METHOD OF IMPROVING THE PURITY OF ALKALI-METAL HYDROXIDES

Abstract: In a method for increasing the purity of alkali metal hydroxides, alkaline earth metal compounds are used to render contaminants insoluble, thereby lessening the entry of contaminants into a solution of alkali-metal hydroxide, or promoting removal of contaminants from a contaminated solution of alkali-metal hydroxide. The invention is particularly applicable to increasing the purity of alkali metal hydroxides regenerated from the spent liquor resulting from sulphur-free alkaline pulping of wood or of other natural material containing cellulose fibres.

Description: - I - METHOD OF IMPROVING THE PURITY OF ALKALI-METAL HYDROXIDES This invention relates to a method of increasing the purity of alkali-metal hydroxides. Sodium hydroxide and other alkali-metal hydroxides produced by known means may contain, as well as some carbonate, inorganic contaminants derived from a variety of sources associated with the manufacture, storage, use or regeneration of those hydroxides. These sources of contaminants include raw materials, chemical reagents, water supplies and materials of construction of industrial plant. In one known category of prior art, - Z - the alkali-metal hydroxide is manufactured or regenerated by a process in which material comprising or containing alkali-metal carbonate, alkali-metal bicarbonate or alkali-metal organic salt or salts is heated together with an amphoteric oxide of a transition metal to obtain a salt or mixed oxide of general formula (M-0) . ( R », 0 c^d in which M is an alkali-metal (e.g. Na, K) and R is a transition metal (e.g. Ti, Fe), said salt or mixed oxide then being hydrolysed to yield the alkali-metal hydroxide. Th s category of prior art for the manufacture or regeneration of alkali-metal hydroxide is discussed in the patent and technical literature (e.g. Australian Patents 450,309, 486,132, 519,156? Kiiskila, F., Paperi Puu 62(5), 339-350 (1980) and is hereinafter referred to as Prior Art Category A. The method of the invention is applicable particularly to Prior Art Category A although not exclusively thereto. This present invention is applicable y≥t more particularly although again not exclusively when an embodiment of Prior Art Category A is employed for the regeneration of sodium hydroxide from the spent liquors (e.g. black liquor) resulting from sulphur-free alkaline pulping of wood or of other lignocellulosic material and where the transition-metal amphoteric oxide used in said regeneration is ferric oxide. In spent sulphur-free alkaline pulping liquor there may be some residual sodium hydroxide but usually the majority of the sodium present is in the form of a mixture of the carbonate and organic salts. When ferric oxide is used according to the aforementioned embodiment of Prior Art Category A to regenerate sodium hydroxide from spent sulphur-free alkaline pulping liquor, that liquor is concentrated by evaporation then mixed with the ferric oxide and fired in air to a temperature usually within the range 750°C to 1000°C to combust the organic substances and to form sodium ferrite. The sodium ferrite is hydrolysed in hot water to give sodium hydroxide solution and solid ferric oxide which after separation from each other may be reused in the pulping and alkali-regeneration sequence. The sodium hydroxide solution usually contains a minor proportion " of unreacted sodium carbonate. The combustion during the firing stage is assumed to convert organic salts of sodium into sodium carbonate which , together with the sodium carbonate initially present in the spent pulping liquor then reacts with the ferric oxide to form predominantly ^-sodium ferrite and carbon dioxide gas. This latter reaction is represented by the equation:- N 2 C0 3 + Fe 2 0 3 2 NaFe0 2 + C0 2 The subsequent hydrolysis of the 4-sodium ferrite is represented by the equation:- 2 NaFe0 2 + H 2 0 2 NaOH + e 2 0 3 Where an alkali-metal hydroxide is manufactured or regenerated according to one or other embodiment of Prior Art Category A the transition-metal amphoteric oxide may make a particularly substantial contribution of inorganic contaminants, possibly rendering the product undesirable or unsuitable for use in certain applications. The chemical elements present as contaminants in the alkali-metal hydroxide may be at least in part in soluble or colloidally-dispersed forms not readily amenable to purely mechanical me«ns of removal such as filtration and may include the transition-metal whose amphoteric oxide is used and also elements originally present as impurities in that - + - amphoteric oxide. Impurities in the transition-metal • amphoteric oxide may be particularly abundant when this is in the form of a naturally-occurring ore (e.g. hematite, ilmenite) , even if that ore has undergone a degree of beneficiation. An ability to use naturally-occurring ores is desirable because these are cheaper than synthetic or highly purified forms of the transition-metal amphoteric oxides. Aluminium, silicon, iron and manganese