INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1952
(7) Bridges, G. L., Pole, G. R., Beinlich, A. W., and Thompson, H. L., Chem. Eng. Progress, 43, 291 (1947). (8) Clark, E. L., Kallenberger, R. H., Browne, R. Y., and Phillips, J. R., Ibid., 45,651 (1949). (9) Emmett, p. E.. "Twelfth Report of the Committee on Cat&'sis," p. 64, New York, John Wiley & Sons, 1940. (10) Emmett, P. H., and Br auer, s., Trans. Elect~ochem.800.t 71, 383 (1937). (11) Fiecher, F.3 and,'rropsch, H., ffes. Abkndl. Kenntnis Kohl% 10, 333 (1930). (12) Hall, W. K., Tarn, W. H., and Anderson, R. B., J . Am. Chem. Soc., 72, 5436 (1950). (13) Koelbel, H., and Engelhardt, F., Erd#E u.KohEe, 2,52 (1949). (14) Krieg, A., Dudash, A. P., and Anderson, R. B., TND. ENG. CHEM., 41, 1508 (1949).
Y
401
(15) Larson, A. T., and Richardson, C. N . , Ibid.,17, 971 (1925). (16) Pichler, H., Technical Oil Missions Reel 101, Documents PG-21, 559-NID, and PG-21,574-NID. (17) Pichler, H., U. S. Bur. Mines, special rept. (18) Pichler, H., and Merkel, H., U. S. Bur. Mines, Tech. Paper 718 (1949). (19) Scheuermenn, A., in report to Zorn, H., PB-97,368; FIAT Final Report 1267 (1949). (20) Starch, H. H,, et u. 8. B ~~ ~ . i~ ~~papm ~ ~709 h (1948). .~ , (21) Thiele*E*W'* IND' 31v (Ig3')* RECEIVED September 25, 1950. Presented before the Division of Gas and Fuel Chemistry at the 118th Meeting of the AMERICAN CEEMIUAL SOCIETT, Chicago, Ill.
Vanadium Recovery from Chromate liquors
I -
development
I
T. S. PERRIN, 1. N. JENKINS,
AND
R. G. B A N N E R
DIAMOND ALKALI CO., CLEVELAND, OHIO
M
OST all chromium compounds are made from sodium or potassium dichromate and the primary source is chromite ore. Chromite ore contains mainly iron chromite, Fe0.Cr203,but it also contains considerable magnesium oxide and aluminum oxide. Magnesium oxide replaces a part of the FeO and aluminum oxide replaces a part of the CrzOs in the spinel-type mineral. Silica and vanadium oxide are the outstanding minor contaminants of the ore. The chromate industry uses a chromite ore, which usually contains 44 to 48y0 CrrOoand which is low in silicon dioxide. The vanadium oxide content averages approximately 0.50%. Calcium oxide will average 5%. The production of sodium dichromate is brought about by roasting the pulverized ore with soda ash and, if desired, lime. The roast is leached with water and the alkaline leach liquor is partially neutralized with sulfuric acid to precipitate hydrated alumina. The hydrated alumina is filtered off, leaving a filtrate, which for the purpose of this article will be called neutral liquor. Additional sulfuric acid is added to the neutral liquor for converting the chromate to dichromate and, upon concentration, sodium sulfate crystals are formed and removed, producing red liquor. With further concentration and cooling, NazCrZOr.2Hz0is crystallized out, leaving a mother liquor. Vanadium occurs to only a small extent in the ore and, in general, about half of that in the ore is carried into the leach liquor as NaV03, sodium metavanadate, and about one half to two thirds of the sodium metavanadate in the leach liquor is carried into the neutral liquor, some being removed with the alumina hydrate. The percentage of NaV03 in chrome liquors will be expressed as the per cent vanadium pentoxide on the basis of the amount of NazCrzO~.2Hz0 contained in the liquors. The neutral liquor, depending upon the type of roasting and leaching used, may contain from practically no vanadium pentoxide to 0.3% on the above basis. However, in most commercial practices the vanadium pentoxide content of various chrome liquors will range from 0.07 to 0.16%. Verv little of this contaminates the dichromate crwtals unless the mother liquor is recycled. With recycling of mother liquor to the next batch of liquor to be crystallized there is a build-up of vanadium pentoxide and the crystals then become contaminated.
