Use of Calcium Fluoride in the Bayer Process

Abstract

A Bayer process includes contacting bauxite and calcium fluoride with sodium hydroxide and forming a slurry, with the calcium fluoride may provide at least 1 wt. % of the calcium added to the process, and precipitating alumina from the slurry.

Claims

1. A Bayer process that includes: contacting bauxite and calcium fluoride with sodium hydroxide and forming a slurry, with the calcium fluoride providing at least 1 wt. % of the calcium added to the process, and precipitating alumina from the slurry.

2. The process defined in claim 1 includes the following steps: comminution, digestion, flash cooling, clarification, precipitation, and calcination steps, and may include optional pre-desilication and spent liquor evaporation steps.

3. The process defined in claim 1 includes adding calcium fluoride in a solid form or as a suspension or solution in liquid.

4. The process defined in claim 2 including adding calcium fluoride in any one or more than one of the comminution, pre-desilication, digestion, flash-cooling and clarification steps of the process.

5. The process defined in claim 1 wherein the calcium fluoride provides at least 10 wt. % of the calcium added to the process.

6. The process defined in claim 1 wherein the calcium fluoride provides at least 25 wt. % of the calcium added to the process.

7. The process defined in claim 1 wherein the calcium fluoride provides at least 50 wt. % of the calcium added to the process.

8. The process defined in claim 1 wherein the calcium fluoride provides 100 wt. % of the calcium added to the process.

9. The process defined in claim 4 includes adding calcium oxide in any one or more than one of the comminution, pre-desilication, digestion, flash-cooling and clarification steps.

10. The process defined in claim 1 including precipitating a calcium-phosphorus-fluorine-containing compound and/or a calcium-phosphorus containing compound from the slurry as a consequence of the addition of the calcium fluoride and thereby reducing the phosphorus concentration in the slurry.

11. The process defined in claim 10 wherein the calcium-phosphorus-fluorine-containing compound is in the form of carbonate-apatite including (Ca, Na).sub.5(PO.sub.4.CO.sub.2OH)(OH, F, Cl) or Ca.sub.5(PO.sub.4).sub.3(F, Cl, OH).

12. The process defined in claim 11 wherein the calcium-phosphorus-fluorine-containing compound is fluorapatite (Ca.sub.5(PO.sub.4).sub.3F).

13. The process defined in claim 1 wherein the calcium fluoride has a purity ranging from 40-100%, suitably 60-90%.

14. The process defined in claim 1 wherein the calcium fluoride is obtained from a low caustic leaching and liming (LCLL) process, such as a LCLL process of a spent potlining treatment plant.

15. The process defined in claim 1 wherein the calcium fluoride is obtained from any suitable source.

16. A Bayer process that includes: contacting bauxite and calcium fluoride with sodium hydroxide and forming a slurry, with the calcium fluoride providing at least 1 wt. % of the calcium added to the process, precipitating a calcium-phosphorus-fluorine-containing compound and/or a calcium-phosphorus containing compound from the slurry as a consequence of the addition of the calcium fluoride and thereby reducing the phosphorus concentration in the slurry, and precipitating alumina from the slurry.

17. A Bayer process that includes: contacting boehmite-containing and/or gibbsite-containing bauxite and calcium fluoride with sodium hydroxide and forming a slurry, with the calcium fluoride providing at least 1 wt. % of the calcium added to the process, and precipitating alumina from the slurry.

18. The process defined in claim 1 wherein the calcium added to the process includes calcium oxide.

19. The process defined in claim 1 wherein the calcium added to the process includes low lime excess CaF.sub.2 containing 80-81% CaF.sub.2, 8-9% CaCO.sub.3, and 3-4% Ca(OH).sub.2.

20. The process defined in claim 1 wherein the calcium added to the process includes high lime excess CaF.sub.2 containing 64-65% CaF.sub.2, 5-6% CaCO.sub.3, and 17-18% Ca(OH).sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention is described by way of example only, with reference to the accompanying drawings, wherein:

[0046] FIG. 1 is a small-scale experiments flowsheet (single-stream configuration);

[0047] FIG. 2 is a Bayer process flowsheet (double-stream configuration) in accordance with one embodiment of the invention which illustrates options for adding CaF.sub.2 and/or CaO, with the flowsheet illustrating the digestion process, in double-stream configuration, noting that the sweetening and double-digestion steps are not illustrated in the figure;

[0048] FIG. 3 presents digestion results with “LLE” CaF2 (mean values±std. dev)−A/C ratio, noting that the 100% CaO case is specific to each series of tests since it is replicated to make sure to have the exact same conditions, and therefore, the 100% CaO case results might differ from a co-dosage test to another.

