CA1314905C - Process for the preparation of oxalic acid and sodium hydrogen oxalate from crude sodium oxalate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for producing oxalic acid, sodium oxalate or mixtures thereof comprises treating crude sodium oxalate solids or slurry, such as produced by the Bayer process in treating bauxite. The crude sodium oxalate solids or slurry include at least aluminum metal ions as well as other metal ions common to the Bayer process.
The removal process comprises:
dissolving the crude sodium oxalate in an aqueous solution to form a solution of sodium oxalate;
separating insolubles from the solution;
passing the solution through an ion exchange column to convert sodium oxalate, the ion exchange column having a bed of cation exchange resin of the acidic type, the solution as it passes over the bed of resin exchanging sodium ions of the sodium oxalate with hydrogen ions of the resin to produce sodium hydrogen oxalate acid, oxalic acid or mixtures thereof depending upon the strength of acidity of the resin, the exchanged sodium ions and the Al metal ions remaining on the bed of resin: and recovering the sodium hydrogen oxalate, oxalic acid or mixtures thereof as the effluent solution from the ion exchange column.

Description

1 31 ~Q5 PROCESS F~R T~E PREP~RATION OF OXALIC ACID AND
SODIUM HYDROGEN OXAL~TE FROM CRUDE SODIUM OXALATE
FIELD OF THE INVENTION
This invention relates the production of oxalic acid, sodium hydrogen oxalate or mixtures thereof by treating crude sodium oxalate solids or slurry such as derived from the Bayer process for treating bauxite in the manufacture of aluminum.
BACKGROUND OF THE INVENTION
In the well-known Bayer process, hot concentrated sodium hydroxide solution is used to attack Bauxite.
The action of the sodium hydroxide solution is to solubilize alumina and thus separate it from the other oxides present in the mined bauxite. In the process of dissolving alumina with sodium hydroxide to form aluminates, organics in the bauxite are attacked and degraded into various compounds, such as the degradation of humic acids into oxalates and other organic moieties.
The oxalates are soluble in the Bayer liquor. Over time, the recirculation of the Bayer liquor used in the processing of bauxite accumulates these impurities and when tha concentrati.on of the impurities reach their limits of supersaturation, they drop out of the solution as solids. A particular concern is with sodium oxalate which precipitates in the form of needles on the aluminum hydroxide seeds. These fine needles act as seeds for the precipitation of the aluminum hydroxide product, causing an increase of fine particles that have to be returned to the Bayer process for reprocessing.
This significantly affec~s the overall efficiency of the Bayer process in isolating aluminum hydroxide from the bauxite.
The sodium oxalates are therefore undesirable and must be removed from the Bayer liquor when its concentration approaches the limit of supersaturation.
The crude sodium ox~late is removed by evaporating to 1 3 1 ~'~05 increase the sodium hydroxide concentration, which decreases the solubility of sodium oxalate, crystalli2ing the sodium oxalate, and removing the crystals by decantation, filtration and other techniques. The sodium oxalates are haæardous to the environment and the public, so that they cannot be readily discarded and must therefore be treated before release to the environment.
It is Xnown that oxalic acid can be recovered from the crude sodium oxalate of the Bayer process by reacting the slurry containing the sodium oxalates with lime at 90C. The reaction produces a calcium oxalate which is precipitated and readily separated from the slurry. The calcium oxalate can then be reacted with 96~ sulfuric acid at elevated temperatures to produce a calcium sulfate byproduct and oxalic acid which can be recovered by crystallization of the oxalic acid from the solution. This technique i5 disclosed in Western German Patent Publication No. DE 2,553l870. This type of process, however, is complicatecl and consumes considerable lime, sulfuric acid, disposal of calcium sulfate byproduct, dangers in handling of concentrated sulfuric acid and is an energy intensive process. It is there*ore desirable to develop a process for treating the crude sodium oxalates of the Bayer process, derive oxalic acid and sodium hydrogen oxalate from the oxalates of sufficient purity to be useful industrially.
Alternative techni~ues for treating of sodium oxalate, albeit for different purposes to upgrade technical grade sodium oxalate, is disclosed in USSR
Patent 401131 issued May 5, 1976. The patent discloses an electrolysis process ~or upgrading technical grade sodium oxalate. The process is carried out in a four-chamber electrolytic cell where the chambers areseparated by ion exchange membranes. In the process of 1 3 1 4qO5 electrolysis, the sodium ions of the sodium oxalate are exchanged for hydrogen ions to prepare oxalic acid of high purity. Iron cathode and graphite anode are used in establishing the electrochemical reaction.
Polish Patent 129059 issued June 28, 1985 discloses a method of regenerating a cation exchanger used in the process of purifying technical grade sodium oxalate to prepare oxalic acid. The patent states that it is known to use a cation exchanger to convert sodium oxalate into sodium hydrogen oxalate and oxalic acid. To assist in the dissolution of the sodium oxalate in a hot solution, oxalic acid is included. The patent discloses the technigue for regenerating the column used in preparing oxalic acid by the ion exchange treatment of the sodium oxalate. Such regeneration is carried out with a 7 nitric acid solution followed by a washing with demineralized water at 70 to 90C and a subsequent wash with water containing 2% to 5% of nitric acid.
SUMMARY OF THE INVENTION
According to an aspect of the invention, a process for producing oxalic acid, sodium hydrogen oxalate or mixtures thereo~ comprises treating crude sodium uxalate solids or slurry whiGh is produced by the Bayer process in tr~ating bauxite. The crude sodium oxalate solids or slurry include at least aluminum metals common to the Bayer process. The removal process comprises:
i) dissolving the crude sodium oxalate in an aqueous solution to form a solution containing sodium oxalate; and ii) passing the solution through an ion exchange column to convert sodium oxalate.
The ion exchange column has a bed of cation exchange resin of the acidic type. The solution, as it passes over the bed of resin, exchanges sodium ions of sodium oxalate with hydrogen ions of the resin to produce sodium hydrogen oxalate, oxalic acid or mixtures .~

