GB2037722A - Removal of organic compounds during the Bayer alumina process - Google Patents

Abstract

Organic compounds are removed from alum earth production cycle performed according to the Bayer process by heating the liquor to be recirculated, obtained after precipitation to 120- 350 DEG C, and introducing oxygen or an oxygen-containing gas into the liquor until a partial oxygen pressure of 3 to 30 atmospheres is attained, and the solid decomposition products of the organic impurities are separated from the liquor.

Description

SPECIFICATION Process for the removal of organic compounds from alum earth production cycle The invention relates to a new process for the removal of organic compounds from the alum earth production cycle performed according to the Bayer process.

The term "alum earth production according to the Bayer process" refers both to the so-called European and the so-called American Bayer processes.

The term "thin liquor" used in the specification and claims refers to the mother liquor obtained after the separation of alum earth hydrate, whereas the term "thick liquor" refers to the recirculated and concentrated thin liquor. The term "digesting liquor" refers to the liquor ready for the digestion of bauxite, which is to be introduced into the digesting apparatus. In some instances the thick liquor is applied directly as digesting liquor, whereas in other instances the thick liquor is subjected first to other pre-treatments, such as purification, adjustment of concentration, etc. In these latter instances the digesting expressions applied in the specification are the same as generally used in connection with alum earth production.

Several difficulties are caused in the slum earth production cycle by the organic impurities which enter the cycle in part with the raw material and in part with other additives. Of the latter the additives applied in the separation of red mud, added to the slurry in order to facilitate filtration or sedimenting, are to be mentioned first.

The organic impurities are substances most diverse in chemical nature. The major problems arise in connection with humic acids, which increase the viscosity of the circulating liquid, prevent the thin liquor from concentration and increase the amount of heat necessary for evaporation. Humic acids represent 2 to 10% ofthe total organic impurities calculated on the basis of the carbon content. Oxalates represent another important group of organic impurities. These compounds remain in the aluminate liquor until precipitation, and they precipitate in major part together with alum earth hydrate as impurity. The amount of oxalates remaining in the mother liquor is only 0.3 girl. About 10 to 15% of the organic impurities (calculated on the basis of the carbon content) consist of formates.The remaining part of the impurities consists of various other organic compounds. Flour and starch form the majority of additives introduced into the cycle; these are utilized as sedimentation aids.

The organic impurities listed above may cause several problems in the alum earth producing cycle, primarily because - they worsen the sedimenting properties of red mud and to a great extent, -they decrease the efficiency of precipitation and make hard or even impossible the filtration of alum earth hydrate, -due to excessive foaming they disturb the concentration of the liquors and decrease heat transfer.

-they increase the solubility of carbonates to a great extent, strongly inhibiting thereby the separation of ballast salts.

Several methods have been elaborated for the removal of these inconvenient impurities. An extensive review of the known methods is given in the German Offenlegungsschrift No. 2,145,872. Such known methods are e.g. slurrying bauxite with water before digestion and then separating the solid phase from the liquids; roasting bauxite and other raw materials before digestion; furthermore oxidative destruction with chlorine or sodium hypochlorite.

With this latter method, however, the degree of destruction is insufficient. This method cannot be applied in large-scale production, either, since the additional process steps required to remove the organic impurities greatly increase the production costs of alum earth. From economical aspects the method disclosed in the cited reference appears to be more favourable. According to this latter method sodium humates present in the thick liquor of the Bayer process are converted into insoluble calcium compounds with lime milk, removing thereby about 50 to 60% of humic acid impurities.

A related technology is suggested in the German Offenlegungsschrift No.2,518,431. In this method magnesium compounds are added to the liquor to be treated, whereupon a mixture of Mg(OH)2 and Al(OH)3 separates, removing 10 to 25% of the organic substances present. This method, in contrast to that utilizing calcium hydroxide, can be applied for the treatment of both thick liquor and thin liquor, and it improves the quality of the digesting liquor substantially.