[0049] FIG. 4 presents digestion results with “LLE” CaF.sub.2-breakpoint analysis, noting that the graph illustrates breakpoint analysis curves based on alumina extraction for tests run using either “LLE” CaF.sub.2 or lime, both compared to a “witness test” without lime addition;

[0050] FIG. 5 presents digestion results with “LLE” CaF.sub.2-fluoride concentration (mean values±std. dev), noting that the 100% CaO case is specific to each series of tests since it is replicated to make sure to have the exact same conditions, and therefore, the 100% CaO case results might differ from a co-dosage test to another;

[0051] FIG. 6 presents digestion results with “LLE” CaF.sub.2-phosphorus concentration (mean values±std. dev), noting that the 100% CaO case is specific to each series of tests since it is replicated to make sure to have the exact same conditions, and therefore, the 100% CaO case results might differ from a co-dosage test to another;

[0052] FIG. 7 presents digestion results with “BILE” CaF.sub.2-phosphorus and fluoride accumulation (mean values), with the graph illustrating the mass variation per gram of digested bauxite of both phosphorus and fluoride in liquor for tests run using “HLE” CaF.sub.2 and CaO in various proportions;

[0053] FIG. 8 presents digestion results with “LLE” CaF.sub.2-phosphorus and fluoride accumulation (mean values), with the graph illustrating the mass variation per gram of digested bauxite of both phosphorus and fluoride in liquor for tests run using “LLE” CaF.sub.2 and CaO in various proportions; and

[0054] FIG. 9 presents digestion results with “LLE” CaF.sub.2-carbonate concentration (mean values±std. dev), noting that the 100% CaO case is specific to each series of tests since it is replicated to make sure to have the exact same conditions, and therefore the 100% CaO case results might differ from a co-dosage test to another, and with the graph illustrating the carbonate concentration in the pregnant liquor for tests run using “LLE” CaF.sub.2 and CaO at various proportions, compared to the base case using only CaO.

DESCRIPTION OF EMBODIMENT

[0055] The invention is Bayer process that includes contacting bauxite and calcium fluoride with sodium hydroxide and forming a slurry and precipitating alumina from the slurry.

[0056] FIG. 2 is an embodiment of the invention.

[0057] The Figure is a Bayer process flowsheet (double-stream configuration) which illustrates options for adding CaF.sub.2 and/or CaO, with the flowsheet illustrating the digestion process, in double-stream configuration, noting that the sweetening and double-digestion steps are not illustrated in the Figure.

[0058] The options include any one or more than one of the comminution, de-silication, digestion, flash cooling and clarification steps. The selection of the step or steps in any given situation will depend on a range of factors, such as feed material, plant operating conditions, etc, known to the skilled person.

[0059] As noted above, the invention is based on test work carried out by the applicant that showed that at least a part of the calcium oxide (lime—CaO) that is used in the Bayer process can be replaced by calcium fluoride (CaF.sub.2) without having a detrimental impact on the process and having process (including by-product recycling) and cost advantages.

[0060] The opportunity presented by the invention can be understood by considering the spent potlining treatment plant of a Group Company of the applicant that is located in the Saguenay area (Canada) and the nearby Vaudreuil alumina refinery of a Group Company of the applicant. The spent potlining treatment plant generates calcium fluoride as a by-product of a low caustic leaching and liming (LCLL) process. Using this calcium fluoride (CaF.sub.2) in an alumina refinery represents an opportunity to beneficially use this by-product and achieve a reduction in calcium oxide requirements and associated costs in the Bayer process. There is an upstream benefit of reducing CO.sub.2 emissions otherwise incurred during calcium oxide production.

[0061] The purpose of the test work was to investigate proof of concept of the invention at a small scale and a laboratory scale, i.e. proof of concept of the use of CaF.sub.2 in digestion as an alternative or complementary to CaO.

[0062] The test work focused on the control of the phosphorus concentration in the Bayer liquor in the plant and thereby to prevent high phosphorus levels in the smelter grade alumina produced in the plant. The invention is not confined to this purpose for the replacement of lime in the Bayer process.

[0063] The test work investigated whether and to what extent CaF.sub.2 could replace CaO.

[0064] The test work included a series of small scale digestion tests summarised in FIG. 1.

[0065] With reference to FIG. 1, the digestion tests included a pre-desilication step, a digestion step, and a solid/liquid separation step.

[0066] The tests were carried out using various proportions of CaF.sub.2 and CaO to investigate the impact of CaF.sub.2 on digestion (alumina extraction, desilication reactions, phosphorus control, fluoride accumulation, and other impurities release, such as carbonate, in the liquor).

