0.8 not less t h a n 1 - 0.8 = 0.2 of the original soluble material will be left unextracted. t h e first t r e a t m e n t will, therefore, b e different from This is the only instance where the mathematical conception of the percentage extraction as a function of the solvent ratio seems t o apply directly t h a t for t h e other treatments. For instance, if a mass t o continuous extraction; continuous extraction may be considered as discontinuous extraction in an infinite number of cells and, therefore, even in of d r y beads is treated three times with fresh solvent, theoretical1 perfect continuous extraction, the solvent ratio must be unity or greater g h e amount of solvent drawn from the system must be equal t o each time with twice as much solvent as t h e beads or greater than the amount left on the extracted material) in order t o obtain retain after draining, t h e solvent ratio will be I for complete recovery of soluble material. I n these cases of multiple extraction t h e a m o u n t of t h e first t r e a t m e n t a n d 2 for t h e other two treatments;* solvent i n t h e final solution is proportional t o t h e sol- a n d only j of t h e 6 volumes of solvent used will be vent ratio only a n d is not affected b y t h e number of f o u n d in t h e final solution, t h e other I volume being t r e a t m e n t s and, therefore, t h e following examples retained b y t h e beads. Or i n multiple extraction, if can be derived from t h e table. Three volumes of twice as much solvent is used %s is retained b y t h e solvent i n 5 t r e a t m e n t s give better extraction (99. 73 beads t h e solvent ratio will be I i n t h e first t r e a t m e n t per cent) t h a n 6 volumes in 3 treatments (99.61 per t h e beads receive a n d 2 i n t h e other treatments, howcent). T w o volumes of solvent in 6 treatments i n ever m a n y there m a y be. t h e a m o u n t of extracted salt is
+
+
a2
+a
when
+ + + +
;
series give better extraction (99. 2 1 per cent) t h a n 4 volumes in 3 t r e a t m e n t s (98.82 per cent), or t h a n I O volumes i n 2 t r e a t m e n t s (99.10 per cent). Other examples can be figured out i n t h e same way. COMPARISON
OF
MULTIPLE
EXTRACTION
WITH
RE-
P E A T E D WASHINGS WITH FRESH SOLVENT
W i t h these t w o tables showing t h e results obtainable in extraction b y repeated washings with fresh solvent a n d b y multiple extraction, respectively, we are now i n position t o make comparisons between these t w o methods. For convenience i n making t h e com-
1 This is not the most efficient way t o use 6 volumes of solvent in three extractions, since better extractions can be obtained by dividing the solvent into three such parts t h a t the solvent ratio is the same each treatment (see footnote p. 867). For instance, if 2 2 / 3 volumes are used in the first treatment and 11/3 volumes in each of the others, the solvent ratio would be &/3 for each treatment and the unextracted salt would be ( 8 / d 3 = 5 2 per cent; while with solvent ratios of 1, 2 and 2, respectively, for the X $/8 X = I/la = three treatments the unextracted salt would be 5.55 per cent. B u t these differences are very small, especially with high solvent ratios, or large number of extractions and in order t o make the results comparable with multiple extractions equal amounts of solvent will be used for each extraction instead of dividing, so as t o give equal solvent ratios. I n multiple extraction the relative amounts of solvent used in different treatments cannot be controlled because fresh solvent is used only for the last treatment of a charge and the previous treatments can use only this same amount over again.
.