According to the Hungarian patent specification No. 146,023 solid sodium hydroxide is added to the liquor to provide an extremely high alkali concentration, thereby facilitating the decomposition of the organic components. However, the above problems cannot be avoided completely by this method either.

The U.S. patent specification No. 2,806,766 suggests a completely new way for removing organic impurities. According to this method the aluminte liquors are heated to 150 to 2500C prior to their further processing, and the organic compounds are separated in part together with the salts which crystallize upon the decrease of solubility. No oxidizing agent is applied in this method.

A part of the known processes mentioned above has the advantage of requiring specific apparatuses, whereas the others cannot be utilized efficiently in the production of alum earth, since they do not enable the removal of organic impurities to the required degree. Some of these latter methods utilize rather specific reactions, thus they are not capable of decomposing all the disturbing organic compounds or removing them completely from the cycle. Thus there exists a need for a process which can be performed easily and enables one to decompose and remove a substantial part of the organic impurities with quite diverse chemical structures.

The invention aims at the elaboration of an inexpensive and efficient method for the removal of organic compounds appearing in the circulating liquids of the high-temperature digestion of aluminium-containing raw materials.

The new process of the invention ensures a very efficient removal of the organic impurities from the alum earth production cycle. By this new method the organic substances can be removed from the thin liquor and the thick liquor as well. One can also proceed, however, so as to decompose organic compounds in the digestion liquor in a part withdrawn from the main cycle, and remove the decomposition products in the separation steps performed after digestion.

The invention is based on the recognition that a substantial part of the organic impurities can be converted into oxalates and carbonates, easily removable from the cycle, when oxidation is performed under appropriate conditions.

According to the invention one proceeds as follows: (a) the liquor to be recirculated, obtained after precipitation, is optionally concentrated and heated to 120-350"C, preferably 210-300"C, oxygen or an oxygen-containing gas is introduced into the liquor until a partial oxygen pressure of 3 to 30 atmospheres, preferably 10 to 25 atmospheres, is attained, oxygen is finely dispersed in the liquor, and, if necessary, oxygen is supplemented according to the consumption, thereafter pressure is decreased to atmospheric, and the solid decomposition products of the organic impurities are separated from the liquor; or (b) a part of the liquor to be introduced into the digestion step is heated to 120-350"C, preferably 210-300"C, oxygen or an oxygen-containing gas is introduced into the liquor until a partial oxygen pressure of 3 to 30 atmospheres, preferably 10 to 25 atmospheres, is attained, oxygen is finely dispersed in the liquor, and, if necessary, the consumed amount of oxygen is supplemented, thereafter the liquor, pre-treated as described above, is combined in the digesting apparatus with the circulating slurry to be digested, and the solid decomposition products of the organic impurities are removed from the aluminate liquor together with red mud.

The oxidative decomposition of the organic impurities can be performed in any apparatus used conventionally in the production of alum earth. Thus an autoclave of any construction, equipped with an assembly for the introduction of oxygen or an oxygen-containing gas, can be applied, provided that the temperature and pressure conditions mentioned above can be maintained therein.

Oxygen or the oxygen-containing gas should be dispersed completely in order to provide the greatest possible contacting area between the liquor and the oxidizing agent.

Therefore the process is performed in an autoclave equipped with high-performance stirring means.

Hollow stirrers (gas stirrers) proved to be preferable. It is also preferred to apply flow-breaking means.

The oxidative destruction of organic impurities can also be performed in the digesting apparatus itself. In this instance a part of the digesting liquor is removed from the cycle, saturated with oxygen or an oxygen-containing gas, and after this pre-treatment the liquor is introduced into the digesting apparatus.

When digestion is conducted in a tube reactor, specific means, filling agents, screens or baffle plates are applied to ensure a good distribution of the gas in the slurry to be digested.

The gaseous oxidizing agent is preferably circulated. The gaseous oxidizing agent leaving the expansion vessel after the pressure decrease following oxidation is optionally pressurized and admixed with fresh oxidizing agent, and then recirculated into the vessel wherein oxidative decomposition is performed.