thereof depending upon the strength oP acidity of the resin and the bed of resin removing the at least Al ~rom the solution. The exchanged sodium ions and the Al matals remain on the bed of resin.
The sodium hydrogen oxalate, oxalic acid or mixtures thereof are recovered in the ef~luent solution from the ion exchange column.
Optionally, when solid oxalic acid and/or sodium hydrogen oxalate is desired, the effluent is eYaporated, then cooled to crystallize out the product, which is then separated from the liquid.
BRIEF DESCRIPTION QF THE DRAWINGS
Preferred embodiments of the invention are shown in the drawinys wherein:
Figure 1 is a graph showing the solubility of sodium oxalate and oxalic acid in water;
Figure 2 is a graph showing the solubility of sodium oxalate in an aqueous solution containing oxalic acid;
and Figure 3 is a ~lowchart of the process for treating the crude sodium oxalate to produce oxalic acid.
DETAILED DESCRIPTION OF THE PREFE'RRED EMBODIMENTS
According to this invention, a process involving ion exchangs resin is designed to treat waste crude sodium oxalate derived from the Bayer process to render the waste materials into industrially useful compounds of oxalic acid, sodium hydrogen oxalate or mixtures thereof while realizing the unexpected advantage of removing metallic impurities from the waste material and producing industrial grade oxalic acid. All of these features and advantages of the invention are attained in an economical process using readily available processing equipment. The process of this invantion is capable o~ treating crude sodium oxalate, either derived from filter cake or slurry of the Bayer process and converting it into sodium hydrogen oxalate, oxalic acid or mixtures thereo~ depending upon the type of ion exchange resin used. The process is relatively simple, B