T H E J O U R N A L OF I N D U S T R I A L A N D BiVGINEERING C H E M I S T R Y
S e p t . , 191;
869
P E R C E K T A G E R E C O V E R Y A N D VOLUME O F E X T R A C T W I T H D I F F E R E N T S O L V E N T RATIOS A N D D I F F E R E N T N U M B E R OF TREATMEKTS TABLE IV-MATERIAL SATURATED WITH SOLVEKT AT OUTSET x = Multiple Series Extraction. y = Repeated Fresh Solvent Extraction 6 TREATMENTS 1 TREATMENT2 T R E A T M E N T S 3 TREATMENTS 4 TREATMENTS 5 TREATMENTS ----x-----Y,----x7 y - y Vol. P e r c e n t 7 x 7 7-vSol7 x 7 -y--7 x 7 -y--Vol. % Vol. % Vol. % 1701. % Vol. 70 Vol. % 1701. % % Vol. % vent of Sol- Recovery Vol. % Vol. vent I and y Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Sol. Rec. Ratio 6 98.44 1 85.71 1 80.00 1 5 96.88 1 75.00 3 87. 50 4 93.75 83.33 50.00 1 66.67 2 75.00 I. . . . . . . 1 2 96.77 8 98.76 2 9 8 . 4 2 10 9 9 , 5 9 2 9 9 . 2 1 1 2 99.87 2 93.33 6 96.30 66.67 2 85.71 4 88.89 2 2. . . . . . . 9 98.44 3 99.17 12 9 9 . 6 1 3 99.73 15 99.90 3 9 9 . 9 1 18 99.98 3 97.50 6 93.75 75.00 3 92..31 3. . . . . . . 3 a 9 6 . 0 0 4 9 8 . 8 2 1 2 99.20 4 99.71 16 99.84 4 9 9 . 9 3 20 99.97 . . . . . .. . . . 4 80.00 4 95.14 4 ...... ... .. ... ,. . . . .. 5 99.87 20 9 9 . 9 2 . . 5 9 9 . 3 6 15 99.54 5 96 77 10 9 7 . 2 2 83.33 5. ...... 5 ... . . ... .. ,.. 6 9 9 . 6 1 18 9 9 . 7 1 6 9 9 . 9 4 24 9 9 . 9 6 . . ... , . 6 97 67 12 9 7 . 9 6 85.71 6 ...... 6 7 9 9 . 7 5 21 9 9 . 8 1 7 9 9 , 9 6 28 99.98 . . ... .. ... , . ... .. ... 7 98 24 14 98.44 87.50 7. . . . . . . 7 8 9 9 . 8 3 24 9 9 . 8 6 8 99.98 32 9 9 . 9 8 . . ... .. ... .. . . . . . ... 8 9 8 . 6 3 16 9 8 . 7 6 88.89 8 8. .. . . . .. ... ... 9 9 9 . 8 8 27 99.90 9 99.99 36 9 9 . 9 9 . . ... 9 9 8 . 9 0 18 9 9 . 0 0 90.00 9 9. ...... ... 10 . . . 30 9 9 . 1 0 20 40 99.17 9 9 . 9 2 99.91 90.90 10 10 10 10.
...
......
..
......
T A B L EV-MATERIAL NOT SATURATED WITH SOLVENTA T OUTSET x = Multiple Series Extraction. y = Repeated Fresh Solvent Extraction 2 TREATMENTS 3 T R E A T M E N T S 4 TREATMENTS 5 TREATMENTS
1 TREATMENT SolVolume Per cent 7 x 7 of Ex- Recovery Vol. ‘f& vent tract x and 3’ Ext. Rec. Ratio* 0 0 0 I ....... 0 50.00 1 75.00 2. . . . . . . 1 1 2 66 67 8 8.89 3....... i5.00 3 93.75 4....... 3 4 8 0 . 0 0 9 6.00 4 5....... 5 83.33 97.22 ....... 5 6 97.96 85.71 6 7 98.44 87.50 i 8 98.77 88.89 8 9 99.00 90.00 9 * For all treatments after thd first.
6
-y-
7C
7
x
7
-y-
Vol. Vol. % ’ Rec. Ext. Rec. Ext. 0 2 1 50.00 0 5 3 83.33 1 87.50 2 96.30 8 5 91.66 7 9 5 . 0 0 3 9 8 . 4 1 11 9 96.67 4 9 9 . 2 0 14 11 97.62 5 9 9 . 5 4 17 13 98.21 6 9 9 . 7 1 20 15 98.61 7 9 9 . 8 1 23 17 9 8 . 8 9 8 99.86 26 19 9 9 . 0 9 9 99.90 29 Solvent ratio is a - 1 on the
Vol. Ext.
a; after 1
ments
a(a
+
*
2
I
treatments a(a
etc.
+
; 3 treatI)
I n multiple extraction where
I)2’
t h e solvent ratio is a - I for t h e first t r e a t m e n t a n d a for t h e other treatments, t h e amount of unextracted salt after one t r e a t m e n t is 5 ; after z treatments, a after 3 treatments,
r; a
7-y--
-x-----.
... ... ... .... ..