As oxidizing agent e.g. oxygen, air, oxygen-enriched air, or a substance capable of liberating oxygen can be applied.

Method (a) of the invention enables one to remove the organic compounds from both the thin liquor and the thick liquor of the cycle.

The rate of oxidation depends strongly on the temperature. At 1800C only about 20 to 30% of the organic impurities can be destructed within 30 to 60 minutes, whereas if the process is conducted at 260 to 2800C, 95 to 97% of the organic impurities decompose within the same period. Complete oxidation can be achieved above 300"C.

In method (a) of the invention the solid decomposition products of the organic impurities are separated together with the excess of soda, by a conventional method for separating solids from liquids applied in the alum earth industry.

If the organic impurities are decomposed in the thin liquor, the separation of the decomposition products can be facilitated greatly by expansion and/or evaporation cooling. Under such conditions the alkali concentration increases, whereupon the solid decomposition products of the organic impurities precipitate more easily and can be removed.

If the oxidative decomposition of the organic impurities is performed during digestion, digestion is conducted preferably at 230 to 260"C under a partial oxygen pressure of about 10 atmospheres. In this case the decomposition products appearing as solid substances need not be removed in a separate step, since they can be separated from the aluminate liquor simultaneously with the removal of red mud.

In method (a) of the invention either the total amount of the circulating liquor or a part thereof can be subjected to oxidative destruction. The actual method of operation depends on the given technological conditions.

Method (b) of the invention is always performed so that a part of the liquor is withdrawn from the cycle, and, after treating it with the oxidizing agent, the liquor is fed back into the cycle. It should be noted that in this instance oxidation is not necessarily terminated when the liquor reenters the cycle, and oxidation may continue until the pressure of the system is reduced to atmospheric.

In the process of the invention catalysts can be applied optionally in order to facilitate the oxidative decomposition of the organic impurities. As catalysts primarily metals with varying valences can be applied.

Silver, copper and cobalt proved to be particularly preferable.

The oxidation time may vary within wide limits, depending on the various technological parameters and on the amount of the organic impurities present. Oxidation is performed generally for about 5 minutes to about 3 hours, preferably for about 30 to 60 minutes.

Both the oxidation and the removal of the solid decomposition products of the organic impurities can be performed in the conventional apparatus of alum earth production. Thus the process of the invention can be introduced very easily into alum earth production.

The most important advantage of the new process is that it allows the organic impurities of the circulating liquors to be substantially removed. Humic acids, causing the most serious problems, can be removed practically quantitatively. It is also surprising that after the removal of the impurities according to the invention the aluminate liquor contains oxalates in a very low concentration, causing no particular problems in the precipitation step.

The invention is described in the following, by way of example only with reference to the attached drawings.

Figure 1 is the scheme of method (a).

Figure 2 is the scheme of method (b).

The diagram shown in Figure 3 illustrates the percentage decrease in organic carbon content of the liquor as a function of temperature. The liquor was treated for one hour under a partial oxygen pressure of 25 atmospheres. Further details of the process are given in Example 2.

The diagram shown in Figure 4 illustrates the percentage decrease in organic carbon content of the liquor as a function of time. The liquor was treated at 240"C under a partial oxygen pressure of 25 atmospheres.

Further details of the process are given in Example 4.

The diagram shown in Figure 5 illustrates the percentage decrease in organic carbon content of the liquor in the catalytic oxidative decomposition as described in Example 5, as a function of time. The liquor was treated at 240"C under a partial oxygen pressure of 25 atmospheres.

It should be noted that Figures 1 and 2 contain at some places two or three pieces of apparatus with the same function, marked with the same reference number. The invention is, however, not limited to the use of the specific number of identical apparatus shown in the figures. By including in the figures more than one piece apparatus with the same function we intended to express that according to a preferred method more than one of these pieces of the apparatus are utilized in the process of the invention.

