industrial chemistry for teachers
Tantalum Recovery by
GUY B. ALEXANDER 8811 Grarnercy Lane Laurel, M a r y l a n d 20810
W h e n you hear the word, yield, what do you think :lhout? The amount of grain a farmer get,s from an acre of land? Something you calculated from an experiment you did in the laboratory? To the manager of a chemical plant, yield may be the percent of a ranr material converted to a useful product. To him, yield is important, and he's uncomfortable, or made uncomfortable, when it is lo\\-. Let's examine why, using tantalum as an example. Suppose tantalum ore is selliug for $10 per lb of contained tantalum oxide. Suppose you are the manager of a plant producing TazOs a t the rate of 25,000 lb per mont.h, so one month's supply of ore, assuming 100% yield, costs a qui~rt,erof a million dollars. I n act,ual cases, yields are never 100%. Let's say your plant averages 90%. Each month you lose 825,000 in Ta205 values. 111 a year t,hat's 8300,000. How hard would you work for $300,00O/year? How would you feel, if, some Monday morning your boss said to you, "Look, your yield last mont,h dropped t o 60%. That's a 8100,000 loss. Where did the tantalum go'?" Wit,h pressures of t,his kind, it is easy t o understand \vhy industry is const~arrt~ly searching for lower cost, more efficient processes. As an example of a highly efficient process, let's examine the priuciples used in extracting t,antalum from its ores. Ores containing tantalum nearly all contain columbium, iron, and manganese. T o recover tantalum, it is necessary t o separate these, and sometimes other elements. The oldest methods for separating tantalum and columbium is based on a fractional crystallization, based on differences in solubilities of K2TaF7 and K2CbOF5.The process was involved (hence expensive), yields were poor, often in the range of about 70%, and purity of the product was usually below 99%. Present day processing is based on liquid-liquid ext,raction. The ore is put in solut,ion in H F .
Ouce in solution, tautalum is separated from the other components of the ore by selective extraction, using an organic solvent, methyl isohutyl ketone, MIBK. To understand liquid extraction and the principles on which it is based one must uuderstand the behavior of
Liquid-Liquid Extraction two phase systems. MIBK and water are essentially insoluble in each other. As H F is added, they become morc miscible, until eventually the system becomes homogeneous in all proportious of MIRK and water. The H,O-MIRK system is rcpresented ill Figure 1.
Figure 1. A repre3ent. 100% H~20and E 100% MIBK. Points between A and E representvoriom percentages of the two liquids depending on how close they ore to A or E. The portion A B rcprerenh solutions of MIBK in woter; the portion D E, woter in MIBK, B D is a heterogeneous region; that is, there are two phases present.
If MIBK and H 2 0are mixed in the proportions reprcsented by any composit,ion on the line BD, the mixture will separate into tn-o phases. Suppose the overall chmposit,ion of the mixture was C. Such a mixture would separate int,o two phases, one, a water-rich phase, of composition R, and the other, an MIBK-rich phase, of composit,ion D. Phase R, being the more dense, will set,tle to t,he bott,om of the container aud phase D will float. The amount of each phase will depeud on the position of C relat,ive to B and D. If C is equidistant between B and D, t,hen there will be equal amounts of B and D. If C is closer to D, t,here will be more of phase D. In general, the relative amount,s are Phase B - CD Phase D CR
.1his proportiouality or mixing rule is t.rue if the com>
posit,ions arc expressed in mole fractions and only npproximat,ely true if the units are expressed in volumes. This is because of the volume contraction on mixing liquids. A three component system, such as HF-HIO-MIBK is often represented on a triangular graph. I n Figure 2, thc base of t,he trianglc rcprcsent,~mixtures of H20MIBK free of H F . The base correspouds to the line in Figure 1.The apex, x,represents 100% H F . Lines parallel to t,he base between t,he base and the apex represent intermediate percentages of HF. T o find the H F concent,ration of a point in the triangle, C, one can intcrpolate between two neighboring lines, parallel to the base. In a similar fashion the O/U H 2 0 and %,'MIRK in t,he mixture C can be determined. I n the H20-HF-MIBK phase diagram there is a Volume 46, Number
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MIBK
Figure 2. Three-phase system, HF.HzO.MIBK x = 100% HF; y = 100% MIBK; 1 = 100% H 2 0 .
Y
Figure 3. HF-RO-MIBK phage dimgram. of is the binodal cuno. The region inside n t is two-phase; the region outside is ringle-phase.
homogeneous or one-phase region and a heterogeneous or two-phase region. The shaded area in Figure 3 is the two-phase region, and the curve rst, separating the two regions is called t,he binodal curve. Any mixture of the three components which falls in the composition range represented by t,he shaded area will separate into two phases, whose compositions will be represented by the intersection of the tie line BD through the point C with the binodal curve rst (Fig. 4). The relative amounts of two phases will be D Phase B
-Phase =-
CB CD
Let us consider the hypothetical ease where pure MIBK is added to a solution containing HF-H20 of composition M (Fig. 5). The following things happen: At first, MIBK will dissolve. On continued additions of MIBK, the compositions move along the line MN toward N. When the composition of the mixture reaches the binodal curve, the solution will get cloudy and a second phase will appear as a thin layer on the top. As more MIBK is added the thickness of this layer will increase, and, eventually, if enough MIBK is added the composition will emerge from the two-phase region, the bottom phase will disamear, .. . and the solution will he, once again, homogeneous. If an amount of MIBK is added to HF-H20 composition M to give an overall composition C, then, as indi-
Figure 5. HF-H20-MIBK phase diagrmm showing the hypolheticol addition of MIBK to o mixture of HF-Hz0 of composition M. The mixture posses from single-phore to two-phase ond bock to ringie-phase again as it opprooches pure MlBK
158 / Journal o f Chemical Education
Figure 4. HF-HIO&UBK phore diogrom. BD is a tie line. A mixture of overoll composition C will reporate into two phores, one 0 water rich, lowar phare of composition 8, where B i s the intersection of the tie line through the point C with the binodal curve and on upper, MIBK rich phare D, the other paint of intersection of lhe tie line with the binodd curve.