FoRiiuLAs-with these conclusions in regard t o t h e solvent ratios when extracting d r y material, t h e formulas expressing t h e percentage extraction under varying conditions can be developed just as was done for wet material. T h e details need not be worked out again a n d t h e results only will be given. I n repeated treatments with fresh solvent where t h e solvent ratio is a I for t h e first t r e a t m e n t a n d a for t h e other t r e a t m e n t s , t h e formula expressing t h e a m o u n t of unextracted soluble material after one t r e a t m e n t is
---y---
-x-
Vol. % Vol. % Vol. %, Vol. Rec. Ext. Rec. Ext. Rec. Ext. Rec Ext. 75.00 0 0 0 1 3 87.50 0 94.44 1 93.75 7 98.15 1 9 6 , 8 8 9 97.92 2 9 8 . 7 6 11 99.48 2 9 9 . 5 9 14 9 9 . 0 0 3 99.61 15 9 9 . 8 0 3 99.90 19 4 9 9 . 8 4 19 99.91 4 99.97 . . 99.44 .. 9 9 . 6 6 5 9 9 . 9 2 23 9 9 . 9 5 , . 6 9 9 . 9 6 27 99.97 . . 99.78 .. .. 9 9 . 8 5 7 9 9 . 9 8 31 9 9 . 9 8 , . .. 9 9 . 8 9 8 9 9 . 9 8 35 99.99 9 ,.. .. 99.92 .. ... first treatment.
yo
A* U2’
etc.
Table V gives t h e computations from these formulas for different solvent ratios a n d different numbers of t r e a t m e n t s , t h a t is, i t gives a comparison between repeated washings Kith fresh solvent a n d multiple extraction when t h e original material is not s a t u r a t e d with t h e solvent on t h e s t a r t . T h e figures in Table V show relationships similar t o those of Table I V a n d t h e general conclusions on t h e advantages of multiple extraction hold here also. As a general comparison between Tables I V a n d V i t can be said t h a t , under similar conditions as t o t h e a m o u n t of original solvent used a n d number of t r e a t m e n t s , a somewhat less complete extraction b u t a greater concentration of solution is found, when t h e original material is not saturated with solvent. OTHER METHODS O P MAXIPULATION
I n t h e foregoing it has been intimated perhaps t h a t t h e solvent ratio was variable at will b y simply varying t h e a m o u n t of solvent used on a charge of material,
5%
Rec. 93 i 5 99.38 99.87 99.96 ,..
...
...
...
...
...
.
.
I
6 TREATMENTS -x---
-y-
Vol. % Vol. % Ext. Rec. Ext. Rec. 0 0 5 96.88 1 9 8 . 4 4 11 9 9 . 7 9 2 9 9 . 8 6 17 99.97 3 99.97 . . ....
..
.. ..
..
. . . . . . . . . .
. . . . .
. . . . . . . . . .
. . . . .
... ... ... ... ,..
,..
b u t in practice i t has been found t h a t t h e a m o u n t of solvent used per charge is determined largely by t h e character of t h e material t o be extracted, t h e most practicable a m o u n t being about t h a t much required just t o cover t h e charge ( t o fill t h e interstices between t h e solid particles). If much more t h a n this a m o u n t is used t h a t portion of t h e solvent above t h e charge is not used efficiently a n d tends t o be less concentrated t h a n t h a t in contact with t h e charge. If much less t h a n this a m o u n t is used t h e upper p a r t of t h e charge not i n contact with solvent is not as thoroughly extracted as t h e lower part of t h e charge. Therefore, in practice t h e solvent ratio is fixed within fairly narrow limits since t h e two terms of t h e ratio volume drained OJ (volume required t o cover minus volume retained after draining) a n d volume retaiiied cijter draining both depend upon t h e nature of t h e material t o be extracted. TWO
COh-SECUTIVE
TREATMENTS WITH
FRESH
SOL-
vEm-The same general effect on concentration of solution a n d on percentage of complete extraction as is obtained b y changing t h e solvent ratio may be obtained, however, b y changing t h e method of applying t h e solvent without changing t h e solvent ratio. I n certain cases where t h e value of t h e recovered product is high a n d t h e cost of t h e recovery process after extraction is relatively low i n comparison with t h e cost of extraction or i n cases where t h e insoluble part of t h e charge is t h e valuable p a r t , it might be desirable t o increase t h e percentage recovery of soluble material a t t h e expense of decreased concentration of solution without increasing t h e a m o u n t of extracting apparatus. These results can be brought about b y giving each charge t w o treatments with fresh solvent instead of only one, as has been t h e case in t h e methods previously studied. T h e concentrations of t h e solutions are practically t h e same as those obtained by doubling t h e solvent ratio, b u t t h e percentage recovery is better i n all cases,’ as shown in Table VI. 1 For the same reason t h a t two treatments each with one volume of fresh solvent is better than one treatment with a double volume (see p. 866)
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
870
TABLEVI-PERCENTAGE EXTRACTION W H E N EVERY CHARGEIS WASHED TWICEWITH FRESH SOLVENT -NUMBER TREATMENTS PERCHARGE-
--
4 6 3 ?umber Times Each Solvent Gsed& 2* 2 2&3* 3 3 & 4* 75100 ‘8 5 . 7 1 91.67 95.00 96.97 98.15 88.89 9,6 . 0 0 98.55 99.47 99.81 99.93 93.75 98.36 99.57 99.89 99.97 99.99 96.00 99.17 99.83 99.96 99.99 5 . . . . . . . . . . . . . 97.22 99.53 Y Y . Y Y . . , , . . 99.92 6. . . . .. . . . . . . . 9z.96 99.70 yY.96 _ _ . ... ... 7 . . . . . . . . . . . . YX.44 99.80 99.98 ... ... *Every alternate lot of solvent is used in one more treatment than t h e previous lot. **With t h e same solvent ratio the volume of solution is twice as great in this case as in the case where every charge is washed only once with fresh solvent, as in Table 111.