In Figure 1 the liquor arriving through line 1 is forwarded by pump 2 into pre-heating autoclaves 3, where it is pre-heated by the expansion vapours arriving through line 4. The liquor is then passed into autoclaves 5, where it is heated to the temperature of oxidation by steam arriving through fresh steam line 6. Oxygen is fed into the liquor through hollow shafts 7 installed into autoclaves 5. The liquor is then passed into autoclave 8, and, after a repose period, into expansion vessels 9. In the expansion vessels the oxygen separates from the liquor together with the vapours and enters the bundle of heating tubes mounted into autoclave 3 through vapour line 4. Here the vapours condense, and oxygen is led through lines 11, together with the condensed water, into separating vessel 12.Water is removed from this vessel through separators 13 into waste water conduit 14, whereas oxygen is passed into oxygen collecting conduit 17 through lines 15 and valves 16. For repeated use, oxygen is fed into oxygen pump 18, and the amount of oxygen is supplemented to the reqired value from an external oxygen source (e.g. a pressure bottle) through valve 20.

The expanded liquor is led from expansion vessels 9 through line 21 into container 22, from which it is passed by pump 23 onto salt filter 24. The salt-free liquor is removed through line 25, whereas the salt is removed through line 26 for further use. When oxidation is performed at 200"C, it is preferred to adjust the pressure at the discharge pipe end of pump 2 to 30 to 40 kp/cm2, and to maintain a pressure of 32 to 45 kp/cm2 at the discharge pipe end of pump 18.

In Figure 2 the liquor arriving through line 1 is forwarded by pump 2 into pre-heating autoclaves 3, where it is heated to the required temperature by the expansion vapours arriving through line 4. The pre-heated liquor is led from pre-heater 3 into autoclave 5, where it is heated to the temperature of oxidation, and oxygen or an oxygen-containing gas is introduced into the liquor through hollow shaft 7. Thereafter the liquor is led into autoclave 30, where the liquor is combined with the pre-heated slurry to be digested. The slurry to be digested is passed through line 27 and is pumped by pump 28 through pre-heaters 29, also heated by the expansion vapours arriving through line 4. The mixture of the slurry and the liquor is heated to the temperature of digestion by the steam arriving through fresh steam line 6.The slurry is then fed into autoclave 31, and, after a repose period, into expansion vessels 32. Here the vapours and the oxygen separate from the expanded slurry. The vapour - oxygen mixture is led through lines 4 into heating elements 10, mounted into autoclaves 3, or into heat exchangers 29, where the vapours condense. The mixture of waste water and oxygen is led through line 11 into separating vessel 12. Water is moved from this vessel through separators 13 into waste water conduit 14, whereas oxygen enters oxygen collecting conduit 17 through lines 15 and valves 16. The further treatment of oxygen is the same as given in connection with Figure 1. The expanded slurry is led from expansion vessel 32 through line 33 into diluting tank 34, wherefrom it is removed for further processing.

When oxidation is performed at 2000C, it is preferred to adjust the temperature to 240 to 2500C after autoclave 31, to adjust the pressure at the discharge pipe end of pump 2 to 50 to 60 kp/cm2, and to maintain a pressure of 52 to 65 kp/cm2 at the discharge pipe end of pump 18.

The process of the invention is elucidated in detail by the aid of the following non-limiting Examples. In the examples the symbol "Corg" refers to the amount of organic compounds expressed as carbon, and Na2Oc is the caustic sodium content, i.e. the sum of the amount of sodium present in the form of NaOH and NaAl(OH)4.

Example 1 A digesting liquor with a Corg content of 6.62 g/l, Na2Oc content of 200 9/l and Al203 content of 100 g/l was treated in the heat recuperative equipment shown in Figure 1.

Oxygen gas was fed into the liquor, and the partial oxygen pressure was adjusted to 25 atmospheres. The amount of oxygen consumed in the reaction was supplemented continuously. The mixture of liquid and gas was heated first to 120"C and then gradually to 300"C, and maintained for one hour at the final temperature.

In one series of tests no gas stirrer was applied (Figure 3, curve 1), whereas in the other series all autoclaves were equipped with gas stirring means (Figure 3, curve 2).