are among elements "that may be present in sufficiently large amounts as contaminants in alkali-metal hydroxide obtained via Prior Art Category A to be undesirable when the alkali-metal hydroxide is used in applications such as the manufacture of papermaking pulp from lignocellulosic materials, the manufacture of rayon or the manufacture of soap. For example, in the manufacture of papermaking pulp from lignocellulosic materials aluminium and silicon can form sludges and scale deposits containing minerals such as cancrinite and analcite in heat-transfer equipment in contact with the spent pulping liquor. Iron and manganese may also incorporate in such mineral deposits but in addition may cause discolouration of the pulp, may catalyse oxidative degradation of the pulp fibres on aging or when oxygen is used as a reagent in the pulping or pulp-bleaching operations and may catalyse decomposition of hydrogen peroxide when this is used as a pulp-bleaching agent. • The present invention concerns a method devised primarily to lessen the aluminium content of contaminated alkali-metal hydroxides, but we have found that application of the method may also effect a lessening of other contaminants including iron. This present invention is characterised by the minimisation of contaminants, which may be effected either by preventing contaminants from entering the - 9- - " solution of alkali-metal hydroxide as this is produced, or by removing contaminants from a contaminated solution- of alkali-metal hydroxide. In either case the invention utilises alkaline earth compounds to render contaminants insoluble, thereby lessening the entry of contaminants into the solution of alkali-metal hydroxide, or promoting removal of contaminants from a contaminated solution of alkali-metal hydroxide. In the latter case at least part of the contaminant content previously in dissolved or other difficultly separable forms in the solution is incorporated into a solid phase removable by filtration, sedimentation, centrifuging or like means of solid-liquid separation. Suitable alkaline earth compounds are the carbonates, basic carbonates, oxides, hydroxides or water-soluble organic or inorganic salts .of alkaline earths (e.g. of Mg, Ca, Sr, Ba) . A mixture of two or more of these compounds, or a compound within these categories, but containing more than one alkaline earth, may also be used. Magnesium is the alkaline earth in a preferred embodiment of this invention. The contaminants removable by the process of the invention include aluminium and iron. The form in which aluminium is rendered insoluble by application of this present invention when the alkaline-earth compound used- is one of magnesium is indicated by X-ray diffraction analysis to be predominantly hydrotalcite, whose .formula may be written as MggAl 2 (OH),gCO.-..4H 2 0. Similar aluminium-containing double salts to this may be presumed to form when compounds of the remaining alkaline earths are used in the application of this invention, but when compounds of any of the alkaline earths are so used the possibility must also be considered of the formation of other categories of insoluble or sparingly-soluble aluminium compounds or of sorption or occlusion of aluminium-containing compounds or ions onto or by insoluble or sparingly soluble alkaline-earth compounds. The forms in which contaminants other than aluminium are rendered less soluble in the alkali-metal hydroxide ' solution by application of this present invention are yet to be resolved with certainty. In relation to Prior Art Category A, the compound of an "alkaline-earth and preferably of magnesium to be used according to this present invention may be introduced into or brought into contact with the alkali-metal hydroxide solution after this has been separated from the reformed amphoteric oxide, but in a particularly preferred embodiment of the invention may be introduced at an earlier stage so that it is present during and/or immediately after the hydrolysis operation, and its solid reaction product then accompanies the reformed amphoteric oxide when this is separated from the alkali-metal hydroxide solution. Because hydrotalcite contains carbonate, it is necessary that for aluminium to be removed in the form of this or analogous compounds of other alkaline earths a source of carbonate must be present during the application of the method of the invention. In the case of Prior Art Category A, the residual alkali-metal carbonate normally present is usually sufficient. Because of overall considerations of cost, of effectiveness and of avoidance of introduction of a contaminant soluble anion into the alkali-metal hydroxide solution, magnesium oxide is the alkaline-earth compound used in a preferred embodiment of this present invention. Economical means of obtaining magnesium oxide suitable for use in that embodiment include the calcining of naturally-occurring carbonate minerals such as magnesite and dolomite. When calcined dolomite is used, aluminium is rendered insoluble mainly by reaction of the