Chronlates are used both in the form of red liquor and as crystals, and the vanadium contamination of either is undesirable for many uses. The outstanding objections to vanadium are in the tanning industry and the pigment industry. The presence of vanadium in chromium compounds used for tanning leather of certain types causes off-colors and its presence in chromium compounds used for making pigments gives inconsistent shades. Large consumers of chromates in the tanning and pigment industries would like the vanadium pentoxide content not to exceed O . O l ~ oand some specify that it not exceed 0.03%. Since these two fields consume a majority of the chromate production a method of removing vanadium is very desirable. The only method found in the literature (1)for removing vanadium on a commercial scale from chromate compounds is based upon coagulation of a vanadium complex, NaZHzV6Olr,from a chromate solution by p H adjustment, agitating, and heating. This method is commonly used (9, 3) in the production of vanadium pentoxide from vanadium ores. It has disadvantages and limitations when used for removing vanadium from chromate production liquors. The coagulation is slow and ineomplete in solutions having concentrations below about 15 grams of vanadium pentoxide per liter. For that reason large storage space is required. Also the vanadium concentration must be allowed to build up or poor removal must be tolerated. This method also consumes additional materials for lowering the p H before removing the vanadium and for raising the p H after removal of vanadium. The precipitate formed from solutions of low vanadium concentration is colloidal and somewhat difficult to filter. The method described below and shown in the flow diagram in Figure 1 is intended to overcome these difficulties. Recovery of a large part of the vanadium is in a form that contains about 85% vanadium pentoxide. PRINCIPLE OF THE PROCESS
I -
The difficult problem involved in removing vanadium contamination from chromate liquors is readily understandable when it is considered that the sodium metavanadate occurs in the solution to the extent of only 1 part vanadium to 3 or 4 thousand
INDUSTRIAL AND ENGINEERING CHEMISTRY
402
parts of solution. Also about 1000 times as much sodium chromate is mixed with the vanadium. The chromate solution must not be contaminated with other substances and the removal must not upset the chromate production process. Naturally, the process must be economical. With these factors in mind an extended investigation was made using adsorbents, solvents, precipitants, and additives (in case of roasts) in all the stages of production. The best results were incorporated in a process shown diagrammatically in Figure 1.
R E T U R N TO CHROMATE P R O C FOR N E U T R A L I Z I N G LEACH L l O AL
PLATE 0 FRAME FILTER
W&SH
RETURN TO CHROMATE PROC. -NEUTRAL LIOUOR I A N K S
Figure 1. Flow Sheet for Removal of Vanadium from Dichromate Liquors
The process is based upon obtaining the vanadium in a more concentrated solution with respect to vanadium and less concentrated with respect to chromium than i t occurs in neutral liquor and then coagulating the vanadium from the concentrated solution by adjusting the pH, agitating, and heating. The concentrating step is affected by stirring a lead sulfate slurry with neutral liquor, whereby a mixture of lead vanadate and lead chromate is formed, separating the mixed precipitate of lead vanadate and lead chromate from the neutral liquor by settling and filtering, and extracting the lead vanadate-lead chromate cake with sulfuric acid. The filtrate of purified neutral liquor is returned to the chromate production process a t the point where neutral liquor would normally be processed. The lead sulfate resulting from treatment of the lead vanadate-chromate with sulfuric acid is recycled for further use. The extracted vanadic and chromic acids are adjusted with respect to p H by adding leach liquor. Then hiazHzV6O1,is coagulated from the solution by heating and agitating. The acid filtrate is returned to the chromate production system and used to neutralize leach liquor. EXTRACTION OF VANADIUM FROM NEUTRAL LIQUOR
The removal of vanadium from neutral liquor depends upon the relative solubilities of lead sulfate, lead chromate, and lead vanadate in the neutral liquor which has a pH of about 8.5. The solubilities decrease in the order listed, lead vanadate being by far the least soluble. Since there is much more chromate than vanadate present, a mole ratio of lead sulfate to vanadium pentoxide far higher than 1 to 1 is needed to give efficient vanadium removal. The lead sulfate must be preslurried to prevent the formation of lumps which would cause incomplete reaction of the lead sulfate. The time of reaction and the use of recycled lead sulfate are also factors that must be considered. The effect of the different factors influencing the reaction are shown in Tables I to IV. The recommended method for conducting the reaction is mixing thoroughly for 10 minutes a preslurried amount of lead sulfate equivalent to an 18 to 1 mole ratio of lead sulfate to vanadium pentoxide, with neutral liquor maintained a t a temperature of 80" C. and having a density of 30" BB. or greater.