The test results summarized in these curves indicate that at 240"C 12% or 48%, respectively, of the original Corg content was oxidized into carbon dioxide (appearing as carbonate), whereas at 3000C the ratio of oxidized Corg was 32% or 92%, respectively. Consequently, the use of stirring means increases the efficiency of decomposition to 3-4 times.

Example 2 Thick liquor originating from an alum earth plant processing karstbauxite (sample 1) and thick liquor originating from an alum earth plant processing laterite bauxite (sample 2) were oxidized with oxygen gas at a final temperaing from an alum earth plant processing laterite bauxite (sample 2) were oxidized with oxygen gas at a final temperature of 180 C or 250"C in an autoclave equipped with gas stirring means. The Corg content of the treated liquor, the amount of Corg removed in salt form, furthermore the ratios of some characteristic fractions are listed in Table 1.

TABLE 1 Total amount Amount of Corg, 10-2 g/1 Parameters of oxidation of Corg, g/l Oxidized (temperature, pressure, time) in the liquor in the carbon salt g/l Humic Fatty Oxalic separated acid C16 acid acid from 11.

of liquor Sample 1 Before oxidation 3.85 11.2 0.1 21.6 - 0.00 180 C, 25 atm., 30 min. 3.07 1.0 0.01 2.6 46.8 0.31 250 C, 25 atm., 15 min. 2.84 0.3 0.01 0.0 57.0 0.44 250 C, 25 atm., 30 min. 2.40 0.3 0.01 0.0 71.0 0.74 Sample 2 Before oxidation 14.7 26.5 0.12 18.2 - 0.00 250 C, 25 atm., 30 min. 6.8 0.3 0.01 0.0 465 3.25 The data of Table 1 indicate that in sample 1 the amount of humic acids and oxalic acid decreases to one-tenth of the original already after a treatment performed at 180"C. When oxidation is performed at 250 C, none of the samples contain detectable amounts of humic acids and oxalic acid. It is remarkable that the oxalate content of the salts separated from the liquor is 2 to 26 times higher than the original oxalate content of the liquor.

Example 3 A thin liquor with a Na2Oc content of 116 gull, Al203 content of 45 gil and Corg content of 4.23 g/l was oxidized in an autoclave equipped with gas stirring means for 60 minutes under a partial oxygen pressure of 10 atmospheres. The temperature was varied in the individual tests between 1 200C and 300"C. The amounts of organic impurities remaining in the liquor were as follows: Tempera- 120 160 200 240 280 300 ture,oC C0rg,gil 3.8 3.6 2.9 1.3 0.7 0.1 The purified liquor was separated from the oxygen, then it was passed directly to the evaporation step (thereby utilizing the heat of the mixture), and the separated decomposition products were removed in the usual way together with the ballast salts.

Example 4 A thick liquor with a Corg content of 6.62 g/l, a Na2O content of 200 gil and an A1203 content of 100 g/l was heated in an autoclave to 240"C. Thereafter oxygen gas was introduced into the liquor until a partial oxygen pressure of 25 atmospheres, thereby producing a gas-liosol. The amount of oxygen consumed in the reaction was supplemented continuously. Oxidation was conducted for 5, 10,20,30, or 60 minutes.

The separated salts, containing a substantial amount of organic substances, were filtered off after the treatment. The results of the tests are summarized in Figure 4. Curve 1 of Figure 4 shows the amount of organic substance oxidized into carbonate, whereas curve 2 shows the amount of organic substance oxidized and removed together with the salt (i.e. the total amount of organic substance removed). These amounts are given in percentages related to the organic substance content of the starting liquor. 75 to 98% of the total organic substance content of the separated salt was oxalate.

Figure 4 indicates that 48% of the total organic substance content of the liquor can be removed within one hour.

Example 5 A digesting liquor with a Corg content of 6.62 girl, a Na2Oc content of 200 g/l and an Al203 content of 100 g/l was treated in an autoclave equipped with gas stirring means.

