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Na2cr04 from Domestic Chromite Concentrates by an Alkali-Fusion Method

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Na2cr04 from Domestic Chromite Concentrates by an Alkali-Fusion Method R I 1916 T Nor REMOVE FRCl1 r~·ReS~:r~~ n~:nter . ~ E. 315 Montgomery Ave. Spokane, WA 99207 Bureau of Mines Report of Investigations/1988 LI BRARY Na2Cr04 From Domestic Chromite Concentrates by an Alkali-Fusion Method By Gary L. Hundley, R. E. Mussier, R. A. Holmes, and R. S. Olsen UNITED STATES DEPARTMENT OF THE INTERIOR .. Report of Investigations 9167 Na2Cr04 From Domestic Chromite Concentrates by an Alkali-Fusion Method By Gary L. Hundley, R. E. Mussier, R. A. Holmes, and R. S. Olsen UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Hodel, Secretary BUREAU OF MINES T S Ary, Director Library of Congress Cataloging in Publication Data: Na2Cr04 from domestic chromite concentrates. (Bureau of Mines report of investigations : 9161) Bibliography: p. 12. Supt. of Does. no.: I 28.23:9167. 1. Chl'omium--Metallurgy. 2. Sodium chromate. 3. Chromite. 4. Sodium hydroxide. 5. Fused salts. 6. Leaching. 1. Hundley, Gary L. II. Title: Alkali-fusion method. III. Series: Report of investigations (United States. Bureau of Mines) ; 9167. TN23.U43 [TN799.C5] 62 s [669'.734] 88-600002 gas CONTENTS Abstract ......•...•..••......•..•....................•..... 1 Introduction ••••••••••••••••••••••••••••• · .............. 2 Raw materials ..••••.••••.•.••.•.•...•••.. 4 Countercurrent leaching •••••••••••••••••• .................... 5 Equipment and procedures ••••••••••••••• 5 Results and discussion ••••••••••••••••••• 6 Solution purification •••••••••••••••••••• 7 Crystallization •••••••••••••••••••••••••• ·. .. .. .. ............. 9 Equipment and procedures ••••••••••••••••• 9 Results and discussion •••••••••••••••• 10 Synthetic leach solutions ••••••••• · ............................ , .. 10 Actual leach solution ••••••••••••• 11 Summary and conclusions •••••••••••••••••••• . , .................. 11 Ref erences •.........••.•...•...••...•.•.•.• 12 ILLUSTRATIONS 1. Chemical processing of chromite •••••••• 3 2. Countercurrent leach flowsheet ••••••••• .................... 5 3. Vacuum evaporative crystallizer •••••••• • •••••••••••••••••••••••••• CI •• 0 ••••• 9 TABLES 1. Head analyses of chromite concentrates ••••••••••••••••••••••• .. .. 4 2. Results of two-s tep, three-stage countercurrent leach tests •• ........ " 6 3. Results of solution purification tes t s ......... .. .. ~ . • •••• (It 8 4. Results of vacuum crystallization tests ........ .. ...... .. .., ..... 10 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT °c degree Celsius kg kilogram cm 3/g cubic centimeter per gram L liter g gram min minute gil gram per liter mL milliliter h hour mL/min milliliter per minute in inch pct percent in Hg inch of mercury ppm part per million (atmospheric pressure) » ZE£2L!! Na2Cr04 FROM DOMESTIC CHROMITE CONCENTRATES BY AN ALKALI"FUSION METHOD By Gary L Hundley,1 R. E. Mussler,2 R. A. Holmes,3 and R. S. Olsen4 ABSTRACT The Bureau of Mines has devised a procedure to recover chromium chemi­ cals from concentrates derived from low-grade domestic chromites. These domestic chromites contain silicon and aluminum impurities at levels that are too high to permit processing by present industrial processes. The Bureau procedure consists of reacting chromite with molten NaOH under oxidizing conditions to form sodium chromate (Na2Cr04)' The reac­ tion product is leached with methanol to recover the majority of the unreacted NaOH, then with water to extract the Na2Cr04 and the remainder of the NaOH. The Na2Cr04 product is recovered by evaporative crystal­ lization from the aqueous solution. This report presents laboratory results of studies to determine the optimum leaching conditions for the reaction products of several domestic chromite concentrates. Results of solution purification and crystallization studies are also presented. The concentrates are from a variety of sources including the Stillwater Complex in Montana, Red Bluff Bay and the Kenai Peninsula in Alaska, and a Ni-Co laterite from southern Oregon. Sodium chromate crystals were produced that contained less than 0.01 pct NaOH. Chemical engineer. 2Research chemist. 3Chemist. 