[0067] With reference to FIG. 1, bauxite passing a 35-mesh sieve, refinery spent liquor, and various proportions of dry CaF.sub.2 and CaO were supplied to the pre-desilication step and formed a desilication slurry at 50-100° C. and 45% solids on a weight basis.

[0068] The resultant desilication slurry was transferred to the digestion step via a heating step. A given volume of spent liquor at 80° C. was added to the desilication slurry upstream of the heating step.

[0069] The heating step heated the slurry to the target digestion temperature. Digestion was carried out at 140° C. to achieve a target alumina to caustic soda (A/C) ratio.

[0070] A resultant digestion slurry was then cooled in an iced-water bath until atmospheric pressure was reached.

[0071] The resultant cold slurry was transferred to the solid/liquid separation step and produced a pregnant liquor and washed solids. The pregnant liquor and washed solids were subjected to a series of analysis tests.

[0072] The test work was carried out using two types of CaF.sub.2, namely: so-called “low lime excess CaF.sub.2” (LLE CaF.sub.2) and “high lime excess CaF.sub.2” (HLE CaF.sub.2), both derived from the low caustic leaching and liming process of a spent potlining treatment plant.

TABLE-US-00001 Compound LLE CaF.sub.2 (wt. %) HLE CaF.sub.2 (wt. %) Al.sub.2O.sub.3 1-2 8-9 SiO.sub.2 1-2 1-2 CaCO.sub.3 8-9 5-6 Ca(OH).sub.2 3-4 17-18 Others 4-5 3-4 CaF.sub.2 purity 80-81 64-65

[0073] The test work was carried out using CaO sampled at the plant with the following composition:

TABLE-US-00002 Compound wt. % SiO.sub.2 0-2 MgO 0-3 CaO 70-95 LOI and other impurities  0-30

[0074] The results of some of the test work are summarized in FIGS. 3-9 and discussed below.

FIG. 3—Alumina to Caustic (A/C) Ratio

[0075] FIG. 3 is a graph illustrating the alumina to caustic (A/C) ratio for digestion tests run using CaF.sub.2 and CaO in various concentrations of CaF.sub.2 and CaO.

[0076] Points to note from FIG. 3 are as follows: [0077] Promising results in that there was no negative impact on A/C ratio when CaF.sub.2 was added to digestion. [0078] The tests for paired data comparing the mean value of the base case (100% CaO) to the results with various proportions of CaF.sub.2 showed no significant differences, with p values ranging from 0,749 to 0.06 and a CI of 95%. [0079] The A/C ratio for all of the tests was close to the target A/C ratio, although a few tests showed a standard deviation of more than 5 ratio points (0.005).

FIG. 4—Breakpoint Analysis

[0080] FIG. 4 presents digestion results with “LLE” CaF.sub.2. The graph provides breakpoint analysis curves based on alumina extraction for tests run using either “LLE” CaF.sub.2 or lime, both compared to a “witness test” without lime addition.

[0081] The graph indicates promising results in that there was no negative impact on A/C ratio and A/C solubility limit margin when CaF.sub.2 was added to digestion, which supports the results shown in FIG. 3.

[0082] The 100% CaF.sub.2 addition test showed similar, almost identical, results as the base case (100% CaO).

FIG. 5—F.SUP.− Concentration in Pregnant and Spent Liquors

[0083] FIG. 5 is a graph illustrating the F.sup.− concentration in pregnant and spent liquors for digestion tests run using low lime excess CaF2 (LLE CaF2) and CaO in various proportions.

[0084] FIG. 5 illustrates the effect of varying the amount of CaF.sub.2 on the F.sup.− concentration in the spent liquor before and after the digestion step.

[0085] Desirably, accumulation of F.sup.− in the liquor is minimized and there is little or no difference between the base case and the co-dosage tests, knowing that bauxite contains small concentrations of fluorine and that it might be deported into the liquor during digestion.

[0086] Points to note from FIG. 5 are as follows: [0087] Promising results for the tests with 33/67 wt. % CaF.sub.2/CaO addition, and 50/50 wt. % CaF.sub.2/CaO addition. [0088] The concentration of fluoride in the spent liquor was similar or greater than in the pregnant liquor in most of the tests (not 75% neither 100%). [0089] The concentration of fluoride in the pregnant liquor for the tests 75/25 and 100% CaF.sub.2 was significantly higher than the base case.

FIG. 6—Phosphorus Concentration in Pregnant and Spent Liquors

[0090] FIG. 6 is a graph illustrating the phosphorus concentration in pregnant and spent liquors for digestion tests run using low lime excess CaF.sub.2 (LLE CaF.sub.2) and CaO in various proportions of CaF.sub.2 and CaO.

[0091] The graph illustrates the effect of varying the amount of CaF.sub.2 on the phosphorus concentration before and after the digestion step. Desirably, the control of phosphorus in the pregnant liquor is maintained or improved when adding CaF.sub.2.