~t~qo5 the product purity (oxalic acid~ is acceptable, and the economics of the process are advantageous.
There are, of course, many references available which describe the classic Bayer process for treating bauxite to yield alumina. For a more complete understanding of the Bayer process, a description is provided in assignee's co-pending application Serial Number 574,176 filed August 9, 1988 which deals with the microbial degradation of oxalates.
In accordance with the process of this invention, an ion exchange resin is used to convert the sodium oxalate isolated from th~ crude sources of the Bayer process into sodium hydrogen oxalate, oxalic acid and mixtures thereof. The ion exchange process is the reversible interchange of ions between a solid and a liquid in which there is no permanent change in the structure of the solid resin bed. The ion exchange resin consists of inert chemical matrix with attached functional groups and mobile counter ions. The cation exchange resins fall into two broad groups; namely, strongly acidic and weakly acidic exchange resins. As is appreciated by those skilled in the art, the most common type of acidic exchanger is made of styrene-vinyldibenzene matrix and sulfonic acid functional groups. Other acidic exchangers consist of polyacrylic-vinyldibenzene and polystyrenedivinyldibenzene matrix resins with either carboxylic~acid or sulfonic~acid functional groups. Both carboxylic and sulfonic acid types of exchanger resins have strong affinity to cations, but have marked differences in their chemical behaviors.
The strong acid sulfonic resins remain ionized in acidic solutions and retain the ability to exchange cations in the solutions. The weaker carboxylic resins exist almost entirely in the free acid form in solutions of pH 4 or less, because the carboxylic acid is but 1 31 4qO5 ~lightly ionized. The counter cations are more tightly bound to the chemical matrix and are thus less mobile and available for exchange with the other cations. As a result depending upon the cation exchange resin used, the result of the ion exchange will vary. For general purposes in understanding the effects of the various types of resins, the reaction of the strongly acidic resin with sodium oxalate may be summarized by the following equations:
RS03 H+ + Na2C24 _ ~ RS03 Na~ + NaHC204 (la) RS03 H+ + Na2HC24 ~ RS03~Na+ ~ H2C204 (lb) The resin is strongly acidic and hence reacts readily with both sodium oxalate (Na2C204) and sodium hydrogen oxalate (NaHC204). This is duP to oxalic acid being weaker than the sulfonic acid, hence the reaction is driven towards the production of the oxalic acid.
When the weakly acidic resin is employed, the exchange i5 limited to the following reaction expressed in the e~uation:

RCO2 H+ ~ Na2C24 ~ -- _ RC02~Na+ + NaHC204 (2a) The ion exchange does not proceed to converting the sodium hydrogen oxalate into oxalic acid, because oxalic acid is as relatively strong as the carboxylic acid and displaces the equilibrium of the process to the production of the sodium hydxogen oxalate of equation t2a)-With the process of this invention depending uponthe acidic str~ngth of the resin used, the products of the ion exchange will either be mixtures of sodium hydrogen oxalate and oxalic acid, when the strongly acidic resin is used, or sodium hydrogen oxalate when the weakly acidic resin is used.

1 31 ~905 Another common constituent of the crude sodium oxalate i5 sodium carbonate. With either the strongly acid or weakly acidic resins, the sodium carbonate is exchanged with a strongly acidic group as follows:

2 RSO3 H~ + Na2C03 ~ 2 RS03~Na+ + CO2 + H2O (3a) and is exchanged with a weakly acidic group as follows:

2 RCO2-H+ + Na2CO3 ~ _ _ 2 ~CO2 Na~ + CO2 + H2O
As is appreciated with the Bayer process, a variety of impurities, including metals, are dissolved by the Bayer process and hence found in the crude sodium oxalate as it exists in tha filter cake, slurry or the liXe. The, metal impurities may exist as hydroxides or other ionic forms. Examples of such impurities include not only aluminum (Al), but others such as calcium ~Ca), iron (Fe~, silicon (Si), vanadium (V) and arsenic (As) and the like. Quite surprisingly, it has been found that with the use of either the weak or strong acid exchange resins, these metal impuxities are removed from the solution containing the sodium oxalate and retained on the exchange resin. This is quite siynificant in providing an effluent from the ion exchang~ column which is free o~ these metal impurities and improves the purity of the oxalic acid derived from the effluent. It is presumed that the metal impurities are exchanged on the resin with the hydrogen.
In the treatment, the crude sodium oxalate, in the .~orm of a filte~ cake or slurry, is mixed with water to dissolve the sodium oxalate~ The insolubles, i~
present, may be removed by some suitable separation, such as filtration. It is appreciated that insolubles may ba left in the solution if they do not significantly reduce the e~fectiveness of the ion exchange column.
The solution containing the dissolved sodium oxalates . '. ` '.~ !