Vol. 44. No. 2
The lead sulfate may be recycied indefinitely without loss of efficiency provided there i s no mechanical loss. A purified neutral liquor containing no more than 0.02% vanadium pentoxide results. ACID EXTRACTION OF LEAD VANADATE-LEAD CHROMATE CAKE
The lead vanadate-lead chromate cake is extracted with sulfuric acid. It is important that this be done under the correct conditions. More sulfuric acid than the equivalent of the lead is needed to completely extract the vanadium and chromium. It was found that the optimum mole ratio of sulfuric acid to lead sulfate should be 1.2 to 1. More than this amount should not be used as it would require more neutralizing to raise the p H in the subsequent coagulation step. Using pure substances, concentrated sulfuric acid partially reduces chromate and vanadate ions to trivalent chromium ions and to vanadyl ions, respectively ( 4 ) . The characteristics of the acid extract and the relative quantities of vanadium that will coagulate and precipitate from the extract of the cake produced by this process indicate that more reduction takes place in the acid extract than in similar solutions made with pure chemicals. These characteristics prevail even when extracting with less concentrated sulfuric acid, very likely because of the presence of impurities in the cake and in commercial sulfuric acid. The temperature of the acid during extraction also affects the extent of apparent reduction. Filtering the acid extract through paper and conducting the extraction in iron equipment intensifies this property. The chromate solution is eventually returned to the production system and, therefore, i t is undesirable for the solution to contain reduced chromium. Only vanadium in its highest valence state will form the complex and coagulate. Therefore, it is important not to reduce the vanadium. In the early stages of this project the arid extract was subjected to oxidation with sodium hypochlorite or, better, lead peroxide, but this step can be avoided by using the proper acid concentration, the proper temperature, filter medium, and construction material hereinafter described. The addition of oxidizing agents to the extract from concentrated acid extraction not only gives better precipitation of vanadium but causes the dark red solution to change to bright red, the latter being characteristic of chromate solutions free of reduced chromium. The use of dilute acid for extracting gives the same effect. A mixture of 1 volume of concentrated sulfuric acid and 2 volumes of water (1 to 2) leaches the cake well but requires the addition of an oxidizing agent to the extract to give good coagulation of the vanadium. An acid to water ratio of 1 to 3, when used at the proper temperature, leaches and coagulates the vanadium
TABLE I.