cated above, the two phases B and D will form. If the tie lines are known, the composition of the two phases can be determined by drawing the tie line through C and extending it until it intersects the binodal curve. Note that the ratio of HzO to H F will be higher in B than it was in the original, in M (Fig. 6). Now suppose that MIBK is added to a solution of HF-H20 which also contains H2TaF7,and that the resulting mixture is in the two-phase region. The mixture will separate into two layers, an upper MIBK-rich layer and a lower, HzO-rich layer. Some HzTaFIwill be found in each layer. In systems of this type, the distributiou of the solute is governed by a number of factors, including the nature of the solute in the two nhases. If the molecular weight of the solute is the same in the two phases, then the ratio of the two concentrations is a constant, i.e., ~
~~~~~
where CI is the concentration of solute in phase 1 and C2 that in phase 2. In the case under discussion, the partition coefficient for H2TaF7is defiued as
where K is the partition coefficient,CE,the concentration of H2TaF7 in the extract (MIBK-rich phase) and Ca is the concentration of H2TaF7 in the raffinate
Figure 6. HF-HzO-MIBK phase diagram. The tie line BD indicotsr the cornporition of the tro pharer f a m e d when MIBK is added to HF-HzO of composition M to form o mixture of composition C.
(II,O-rich phase). Rut, Cx = W*:/V,. Cn = 1Z'~lVn
I\-hcrr M'I.: is tlic \wight of H2TaF7in t,hc extract,, V g , thc volumc of the cxtrtict., WR, t,he weight of H2TaFi i n the raffinate, and ITn, the volume of the raffinate. Let T = total weight of HITaF1in the system =
WE
+ Wn
Whence WR = T -WE
Then
Tantalum1
Figure 7. Schematic diagram of the extraction process, taken from Pierret, J. A,, US. Patent 3.1 17,833, Jnnuory 14, 1964.
But W E / Tis the fraction of the H2TaF7extracted. The way to make this fraction large is to make VR/VE small in relation to K . In general the partition coefficient for H2TaF, in the system H20-HF-MIBK is about 1. K varies somewhat with the acidity, dropping as the H F content of the system drops. An explanation for this might be the reaction H;TaR
+ zHgO;;'ILTa(OH), F,., + zHF
The species H2Ta(OH),h., is not as extractable as H?TaF,. i f thkre were equal volumis of extract and raffinate, and if K r e r e 1, then any H2TaR in the system xvould partitiou itself so that half of it would be in the extract phase and half of it in the raffinate. If now the first raffinate is mixed with fresh MIBK such that equal volumes of a second extract and raffinate are formed, half the remaining HzTaF, would be extracted, leaving only of the original in the second raffinate. After the third extraction % would be left, after the fourth %6, etc. Wit,h this background, let's discuss what happens, in actual practice, to the H?TaF7 solution prepared by dissolving tantalite ore in HF. First the HzTaF7 is extracted into MIBK. Carried with it are H2CbF, and other metal fluorides, impurities. Thc H2TaF7,H2CbFr, and impurities are separated by the countercurrent flow of the two immiscible solvents, for example, through mixer-settler boxes. The basis for the purification of HzTaFl is: (I) the partition co-
efficient of HZTaF7decreases as the H F concentration decreases, and (2) the partition coefficient of H2TaF7is greater than that of the fluoride complexes of the other metals in tantalum-containing ores. Thus, a t high H F concentrations, HlTaF7 and some impurit,ies are extracted into the MIBK-rich phase. At intermediate acid concent,rations, H2TaF7 remains in the MIBK, while the impurities are back-extracted into the aqueous phase. At low acidity H2TaF7is recovered in the aqueous phase, and the MIBK can be re-used in the next extraction-purification cycle. By proper control of acidity and by using sufficient stages in the countercurrent extraction cycle, yields of tantalum in the extraction purification process are essentially quantitative. A schematic of the process is shoxvn in Figure 7. An MIRK solution is fed into the middle of a mixer-settler extraction box, MIBK a t one end and a sulfuric acid solution a t the other. The strength of the acid is so adjusted (about 12 N ) that all metal fluorides exccpt H2TaF7 and HzCbF7 are extracted and removed in the aqueous raffinate. H2TaF7 and H2CbF7are carried to the second step in the MIBK extract. The process of step 1 is repeated, using more dilute HzSOa (IN), in xvhich case the aqueous raffinate contains the HlCbFi and the MIBK extract the H,TaF;. In step 3, the scrub solution is water, and the H2TaF7is essentially quantitatively recovered. When processes like this are operating smoothly, the plant manager can relax, a t least. with regard to this particular part of his operations.
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