1 31 ~905 is then passed through the ion exchange column to convert the sodium oxalate into sodium hydrogen oxalate, oxalic acid or mixtures thereof, depending upon the type of resin used. The oxalic acid or sodium hydrogen oxalate or mixtures thereof are recovered in the solution which emerges from the ion exchange column as an effluent. The dissolved oxalic acid, sodium hydrogen oxalate, and mixtures thereof are useful industrially, and can be used without further treatment. By evaporating and cooling such effluent to cause the oxalic acid and/or sodium hydrogen oxalate to precipitate, the precipitated oxalic acid and/or sodium hydrogen oxalate can be removed from the effluent by filtration or other types of separating procedures.
This optional separation of the sodium hydrogen oxalate and/or oxalic acid is useful when solid forms of these products are desired, such as to facilitate handling and shipping.
It has been realized that, in dissolving the~crude sodium oxalate in an aqueous solution, dissolution is improved by dissolving the crude sodium oxalate in hot water. Furthermore, superior dissolution is obtained by including oxalic acid in the hot water before mixing with the crude sodium oxalate. It is believed that the 25 dissolution of the sodium oxalate in a solution ;
containing oxalic acid produces the chemical species o~
sodium oxalate, sodium hydrogen oxalate and oxalic acid due to the chemical reaction of sodium oxalate with oxalic acid. The relative concentrations present in the solution will depend on the amounts of each originally present and the amounts of sodium oxalate added. It is preferred that the temperature of the hot water containing oxalic acid in which the sodium oxalate is dissolved be in the range of 65 to 90C. Although depending upon the circumstances, it is understood that the sodium oxalate is soluble in water at a temperature 1 3 1 ~905 from a practical range of approximately 20C, i.e., ambient, to approximately 90C.
It i9 understood that the concentration of oxalic acid in the hot water to promote the dissolution of the sodium oxalate may vary considerably, the preferred concentration of the oxalic acid in the water is in the range of 30 to 60 grams per liter. In noting that the presence of oxalic acid enhances dissolution of the crude sodium oxalate, it is sometimes expedient to recirculate all or a portion of the effluent to the solution in which fresh crude sodium oxalate is dissolved. The recirculation of the effluent nGt only assists in dissolving the crude sodium oxalate, but in addition, sodium hydrogen oxalate which remains in the effluent is further converted to oxalic acid. As is demonstrated in the Examples section, such recirculation of the effluent can greatly enhance the concentration of oxalic acid in the effluent of the exchange column.
As is apparent from Figure 1, the solubility of oxalic acid in water is considerably greater than sodium oxalate. As low as 15C, sodium oxalate has a solubility of approximately 32 grams per liter. As is apparent from Figure 2, the presence of oxalic acid in the hot water in which the sodium oxalate is dissolved significantly increases the solubility due to the formation of other chemical moieties.
It is understood that with any type of ion exchange column, regeneration of the resin bed is required from time to time to replace the depletion of the hydrogen ions on the weakly or strongly acidic resin forms.
Should the resin be of the strongly acidic type, the resin is represented by the formula: RS03~Na~ of equation (lb). Whereas with the weakly acidic, the resin is represented by the formula: RC02~Na+ of equation (2a). After exhaustion of the column, treatment of the resins with an excess of a dilute 1 31 4qO5 solution of strong mineral acid reverses the exchange reaction as demonstrated in the following equation providing the resin in the acid form. The reactions with the strong or weak resins are as follows:
RSO3~Na~ + HX _ _ RSO3-H+ + NaX (5a) RCO2 Na+ ~ HX _ RCO2-H+ + NAx (5b) ~ .
In this manner, the resin bed is regenerated for reuse by way of a simple regeneration process.
With reference to Fiyure 3, a flowchart for the preferred embodiment o~ the process of this invention is set out. The sodium oxalate is dissolved in tank 10 with the addition of water where the tank is, in accordance with this embodiment, heated by a suitable heating device to the desired temperature. The hot solution is passed through filter 12 to remove any undissolved matter. The hot solution is then passed through either resin column 14 or resin column 16 to efPect the ion exchange. It is appreciated that the ion exchange process on the resin bed is equally effective for purposes of this invention regardless of whether the solution is cold or hot.
The effluent frQm either resin column 14 or 1~ is transferred to a holding tank 18. The choice of the resin column 14 or 16 is dependent upon which column is in service and which column is in the regeneration mode in accordance with standard procedures in ion exchange technology. In this manner, down time is minimized with respect to treatment o~ incoming crude sodium oxalates. The effluent from the column as held in holding tank 18 is then treated in the reactor 20 with activated carbon to improve the clarity of the solution.
The spent carbon is removed from the solution by way of filter 22, where ~he filter solution is passed on to evaporator ~4. The evaporator 24 removes water to 1 31 ~05 achieve precipitation or separation of the desired sodium hydrogen oxalate or oxalic acid which is passed on in solid form to a settling tank 26. The removed condensate is either passed to the tank 10 as shown, or used in rinsing of either of the resin columns. The product from the settling tank 26 is passed through a filter 28 to remove further solids where part of the liquid is used to displacement wash the resin column to recover the maximum amount of product retained in the column. The solids are then passed through a dryer 30 to provide the dried product in the form of either oxalic acid, sodium hydrogen oxalate or mixtures thereof.
~he resin column 14 or 16 is regenerated by usa of a strong mineral acid, such as sulfuric acid, as contained in tank 32. The acid in solution is passed through the respective resin coIumn 14 or 16 during the regeneration mode and removed and disposed of in the regeneration waste line 34 in accordance with standard column regeneration techniques. The columns are wa~hed with condensate from 24.
The following Examples demonstrate various preferred conditions in the operation of the process of this invention, but are understood not to be limiting with respect to the scope of the invention as defined in the accompanying set of claims.