Solvent Ratio l**. . . . . . . . . . 2............. 3...... . . . . . . . 4.... ........
2
T h e formulas f r o m which t h e table was prepared are shown below.
TABLEVII-PERCENTAGE EXTRACTION BY ONE TREATMENT WITH FRESH SOLVENT FOR EVERY T w o CHARGES -NUMBER TIMES EACHSOLVENTUSEDSol3 4 5 6 7 8 vent -Number Treatments Per Charge------, 2 2 & 3(a) 3 3 & 4(a) 4 Ratio 1 & 2(a) 1 ( b ) . . . . . . . . . 42.86 45.83 47.50 48.48 49.07 2 . . . . . . . . . . . . 65.22 71.93 76.30 79.55 82.00 3 . . . . . . . . . . . . 76.36 83.97 88.43 91.62 93.79 4 . . . . . . . . . . . . 82.57 89.89 93.61 97.49 96.05 5 . . . . . . . . . . . . 86.39 93.12 96.09 97.89 98.81 6 . . . . . . . . . . . . 88.92 95.04 97.42 98.76 99.37 I . . . . . . . . . . . . 90.71 96.27 98.18 99.21 99.63 8 . . . . . . . . . . . . 92.03 97.10 98.66 99.77 99.47 9 . . . . . . . . . . . . 93.04 97.68 99.63 98.98 99.85 10.. . . . . . . . . . . 93.83 98.10 99.20 99.73 99.90 (Q) Every alternate charge gets one less treatment t h a n t h e previous charge. ( b ) With the same solvent ratio t h e volume of solution is only one-half as great as in t h e case where every charge gets one treatment with fresh solvent (Table 111) and only one-fourth as great as in Table VI. Number Solvent of Used TreatNumber ments Times
Number Solvent of Used Treat- Number ments Times 2 1 3 1&2 4 2 5 2&3 6 3 7 3&4
Vol. 9, No. 9
PERCENTAGE UNEXTRACTED*
These denominators form a series in which a n y t e r m = -44n-i A*%-2 I a n d t h e following a r rangement also shows their series relationship :
+
An
+
++ 1)3 - a + 1)4 + 1) (a + - 3Q(Q+ (a + - 4Q(Q+ + ( a + 117 - ~ Q ( + Q 1)d + 3 d ( a + 1) (a + l ) g 6a(a 4- 1)s + 6&(a + 1 ) 2 14 i- 1 1 9 - i a ( a + 118 + 10a2(a + 1)s - a3 (Q
3 & 4
(Q
(Q
7
2(a7
~ Q ( Q
+ 1 4 ~ 3+ 2 . 5 ~ 2+ 13a + 2 + 2a6 + 4a5 + 8a4 + 15a3 + 1 6 d + 7a + 1)
Q2
This method of t r e a t m e n t is really a combination of ordinary multiple extraction with repeated washings with fresh solvent a n d t h e results are naturally a mean between these two simple methods of extraction. OKE
TREATNENT
WITH
FRESH SOLVEST FOR EVERY
CHARGES-The opposite case, where high concentration of solution is desired, even a t t h e expense of incomplete extraction or longer series of extraction, can be obtained b y treating only every alternate charge of material with fresh solvent, every other alternate charge getting i t s final t r e a t m e n t with solvent which has been used once before; in this way two charges of extracted insoluble material are removed from t h e system for every one lot of solvent removed. Table T I 1 shows t h e results obtained b y this method of manipulation. The d a t a in Table VI1 were obtained from * t h e formulas listed beloJT this table. Of course, these two methods of manipulation can also be carried out as well on material not wet with solvent a t t h e outset (see p. 868). b u t it is not thought worth while t o include t h e results obtained under such conditions as t h e y show only t h e same general relationship between “wet” and “ d r y ” extractions a s has already been mentioned on p. 869. TWO
APPLICATION TO COhlIYERCIAL CONDITIONS
For t h e mathematical treatments an ideal case was assumed in which t h e insoluble portion of t h e original material does not prevent t h e complete dissolving of t h e soluble portion or t h e complete intermingling of t h e solution added a n d t h e solution already present, i t s only effect being t o prevent t h e complete draining off of t h e solution. Such conditions are closely approached in t h e concrete examples
*
These numerators form a series in which any term equals the previous term plus the second previousterm multipliedbyaor An=A,-~+AA,-2,
These denominations are a series in which a n y t e r m = An--’ An- 1 AAn- 2 , or t h e series can also be expressed as follows t o show t h e relationship between t h e various terms.