The liquor was heated to 240"C, and a solid catalyst containing 10 mg/l of copper, cobalt or silver was added. Oxygen gas was fed into the liquor until a partial oxygen pressure of 25 atmospheres. The amount of oxygen consumed in the reaction was supplemented continuously.

No catalyst was used in the comparative test (Figure 5, curve 1). When comparing the results it appears that at 240"C 24% of the initial Corg content is oxidized within 20 minutes when no catalyst is applied, whereas in the presence of e.g. a copper catalyst (see curve 4) the amount of oxidized Carg is 62%, i.e. a 2.6-fold increase in efficiency is caused by this catalyst. When using silver (curve 2) or cobalt catalyst (curve 3), the increase in oxidation efficiency is 1.2-fold or 1.3-fold, respectively, under otherwise identical conditions (20 minutes at 240"C).

Example 6 A thick liquor with a Na2Oc content of 280 gill, Al203 content of 140 girl, Corg content of 9.30 g/l and Na2CO3 content of about 10% was treated directly in the digesting step as shown in Figure 2. Thus, according to the two-stream digesting process, the thick liquor was heated separately, and the organic substances were destroyed in this heating step. Pressurized air, enriched in oxygen, was introduced into the thick liquor until a partial oxygen pressure of 5 atmospheres. A gas-liosol was produced in the autoclave by gas stirring means, and the partial oxygen pressure was maintained at a constant value. The thick liquor was heated to 210"C and maintained at this temperature for 100 minutes in order to ensure a more perfect oxidation. Thereafter the thick liquor stream was combined with the stream of bauxite slurry. The oxygen-containing gas liberated in the explansion vessels was recirculated into the process. The oxygen-containing gas was kept in circulation, and only the amount of oxygen consumed in the chemical reaction was supplemented.

25% of the organic substance content of the thick liquor was oxidized into carbonate, whereas the majority of the residual 75% was converted into sodium salts of formic acid, oxalic acid and aromatic carboxylic acids.

These components in the amounts obtained disturb the Bayer cycle much less than the original organic impurities.

Claims (14)

1. A process for the removal of organic compounds from alum earth production cycle performed according to the Bayer process in which at least part of the liquor to be recirculated, obtained after precipitation, is heated to 120-3500C oxygen or an oxygen-containing gas is introduced into the liquor until a partial oxygen pressure of 3 to 30 atmospheres is attained, oxygen is finely dispersed in the liquor, and the solid decomposition products of the organic impurities are separated from the liquor.

2. A process as claimed in claim 1 wherein the liquor to be recirculated is concentrated prior to recirculation.

3. A process as claimed in claim 1 or claim 2 wherein the liquor to be recirculated is heated to 210 to 300"C.

4. A process as claimed in any one of the preceding claims wherein the partial oxygen pressure is 3 to 30 atmospheres.

5. A process as claimed in any one of the preceding claims wherein oxygen is added as it is consumed.

6. A process as claimed in any one of the preceding claims wherein the liquor is recirculated and after oxygenation pressure is decreased to atmospheric pressure.

7. A process as claimed in claim 1, wherein after the pressure release following oxidation, the recovered oxygen or oxygen-containing gas is pressurized further and thereafter it is returned for further oxidative decomposition of organic substances.

8. A process as claimed in claim 1 wherein the oxidative decomposition of organic impurities is catalyzed with a metal of variable valence.

9. A process as claimed in claim 8, wherein copper, silver or cobalt is applied as catalyst.

10. A process as claimed in any one of the preceding claims wherein the liquor to be recirculated is combined in digesting apparatus with the circulating slurry to be digested,and the solid decomposition products of the organic impurities are removed from the aluminate liquor.

11. A process as claimed in any one of the preceding claims wherein oxidation is performed for a period of 5 minutes to 3 hours.

12. A process as claimed in claim 11 wherein oxidation is performed for 30 to 60 minutes.

13. A process as claimed in claim 2 substantially as hereinbefore described in any one of the Examples.

14. Alum earth when treated by a process as claimed in any one of the preceding claims.