Bayer sodium oxalate slurry (11.5 g, containing 52%
Na2C204) was added to a 500 mL beaker containing water (250 ml). The solution was heated on a hot plate for 15 minutes and the insoluble materials were filtered off (0.35 g, 3~). After cooling, the solution was passed through a column (8 mm diameker) containing strong acidic IR-120 Amberlite~ cation exchange resin in the acid form ~50 g), at a rate of 200 to 300 mL hour.
Activated carbon (about 0.5 g) was added to the effluent 1 3 1 ~905 and the slurry was then heated to boil, and then filtered. The solution was evaporated to dryness under vacuum to yield pale yellow solid (3.7 g). The solid was identified as the oxalic acid anhydrous by X-ray diffraction (~ oxalic acid) and assayed by titration with KMnO4 (95~ purity). The yield of the crude product was thus 93% on the basis of the sodium oxalate content of the Bayer sodium oxalate slurry. Recrystallization of the solid in water resulted in a pure white colored oxalic acid crystal in needle form. They were identified as the oxalic acid dihydrate by X-ray diffraction analysis.

Bayer sodium oxalate slurry (6.2 g with 52.2%
Na2C2O4) was added to a beaker containing about 125 mL
of water. After stirring for about 15 minute, the undissolved solid was separated by filtration and the filtrate was passed through a column (3/4" diameter) ~ontaining weakly acidic Amberlite IRC-50~ (in the acid form, 50g), a rate of about 200 to 400 mL/hour.
Activated carbon (about 0.4 g) was added to the effluent and the mixture was then heated to boil, and then filtered. The solution was then evaporated to dryness under vacuum to yield a pale yellow solid (3.27 g) identified as NaHC204.~2O by X-ray diffraction. The yield of the crude was 100%. Recrystallization of the solid in water gave a pure whi*e colored sodium hydrogen oxalate monohydrate, NaHC2O4.H2O in 70% to 90% yield.
Bayer oxalate slurry (6.0 g with 52.5~ Na2C2O4~
wera similarly worked-up as abova; but this time using another weakly acidic resin Amberlite IRC-718~ (in the acid form, 50 g). The crude yield was 3.12 g, corresponding to 100%.