magnesium oxide component rather than of the calcium oxide component. An alkali-metal hydroxide solution may be treated with the alkaline-earth compound at any temperature in the range from the freezing point up to and including the boiling point of said solution. The rate at and extent to which any given impurity already dissolved in an alkali-metal hydroxide solution transfers to the solid phase on contact of said solution with the alkaline-earth compound depends on several factors including the identity, method of preparation and quantity of the particular alkaline-earth compound used and the temperature, duration and degree of intimacy of said contact. The conditions stated in the examples hereinafter presented as illustrative of the manner of operation of this present invention are no indicative of the entire range of conditions under which that invention may be applied, nor do they encompass all embodiments of the invention. Example 1 A sodium hydroxide solution clarified by centrifuging and containing total sodium at 5.56 mole/kg and of 89.1 per cent causticity was produced from evaporated spent sulphur-free alkaline pulping liquor and hematite ore according to an embodiment of Prior Art Category A. Natural dolomite was calcined for two hours at 1100 C to give a granular product containing 25.2 per cent magnesium by mass and allowed to cool. - S - Portions of the sodium hydroxide solution each of approximately 80 g were heated to 80°C, mixed with different amounts of the calcined dolomite, immediately vigorously agitated for 30 minutes while maintaining the temperature at 80°C then filtered without prior cooling. A further similar-sized portion of the sodium hydroxide solution was treated in identical manner except that no calcined dolomite or other additive was used. Chemical analysis of the portions of sodium hydroxide solution after treatment indicated that the portion not brought into contact with an additive contained aluminium and iron at respectively 7.59 g/kg Na and 0.056 g/kg Na whereas in the portions treated with calcined dolomite the levels of those impurities were lower by the percentages shown in Table 1. TABLE 1 Sodium Hydroxide Calcined Percentage Lowering Solution Dolomite of Impurities g g Al " Fe 80.13 1.25 98 99 80.16 0.62 81 99 Example 2 A further portion of the untreated sodium hydroxide solution of Example 1 and of mass 80.05 g was treated in identical manner to that described in that Example except that in place of an addition of calcined dolomite an addition of 0.427 g of laboratory-grade magnesium oxide was used. _ <? _ . Chemical analysis of the treated sodium hydroxide solution indicated that the contents of aluminium and iron were respectively 87 per cent and 99 per cent lower than in the portion of liquor treated without use of an additive. Example 3 A filtered sodium hydroxide solution containing total sodium " at 5.42 mole/kg and of 83.8 per cent causticity was produced from hematite ore and laboratory-grade sodium carbonate according to an embodiment of Prior Art Category A. A 33.85 g portion of this solution was heated to 80°C, mixed with 50.04 g of laboratory-grade magnesium sulphate heptahydrate, immediately vigorously agitated for 30 minutes while maintaining said temperature then filtered without prior cooling. A second similar-sized portion of the sodium hydroxide solution was treated identically except that no magnesium sulphate or other additive was used. Chemical analysis of the portions of sodium hydroxide solutions after treatment indicated that the portion not brought in contact with an additive contained aluminium and iron at respectively 28.6 g/kg Na and 0.248 g/kg Na whereas in the portion treated with magnesium sulphate the levels of those impurities were lower by respectively 80 per cent and 97 per cent. Example 4 A granular solid containing ^-sodium ferrite and with an Fe:Na mole ratio of 1.42 was obtained according to an embodiment of Prior Art Category A by firing in air a mixture of evaporated spent sulphur-free alkaline pulping liquor and hematite ore and allowing the product to cool. Natural dolomite was calcined at 1100°C for two hours and allowed to cool, giving a granular solid containing 25.2 per cent magnesium by mass. One hundred grammes of the solid containing J-sodium ferrite were mixed with 4.40 g of the calcined dolomite and then with 60 ml of distilled water and continuously agitated for one hour at 80°C to hydrolyse the ferrite and leach out the resultant sodium hydroxide into solution. The sodium hydroxide solution was then filtered without prior cooling. A second and similar-sized portion of the solid containing sodium ferrite was treated identically except . that no calcined dolomite was used. Chemical analysis of the two sodium hydroxide solutions gave the results shown in Table 2 indicating that the sodium hydroxide solution prepared in the presence of calcined dolomite contained 79 per cent less aluminium than the other. TABLE 2 Total Na, Causticity Al, mole/L % g/kg Na No calcined 5.96 90.3 7.05 dolomite