E r F E C T OF PRESLURRYIXG
hlole ratio PbSOd t o VzOs. 18 to 1 Temperature. 80' C . Reaction time. 10 minutes Each test independent of others V205 Remaining in Purified Liquor, 7 0 Form of PbSO4 Used 0.22 No PbS04 0.11 D r y PbSO4 0.10 D r y PbSO4 0,008 Preslurried PbSOi 0.008 Preslurried PbSOi 0.015 Preslurried PbSOi 0.013 Preslurried PbSOa
TABLE11. EFFECTOF QUANTITYO F LEAD SULFATEUSED Temperature. 80° C. Time. 10 minutes Each test independent of others Mole Ratio VzOs Remaining in Purified Liquor, % PbSOa to VzOs 0.0 10.0 13.5 18.0 22.5
0.22 0.05 0.015 0.012 0.008
INDUSTRIAL A N D ENGINEERING CHEMISTRY
February 1952
TABLE 111. EFFECTOF REACTIONTIME Mole ratio PbSOl t o VzOs. 18 t o 1 Temperature. 80' C. Each test independent of others VzOa Remaining in Time of Reaction, ' Minutes Purified Liquor, Z 0.22 0 0.028 5 0.012 10 0.008 15
LEADSULFATE TABLE IV. EFFECTOF RECYCLING Mole ratio PbSOr to VnOs. 18 to 1 Temperature. 80' C. Time, 10 minutes ._._ >.-A .* .IL__. 1
-J r e mL :inuepwnueuc 01 ortier8
Amount of HzSOr used for extracting PbSO4 varied from 1 to 1.2 equivalents of PbS04 VzOs Remaining in Purified Liquor, % Unused PbSOd 0.015 1 recycle 0.014 2 recycles 0.028 3 recycles 0.038 0.038 4 recycles 5 recycles 0.020 6 recycles 0.020
satisfactorily; therefore, it is the preferred dilution ratio. The coagulation is not so good without the addition of an oxidizing agent, however. An acid to water ratio of 1 to 4 does not give satisfactory leaching. The temperature of the extracting acid should be 70" C. or less. Vin on cloth or ceramic filters are satisfactory filtering mediums anJlead or stainless steel are satisfactory construction materials. After the cake is extracted with the acid, it is washed with water until the washings are colorIess. The washings are added to the extract. Table V shows the effect of these conditions. Both the leaching and coagulating efficiencies must be considered in judging the extracting process. The use of dilute acid for extraction avoids the addition of an oxidizing agent, which results in an economic saving. The lead sulfate cake resulting from the extraction is slurried with sufficient neutral liquor or water to enable it to be pumped back to the lead sulfate surge tank for reuse.
TABLE V.
EFFECTOF EXTRACTING CONDITIONS
Constant conditions 1.2 equivalents of HeSOr t o PhSOr used for the extraction. Cake slurried for 10 minutes with acid. Coagulation a t 90° C. with mild agitatipn f p r 3 hours. The pH of,the extract adjusted to 2.0. using leaphliquor. Oxidations were done b y addtng solid lead peroxide to the hot acid extract VzOs, 70Recovered from Cake Acid by, extract b y Conditions of Extraction leaching coagulation 1 to 2 acid to water volume ratio a t 95' C. Oxidation of acid extract 100 96 Without oxidation, filtered through fritted glass 100 53 Without oxidation, filtered through paper 100 1 1 to 3 acid t o water volume ratio 94.8 At 95' C with oxidation filtered through glass 90.5 85.4 At95' C" without oxidadon filtered through lass 90 5 86.2 A t 70' C' without oxidatioh. filtered througff glass 100 84.9 At 25' C.:'without oxidation, filtered through glass 100 At 60° C., in stainless steel, without oxidation, filtered lhrough glass 96 86.3 1 to 4 acid t o water volume ratio at 90' C . 73 86.4 Without oxidation, filtered through glass
The results shown in Table V indicate that reduction of chromium, and possibly vanadium, causes poor coagulation and precipitation of the vanadium from the acid extract. Attempts to show the quantities of reduced chromium and vanadium in the acid extract gave inconsistent results, although approximately 1.6% of the chromium was found to be reduced when hot 1 to 1 sulfuric acid was used for the extraction. The relative electrode potentials of Crz07-2, Cr + 3 , and V01-8 and VO f 2 indicate that no reduced vanadium could be present with hexavalent chromium. In order to show the reducing effect of hot sulfuric acid of different concentrations, a series of tests was made. Three 1-gram
403
samples each of ure sodium dichromate and sodium metavanadate were treatelwith concentrated sulfuric acid, 1 to 1 sulfuric acid, and 1 to 2 sulfuric acid and heated near the boiling temperature for 15 minutes. The solutions were then made up to appropriate volumes and aliquots used for titrations. T o the vanadium samples, excess ferrous sulfate was added, the acid concentration adjusted and the excess ferrous sulfate back-titrated with 0.0260 N potas&um permanganate solution using orthophenanthroline ferrous sulfate indicator. From this titration and a blank on the ferrous sulfate, the oxidized vanadium was determined. Continuation of the titration after the orthophenanthroline end point to the permanganate end point gave the total vanadium present. The difference between the total and oxidized vanadium gave the reduced vanadium. The reduced chromium in. the chromium solutions was determined by double preci itation of the hydroxide from a boiling solution, oxidizing the c%romium with perchloric acid, and titrating with 0.0260 N otassium permanganate. The total chromium was determinecfbyoxidizing the solution with perchloric acid and titrating with 0.0260N potassium permanganate. Table VI shows the results of these experiments.