Crude sodium oxalate (115 g of cake containing 60 g of Na2C204) were added to a stirred and heated (70C) solution of 60 g~l oxalic acid (945 mL). After 30 minutes at the temperature, the mixture was quickly filtered to remove undissolved solids (4.5 g ~ 4%) and the filtrate, kept at 65 to 70C, was passed through a heated (70C) 3acketed column (17" with 3/4" diameter) containing 552 g o~ strongly acidic Dowex-W X8~ resin in the hydrogen form, that had been filled with 220 mL
(equivalent of the void volume) of 60 g/L oxalic acid solution. After all the sample solution had passed lG through the column, additional 220 mL of 60 g~L of oxalic acid were charged to the column to displacement wash out the sample solution completely. The combined solutions wee then evaporated to 500 mL after ~reatment with 1 g of activated carbon, and then cooled to room temperature (26C). The oxalic acid crystals precipitated were filtered and dried in air and weighed (32.~ g).

The crude sodium oxalate was analyzed for impurities by inductively coupled plasma atomic emissions spectroscopy (ICPAES) technique. The solid products from the ion exchange treated sodium oxalate was analyzed for presence of the metal impurities. The metal impurities analyzed for, in both the crude ~ ;
material and the products, were aluminum, calcium, iron, silicon and vanadium. The results of these analyses are summarized in the following Table 1.

1 31 ~n~

M~TAL IMPURITIES FOUND IN C~UDE SODIUM OXALATE, OXALIC ACID AND SODIUM HYDROGEN OXALATE IN PERCENT
OXALIC ACID SODIUM HYDROGEN
FROM OXALATE FROM
CRUDE SODIUM STRONGLY WEAKLY
ELEMENT OXALATE ACIDIC RESIN ACIDIC RESIN
IMPURITY % % %
Al 2.56 0.013 0.008 Ca 0.13 0.001 0.003 Fe 0.052 0.01 0.03 Si 0.047 0.007 0.003 V 0.010 O.001 O.001 It is apparent that regardless of use of strong or weakly acidic resins, the impurities are considerably reduced in the desired product which indicates that the metal ions are retained on the resin.

One litre of a solution of crude Bayer sodium oxalate filter cake containing 30 g/L Na2C2O4, heated to 90C, was passed through a 70 cm diameter 300 cm long water jacketed column maintainecl at about 90C
containing 700 g of Dowex 50 W-X8~ resin in the hydrogen form. The resulting e~fluent contained oxalic acid,~ at a concentration of 26,8 g/L. The resin was next treated with 500 mL of water, the ef~luent was called Wash Water; then treated with 500 mL;of a I0% solution of sulfuric acid to regenerate the:resin to the hydrogen form, the rasulting effluent was called spent regenerant; followed by 500 mL of water to give an effluent called rinse water. This process was repeated four times in tests I, II, III, and IV, to yield four product effluents containing respectively 26.8, 52.4, 74~1 and 87.6 g/L oxalic acid. The concentration of oxalic acid in the effluents increased because the sodium hydrogen oxalate resulting from the reaction of Na2C2O4 with the oxalic acid is more soluble than sodium oxalate, and this higher concentration was converted to the hydrogen form; i.e., the oxalic acid by the ion exchange resin.
From these three tests, the four sets of wash water, spent regenerant, and rinse water, were analyzed;
the results are shown in Table 2. The results indicate that the Na, and the Al, Fe, Ca, Pb and As impurities are retained by the resin, and are not displaced by the wash water, but are removed from the resin by the regenerant solution of sulfuric acid, and that the remainder of these displaced cations are displacement washed out of the resin by the rinse water~