used Calcined dolomite 6.41 91.7 1.50 used Example 5 Granular calcined dolomite of Example 1 was ground to powder. A 200.8-g portion of an impure sodium hydroxide solution of total sodium content 6.12 mol/kg and of causticity 83.6 per cent was heated to 100°C, mixed -with 2.53 g of the powdered calcined dolomite and immediately placed under continuous vigorous agitation while maintaining said temperature. At intervals ranging from 3 minutes to 60 minutes from the time of first contact between the sodium hydroxide solution and the calcined dolomite, samples of that solution were withdrawn from the reaction mixture through filters without prior cooling. A second portion of the impure sodium hydroxide solution was heated to 100°C and filtered with no addition of calcined dolomite or other additive. Chemical analysis of the sodium hydroxide solution treated without use of calcined dolomite indicated aluminium and iron contents of respectively 8.-52 g/kg Na and 0.045 g/kg Na. Chemical analysis of the samples of sodium hydroxide solution treated with the calcined dolomite indicated that the contents of these two impurities were lower by the percentages shown in Table 3. TABLE 3 Percentage Lowering Time of of Impurities Treatment (Minutes) Al Fe 3 15 99 5 35 99 8 59 99 12 70 99 18 90 99 24 92 99 30 91 99 45 86 99 60 78 99 It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. Method for improving the purity of alkali-metal hydroxide which is manufactured or regenerated by a process in which material comprising or containing alkali-metal carbonate, alkali-metal bicarbonate or alkli-metal organic salt or salts is heated together with an amphoteric oxide of a transition metal to obtain a salt or mixed oxide of general formula (M_ 20)a. (R-bOc)α. in which M is an alkali-metal and R is a transition metal, said salt or mixed oxide then being hydrolysed and leached to yield a solution of the alkali-metal hydroxide and leaving a solid residue comprising or containing the amphoteric oxide of the transition metal, characterized by adding one or more alkaline-earth compounds to react with potential contaminants of the alkali-metal hydroxide solution to form relatively insoluble compounds and thereby reduce the amount of said contaminants appearing in the alkali-metal hydroxide solution when said solution is separated from the solid residue of amphoteric oxide. 2. Method according to claim 1 wherein the alkaline-earth compound or compounds is or are added prior to the hydrolysis operation. 3. Method according to claim 1 wherein the alkaline-earth compound or compounds is or are added during the hydrolysis operation. 4. Method according to claim 1 wherein the alkaline-earth compound or compounds is or are added after the hydrolysis operation but before separation of the- alkali-metal hydroxide solution from the residual solid comprising or containing the amphoteric oxide of the transition metal. - 1 - 5. Method according to claim 1 wherein the alkaline-earth compound or compounds is or are selected from the carbonates, basic carbonates, oxides, hydroxides and water-soluble organic or inorganic salts of the alkaline earths. 6. Method according to claim 1 wherein the alkaline-earth is magnesium. 7. Method according to claim 1 wherein the alkaline-earth compound is dolomite or calcined dolomite. 8. Method according to claim 1 wherein the material comprising or containing alkali-metal carbonate, alkali-metal bicarbonate or alkali-metal organic salt or salts is the- spent liquor resulting from sulphur-free alkaline pulping of wood or of other natural material containing cellulose fibres. 9 " . Method according to claim 1 wherein the alkali-metal is sodium and/or potassium and the transition metal is iron and/or titanium. 10. Method according to claim 1 wherein the contaminants or potential contaminants of the alkali-metal hydroxide solution are aluminium and/or iron. 11. Method for purifying contaminated alkali-metal hydroxides; characterized in that a solution of alkali-metal hydroxide is contacted with one or more alkaline-earth compounds to precipitate relatively insoluble forms of contaminants and the precipitated compounds are removed from the solution. 12. Method according to claim 11 wherein the alkaline-earth compound or compounds is or are selected from the carbonates, basic carbonates, oxides, hydroxides and water-soluble organic or inorganic salts of the alkaline-earths. 13. Method according to claim 11 wherein the alkaline-earth is magnesium. 14. Method according to claim 11 wherein the magnesium compound is calcined dolomite. ' 15. Method according to claim 11 wherein the alkali-metal is sodium and/or potassium. 16. Method according to claim 11 wherein the contaminants are aluminium and/or iron.

Inventors: BENNETT ALLAN JOHN, GILLETT JOHN DAVID, SCUKOVIC JACK, WUNDER HEIKO, BENNETT, ALLAN, JOHN, GILLETT, JOHN, DAVID, SCUKOVIC, JACK, WUNDER, HEIKO
Applicants: NORTH BROKEN HILL LTD, NORTH BROKEN HILL LIMITED
Application year: 1984
Main classification: C01D1
IPC codes: C01D1, C01G49, D21C11