TABLE VI.
REDUCTION OF VANADATE AND CHROMATE WITH HOT SULFURIC ACID
Reduced Vanadium, % Reduced chromium, %
Coded. His01 2.03 8.90
1 to 1 HzSO4 1.68 0.00
1 t o 2 HzSOI
0.00 0 00
ANALYSIS OF VANADIUM PRECIPITATE
TABLE VII.
%
VzOs
84.30 9.03 2.54 3.13 0.68 0.13 0.10 99.81
NazO FezOa Hz0 (calcd.) SO8 CrO: SiOa
To show whether or not reduced vanadium can exist in the presence of dichromate, a sample of sodium vanadate was reduced 50% with ferrous sulfate, and sufficient sodium dichromate and sulfuric acid were added to simulate the acid extract. The reduced chromium formed by reaction of dichromate with reduced vanadium was determined and its reduced vanadium equivalent was compared with the original amount of reduced vanadium. The dichromate reduced was equivalent to 86% of the reduced vanadium. The extent of reduction is slight except for concentrated sulfuric acid. However, the presence of impurities in the cake and commercial sulfuric acid may cause more reduction when the cake is extracted with less concentrated acid. As pointed out above,
TABLE VIII.
PH 1.0 1.5 2.0 2.8 TABLE
Ix.
EFFECT OF pH
UPON
COAGULATION
17 grams VZOSper liter of extract Heated 3 hours a t 90' to 95" C. Extract oxidized Each test independent of others VnOs, % Coagulated 81.3 94.2 97.5 18.7
EFFECT O F
TIMEO F AGITATIONA N D HEATINQ
17 grama V Z O per ~ liter extract pH. 2 Extract oxidized Temperature. 90' to 95' C. Time of Agitation, VnOs. Hours % Coagulated 36.2 67.0 74.6 95.5 97.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
404
Vol. 44, No. 2
1.6% of the chroniium was reduced when the cake was extracted with 1 to 1 sulfuric acid, No attempt was made to purify the precipitated vanadium. The analysis of the dry crude material indicates that the substance is disodium dihydrogen hexametavanadate, Na2H2VeOl7,as reported in the literature (3). The analysis of the crude product is given in Table VII.
The application of this process for purification of chromate liquors depends upon the end use of the liquors and the quantity of vanadium contained in the liquor. It is possible in the processing of chromite ores to alter the conditions of operation and t o allocate liquors containing more or less vanadium so as to meet the needs of the customers.
COAGULATION OF VANADIUM FROM ACID EXTRACT
(1) Argall, George O., Jr., Quart. Colo. School Mines, 38, No. 4, 36,37
LITERATURE CITED
(October 1943).