[0092] Points to note from FIG. 6 are as follows; [0093] Promising results for all the co-dosage tests in that they show an effective phosphorus control. [0094] The tests for paired data comparing the mean value of phosphorus in the pregnant liquor of the base case (100% CaO) to the 75/25 and 100% CaF.sub.2 addition tests showed no significant differences, with p values of 0,631 and 0,162. The 33/67 and 50/50 tests showed significantly enhanced phosphorus control.

FIG. 7—Mass Variation of Phosphorus in Liquors

[0095] FIG. 7 is a graph illustrating the mass variation of phosphorus in liquors for digestion tests run using purified or high lime excess CaF.sub.2 (BILE CaF.sub.2) and CaO in various proportions of CaF.sub.2 and CaO.

[0096] Points to note from FIG. 7 are as follows: [0097] A good phosphorus control for the tests in co-dosage with CaF.sub.2, compared to the test without lime addition, showing the potential of “BLE”-CaF.sub.2/CaO mixtures to fulfil this function in the Bayer process. [0098] The phosphorus control obtained in co-dosage with CaF.sub.2 was enhanced in most cases or comparable to the one obtained with the base case (100% CaO). [0099] The fluoride accumulation was significantly higher than the base case (100% CaO) in all co-dosage tests, probably due to a different composition of CaF.sub.2 when produced with high lime excess.

FIG. 8—LLE CaF.SUB.2.-Phosphorus and Fluoride Accumulation

[0100] FIG. 8 presents digestion results with “LLE” CaF.sub.2-phosphorus and fluoride accumulation (mean values). The graph illustrates the mass variation per gram of digested bauxite of both phosphorus and fluoride in liquor for tests run using “LLE” CaF.sub.2 and CaO in various proportions

[0101] Points to note from FIG. 8 are as follows: [0102] Good phosphorus control for the tests in co-dosage with CaF.sub.2, compared to the test without lime addition, showing the potential of CaF.sub.2/CaO mixtures to fulfil this function in the Bayer process. [0103] A slight increase in F− in liquor, however not significant, for the tests with 33/67 and 50/50 addition, and a significantly higher accumulation with 75/25 and 100% CaF.sub.2 compared to the base case.
FIG. 9—“LLE” CaF.sub.2-Carbonate Concentration (Mean Values±Std. Dev)

[0104] FIG. 9 presents digestion results with “LLE” CaF.sub.2-carbonate concentration (mean values±std. dev).

[0105] The 100% CaO case is specific to each series of tests since it is replicated to make sure to have the exact same conditions, and therefore the 100% CaO case results might differ from a co-dosage test to another.

[0106] The graph illustrates the carbonate concentration in the pregnant liquor for tests run using “LLE” CaF.sub.2 and CaO in various proportions, compared to the base case using only CaO.

[0107] Points to note from FIG. 8 are as follows: [0108] Promising results in that there was no significant carbonate increase in the liquor when “LLE” CaF2 was added to digestion. [0109] The tests for paired data comparing the mean value of carbonate in the pregnant liquor of the base case (100% CaO) to the CaF.sub.2 co-dosage tests showed no significant differences, with p values ranging from 0,791 to 0,334, and a CI of 95%.

Summary of Results in FIGS. 3-9

[0110] Partial replacement of CaO with CaF.sub.2 was successful from a number of viewpoints of: [0111] A/C ratio target achievement in digestion and no significant difference compared to the base case; [0112] The breakpoint analysis results support the fact that the addition of CaF.sub.2 did not impact adversely the extraction of alumina. [0113] Good phosphorus control for the tests in co-dosage with CaF.sub.2, compared to the test without lime addition, showing the potential of CaF.sub.2/CaO mixtures to fulfil this function in the Bayer process. [0114] A slight increase in F− in liquor, however not significant, for the tests with 33/67 and 50/50 addition, and a significantly higher accumulation with 75/25 and 100% CaF.sub.2 compared to the base case. [0115] Promising results in that there was no significant carbonate increase in the liquor when “LLE” CaF2 was added to digestion.

[0116] It is noted that the results reported above in relation to FIGS. 3-9 are shown only as an example of proof of concept performed in a very smaller scale than the refinery scale, and that they are closely dependent of the type and origin of bauxite used, the impurities present in the refinery liquor, and the conditions of digestion, and other plant operating conditions.

[0117] It is also noted that the results reported above in relation to FIGS. 3-9 provide a level of confidence that the process can be a viable process at a refinery scale and can achieve the advantages mentioned.

[0118] Many modifications may be made to the embodiment of the invention described above without departing from the spirit and scope of the invention.