4Supervisory chemical engineer. Albany Research Center, Bureau of Mines, Albany, OR. 2 INTRODUCTION Chromium is a commodity that is essen­ Na2Cr04 in a methanol solution containing tial to the Nation's metallurgical, chem­ 10 pct NaOH is only 0.013 pct (4). The ical, and refractory industries. The methanol solution is then evaporated to United States has no domestic production recover the NaOH, which is recycled to or economic reserves of chromite, the the fusion reactor. only commercial ore of chromium, and must The residue from the methanol leach is rely on imports to meet national needs. water leached to recover the remainder of The chemical industry consumes approxi­ the NaOH and the Na2Cr04. Silicon and mately 25 pct of the chromite for the aluminum compounds are removed from the production of pigments, chromic acid for aqueous solution by adding a soluble electro plating, and other chemicals used silica compound to adjust the Si-Al ratio for leather tanning, wood preservatives, so the compound, NaAlSi04, will form and catalysts, and corrosion inhibitors precipitate from solution. The final (1 ).5 Na2Cr04 product is recovered from the -Commercial processes presently used to aqueous solution by evaporative crystal­ chemically treat chromite concentrates lization. The mother liquor exiting the include an oxidizing roast of the chro­ crystallizer is evaporated, and the mite with Na2C03 and CaO in a rotary kiln solids are recycled to the fusion reac­ at temperatures around 1,100 0 to tor. The Na2Cr04 product from this pro­ 1,150 0 C. The amounts of reagents and a cedure is a basic industrial chemical and diluent are controlled so that the reac­ can be used to produce the other common tion mixture remains as a solid phase in Cr compounds in commercial use. the kiln (2). Concentrates produced from This report describes experimental domestic ~hromite deposits contain too results for the leaching, solution puri­ much silicon to be processed by this fication, and crystallization steps of method. The silicon forms molten, sticky the procedure. A previous report (5) reaction products with the Na2C03, which describes the experimental results of the cause balls or rings of material to form fusion step for a variety of concen­ in the kiln, hindering its operation. trates. The general equation for the The aluminum content in domestic re­ fusion reaction of the chromite with the sources also is high, resulting in an NaOH is excess consumption of reagents (3). A simplified flowsheet for the Bureau of Mines procedure to recover chromium chemicals from domestic chromite concen­ + 2Na2Cr04 + 1/2 Fe203 + 2H20. (1) trates is shown in figure 1. Briefly, the procedure consists of reacting the Previous work by Chandra (~) showed chromite at 550 0 to 650 0 C with an excess that an excess of NaOH must be used to of molten NaOH under oxidizing conditions maintain a fluid reaction mixture. A to produce Na2Cr04" The product from the NaOH-to-chromite weight ratio of ~4:1 fusion reaction is solidified, crushed, (22.4:1 mole ratio) typically is neces­ and leached with methanol in a counter­ sary to maintain fluidity and to obtain current procedure. The methanol leach good conversion of the chromium in the removes the majority of the excess NaOH chromite to Na2Cr04' Reaction products while only removing a trace of the build up on the surface of the chromite Na2Cr04" This separation can be accom­ particles preventing further reaction so plished because of the large difference the mixture must be kept well agitated to in solubility of the two compounds in remove these surface products. A methanol. For example, the solubility of stirred-pot-type reactor was used in the present work. Studies also are being numbers in parentheses re­ conducted in Japan on a similar procedure fer to items in the list of references at using sodium nitrate as the oxidizing the end of this report. medium rather than air as used in the s 3 Makeup NaOH Oxidation and fusion Air-~ 550°-650° C Solidified fusion product Recycle methanol Methanol leach Solid-liquid Solution Recycle separation vaporation 1--NoOH --til.... ~_....--~::.J Water Recycle NaOH Condenser Soluble silica Solution Purification ; ~~ ~dr~: fa"~ d I mllllllll~1II1II1II I Residue to AI and Si disposal compounds FIGURE 1.-Chemical processing of chromite. Bureau method (L-~). The results of the marginal high-iron concentrates, the Bureau study indicated that chromium con­ chromium conversions were in the 90- to versions in the fusion reactor as high as 94-pct range, and with submarginal 98.9 pct can be obtained for high­ chromites the chromium conversion was in chromium, high-grade concentrates. For the 68- to 81-pct range. 4 RAW MATERIALS The chromite concentrates tested in A concentrate from southern Oregon was this study and presented in a previous derived from a Ni-Co laterite leach resi­ Bureau report (2) were obtained from a due. This material was the residue variety of sources in Alaska, California, remaining after processing in the Bu­ Montana, and Oregon. They were catego­ reau's roast-leach procedure for recover­ rized in one of the following groups: ing nickel and cobalt. Beneficiation of this residue resulted in a high-iron 1. High chromium (metallurgical-grade) chromite concentrate containing 41.5 pct chromite that contained a m1n1mum of Cr203 and 1.7 pct Si02 with a Cr-Fe ratio 46 pct Cr203 with a Cr-Fe ratio >2.0:1. of 1.8 (.!.!). A concentrate from the Mouat Mine in 2. High-iron (chemical-grade) chromite
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