+
A,
~
~
++a 113+ -l ++ l)42a?(a + 1 ) 1 ) s - 3a2(a + 11% + 1)s - 4a?(a + 1)s + ad = (a + - 5az(Q + I ) * + 3Q4(Qf 1 ) = (a f 1 ) 8 + + 6Qr(Q+ = + l ) * - 6Q2(Q ia2(a + 1 ) 6 + 10ad(a +
Qz =
=
(Q
a3 = ( Q ar = (a as = (a as = (a Q7
a8 ~g
+
(Q
Q?
l)3
- a8
of glass beads wet with a solution of salt or encrusted with solid salt if t h e t r e a t m e n t with solvent is accompanied b y thorough mixing, such as might be accomplished b y boiling or mechanical agitation. These conditions could also be approximated in a n y commercial process in which t h e soluble substance is held a t or near t h e surface of t h e particles of t h e original material t o be extracted, b u t when t h e soluble substance is distributed evenly throughout each particle a n d t h e particles are large, t h e n t h e ready dissolving of t h e soluble substance is hindered a n d t h e theoretical extraction might not be obtained. I n this case, however, t h e soluble substance protected from action of t h e solvent b y t h e large size of t h e particle really has no effect in the process whatever a n d should not be considered in a n y s t u d y of t h e course of t h e extraction. For instance, if I O per cent of t h e soluble substance is completely protected from action of t h e solvent t h e course of t h e extraction will still be according t o t h e mathematical formula, except t h a t t h e theoretical limit of possible extraction is now 90 per cent of t h e actual soluble material present instead of I O O per cent; t h a t is, all figures are reduced b y I O per cent. If there is no agitation of t h e solid charge itself,
Sept., 1917
T H E J O V R X A L O F I N D C S T R I d L A N D ENGINEERI,VG C H E M I S T R Y
b u t only circulation of t h e solvent, as b y boiling, it is possible t h a t by packing together of parts of t h e charge and b y t h e flow of solvent through channels around t h e packed portions a certain proportion of t h e charge would never be acted upon b y solvent. This p a r t of the charge unacted on b y solvent would affect t h e course of t h e extraction in exactly t h e same way as t h e soluble substance near t h e center of t h e large particles, i. e . , t h e mathematical formulas would apply b u t all t h e figures would be reduced b y a percentage equal t o t h e percentage of t h e total charge not reached b y t h e solvent. If, however, certain portions of t h e charge not reached b y t h e solvent during t h e first application of solvent t o t h e charge are reached during later applications, t h e n t h e course of t h e extraction does not follow t h e formulas. But it is believed t h a t this action takes place only t o a very small ext e n t in commercial processes and it can be safely asserted t h a t a n y commercial extraction process in which such action of t h e solvent does occur is in general a n inefficient process, inefficient because i t requires a number of separate treatments t o effect t h e complete solution which might better be obtained in a single treatment with longer time or with some other improved condition. This method of looking a t t h e operation of a n extraction process shows t h e advantages t o be obtained b y keeping t h e solutions from t h e different treatments very well separated; imperfect draining of the solvent from a charge before applying t h e next solvent is 6quivalent t o decreasing t h e solvent ratio (because t h e solvent ratio varies inversely as t h e amount of solvent left in t h e charge) and decreased solvent ratio means decreased yields, other conditions being t h e same. In certain cases t h e extraction process is run in a manner t h a t may be called semi-continuous, t h a t is, each charge is boiled or otherwise agitated with its particular lot of solvent while s h u t off from t h e rest of t h e system (discontinuous), t h e n all t h e extractors are connected together and one pumping seraes t o force t h e solvent from each extractor t o the next in series (continuous in t h a t the solvent is being pumped in a t t h e same time it is being d r a n n off). This method of moving the solvent may be the result of an a t t e m p t a t economy (in making one pumping take t h e place of several) or it may be in imitation of t h e continuous method of extraction. K