By adding leach liquor to the acid extract until the p H reaches 2 and heating to 90” C. while gently agitating for 3 hours, colloidal Na2H2V& forms and coagulates into an easily filtered product. The coagulation is best when the vanadium pentoxide content of the acid solution is 15 grams per liter or more. It is important to control the pH and time of reaction as shown by Tables VI11 and IX.
(2) Mellor,
J. W., “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. IX, p. 754, New York, Longmans, Green and Go., December 1939. (3) Ravitz, F.,et al., Metals Technol., 14, 12 (June 1947); Am. Inst. Mining Met. Engrs., Tech. Pub. 2178. (4) Van Wirt, A. E., et al., U . S. Patent 2,357,988 (1944). RECEIVED February 24, 1951.
Performance o development UNDER CONTROLLED AGITATION GEORGE FEICK
AND
HARRY M.ANDERSON
ARTHUR D. LITTLE, INC., CAMBRIDGE 49, MASS.
E
QUIPMENT for carrying out liquid-liquid extractions
may conveniently be divided into two basic classes (9) according as the separation of the two immiscible phases is effected by gravity or by centrifugal force. The most common forms of apparatus employing gravity separations are the countercurrent columns and the various mixer-settler combinations. I n general, the countercurrent columns suffer from the disadvantage that the only energy available for agitation and maintenance of the dispersion is that which is derived from the density difference of the two liquids. Since this energy is usually insufficient to produce any subdivision of the dispersed-phase droplets, there is no mechanism for counteracting the effects of drop coalescence and the resulting decrease in the surface of contact between the two phases The result of this energy deficiency is that a column is a much less efficient device when used for extraction than it is when used for contacting a liquid with a vapor or a gas supplied by forced flow. On the other hand, the mixer-settler combinations provide abundant power for agitation and dispersion but have the drawback that they inherently involve batch operation. It is, therefore, awkward and expensive to carry out extractions in such equipment where the process requires a large number of equilibrium stages I n recent years, several types of apparatus have been devised which combine in some measure the desirable countercurrent feature of the column with the excellent interphase contact of the power-driven mixer ( 2 , 6, 8 IO, IS). Most of these devices consist essentially of a number of mixers arranged vertically to form a column with packed or baffled settling zones between each mixing zone. An experimental study of such a device has recently been published by Scheibel ( 1 2 ) . Although some of these devices are relatively complicated mechanically, their efficiency appears to be high. A different arrangement for combining countercurrent action with mechanical agitation is that of van Dijck ( 8 )who provides
an unpacked column 15 ith a number of close-fitting perforated plates which are so constructed that they may be moved up and down with respect to the column and its liquid contents. Alternatively, the plates niay be made stationary and the liquids reciprocated by means of an external piston and cylinder, Unfortunately, no experimental results with such a column appear to have been published. However, the operation of this device is still stepwise t o some degree rather than being perfectly continuous. From the foregoing considerations, it would seem that a more desirable way of conducting an extraction would be to use a packed column and to supply the desired agitation by moving the liquids up and down. I n this way the packing becomes, in a sense, a stirrer, with the advantage that its energy of agitation is uniformly supplied over the entire volume of the liquids under treatment without interfering in any way with the continuity of the countercurrent action. The desired agitation may be supplied by means of a flexible diaphragm, a piston and cylinder, or other suitable apparatus located outside the column, or perhaps more simply by periodically interrupting the flow of liquids into the column. Such an apparatus has the practical advantage that its moving parts are readily accessible and out of contact with the liquids under treatment. The vigor of the agitation can be kept under close control by regulating the frequency and amplitude of the reciprocating motion. I n order to test these ideas, a small experimental column was built with which the extraction of benzoic acid from toluene with water could be studied under various conditions of agitation. It was found that the drop size of the dispersed toluene phase could easily be controlled over the entire range from about inch in diameter to a very fine emulsion. The performance of this column with and without agitation was compared by calculation of extraction co.efficients, HTU values, and number of equilibrium contacts. By any of these criteria, agitation