h a t e y e r the origin of this method. it is very faulty: the economy in pumping is false economy and the simultaneous movement of t h e different sol\-ents, is tvvhich is advantageous i n continuous here actually disadT-antageous. The main reason for t h e poor results of this method lies in t h e fact t h a t the only way t o obtain a high solvent ratio (and CorresPondinglY high Per cent extraction) is t o drain off one lot of solvent as CoInDletelv as Dossible before apthe next l o t , can readily be Seen that if o n e lot of solvent is used t o force Out another lot there is certain t o be mixing of the tlvo SolT-ents in the process (aside f r o m the ?2ecessars mixing due to the old solvent which is naturally retained b y t h e solid p a r t of t h e
87 I
charge); in very long narrow extractors this mixing might be negligible, b u t in a n y apparatus of commercially practicable size and shape the mixing would be serious. Any mixing of this sort has t h e same effect as decreasing the solvent ratio, t h a t is, i t decreases t h e percentage extraction.' A P P L I C A T I O N TO E X P E R I J I E K T A L
JVORK
Another important use for this conception of extraction mill be found in t h e simplification of experimental work on new extraction processes; with t h e formulas and tables a t hand it may no longer be necessary t o run complete tests in series extraction in order t o determine how many treatments a n d how much solvent are necessary. Instead, a single experimental treatment will determine t h e solvent ratio and percentage extraction and from these figures i t should be possible t o compute t h e results from different number of treatments. The effect of variations in methods of treatment, in time, pressure, temperature, etc.. can also be determined as well in a single extraction as in a series and then t h e results can be expanded t o any length series b y use of these formulas. For instance, suppose t h a t in treating a certain material with solvent, I volume of solvent was retained and 4 volumes were drained off; if 80 per cent of t h e soluble substance were found in t h e 4 volumes of solution then no change in time, pressure, or other factors would be necessary for perfect extraction and t h e results obtainable from various numbers of treatments in series could be found from Table 111, solvent ratio 4. If, however, only j z per cent were found, it T o d d indicate t h a t j z , 80 = 90 per cent would be the maximum t h a t could be expected under such conditions of extraction, however long a series were used. If some change, such as an increase of pressure. gave 76 per cent in first treatment with fresh solvent and with solvent ratio 4. t h e n 7 6 8 0 = 9j.o per cent would be the maximum t h a t could be expected with the new conditions of pressure; t h a t is, t h e maximum increase in yield, due t o t h e increase in pressure, would be j . 0 per cent. The yields obtainable with varying numbers of treatments can be determined from Table 111 b y using, respectively, 90 per cent and 9 j per cent of t h e yield percentages giT-en in t h a t table. coscLusIos -1 mathematical conception of the process of discontinuous extraction has been developed by means of n.hich the relative yields (total and per unit of solvent) obtainable by various methods of extraction are theoretically determined. This conccption. if applicable in practice. will be useful in the s t u d y of commercial processes and in the simplification of experinlentnl On
extraction'
FunEST PRouucTs lrBoR~'T"RY AI.4DlSOS, \vISCOSSIX
______
I n a continuoui extraction process the n d w n t a g e of the continuous movement or the solvent lies in t h e f a c t that different zones or solution in each extractor hax-e different concentrations. t h a t i,, different zone> of the same extractor may act as separate estractors, thus increasing the "number 01 treatments" (in terms of diicontinuous e x t r a r t i o n ~hut in simultaneous movement of solvents in diicontinuous estraction although there may be a "zoning" of t h e solution in each extractor a t t h e end of a pumping period, this is without effect in t h e final results because the "zoned" solution does not pass forward t o the next extractor h u t is immediately rendered homogenous b y agitation. 1