:L5 ANALYSIS OF WASH WATER, SPE:NT P~EGENERANT AND RINSE WATER
Test Sample Elements Present, in ~ts per Million Na Al Fe Ca Pb As 20 I Wash Water Not analyzed Sp2nt Reg~erant46000 2300.008 9.4 0.5 N.D.*
Rinse Wat~ 600 220 0.70.12 0.4 N.D.
II Wa~;h Water5500 11 0.4 0.9 0.06 N.D.
Spent P~generant43000 1630.04 10.0 0.5 N.D.
Rinse Water1030 207 1.4 0.1 0O4 N.D.
III Wash Water5400 9 O.010.7 0.03 N.D.
Spe~t Regenerant43000 1130.0~i 5.6 0.5 N.D.
Rinse Water766 172 1.50.05 0.3 N.D.
IV Wa~h Water7170 12 0.041.5 0.07 N~D.
Spent P~generant43000 125 0.4 8.2 0.5 N.D.
* Not Detected EX ~ PLE 6~
A portion of the 87.6 g/L solution of oxalic acid obtained from Test III of Example 5 was evaporated to about 25% of its original volume to precipitate crystals of oxalic acid. Another portion of this same solution was passed through the ion exchange column in the hydrogen form for a second time. The resulting effluent was also evaporated to about 25% of its original volume to precipitate the oxalic acid. The solids were analyz~d for their impurity contents; the results are shown in Table 3. These results show that both products have low concentrations of impurities, but that the sodium content of the product obtained on a single passage through the column is not high purity oxalic acid, since it contains about 35 ~ of sodium. However, on a second pass through the column, the sodium is replaced by hydrogen, and the sodium content of the resulting oxalic acid is only 0.038%, and the concentration of the other impurities is also reduced.

ANALYSIS OF OXALIC ACID PRODUCTS
15 Sample Elements, in ~
Na Al Fe Ca Pb As Precipitated 3.55 0.041 0.002 0.001 0.007 N.D.
1 pass ~0 After 2nd 0.038 0.027 0.001 0.001 0.002 N.D.
Pass through Column Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (17)

1. A process for producing oxalic acid, sodium hydrogen oxalate or mixtures thereof by treating crude sodium oxalate solids or slurry which is produced by the Bayer process in treating bauxite, said crude sodium oxalate solids or slurry include at least Al common to said Bayer process, said process comprises:
i) dissolving said crude sodium oxalate in an aqueous solution to make a solution containing sodium oxalate;
ii) passing said solution through an ion exchange column to convert sodium oxalate, said ion exchange column having a bed of cation exchange resin of an acidic type, said solution as it passes over said bed of resin exchanging sodium ions of said sodium oxalate with hydrogen ions of said resin to produce oxalic acid, sodium hydrogen oxalate or mixtures thereof depending upon strength of acidity of said acid type resin and said bed of resin removing said at least Al from said solution, said exchanged sodium ions and said Al remaining on said bed of resin;
iii) recovering said sodium hydrogen oxalate, oxalic acid or mixtures thereof as an effluent solution from said ion exchange column.

2. A process of claim 1, wherein said resin is of a weak acid type to convert said sodium oxalate to sodium hydrogen oxalate.

3. A process of claim 2, wherein said resin is a carboxylic acid type.

4. A process of claim 1, wherein said resin is of a strong acid type to convert said sodium oxalate to sodium hydrogen oxalate, oxalic acid or mixtures thereof.

5. A process of claim 4, wherein said resin is a sulfonic acid type.

6. A process of claim 1, wherein said solution of sodium oxalate is at a temperature in the range of 20°C
to 90°C, when passed through said ion exchange column.

7. A process of claim 6, wherein said solution is at a temperature in the range of 65°C to 90°C.

8. A process of claim 7, wherein said step of dissolving said crude sodium oxalate in an aqueous solution includes use of oxalic acid in said aqueous solution to enhance the solubility of said crude sodium oxalate in said aqueous solution to form said sodium oxalate containing solution.

9. A process of claim 8, wherein said solution is at a temperature in the range of 20°C to 90°C when passed through said ion exchange column.

10. A process of claim 9, wherein said solution is at a temperature in the range of 65°C to 90°C.

11. A process of claim 1, wherein said effluent is recirculated through said ion exchange column to improve purity of separated sodium hydrogen oxalate, oxalic acid or mixture thereof.

12. A process of claim 4, wherein said effluent containing sodium hydrogen oxalate and oxalic acid is recirculated to form a solution in which fresh crude sodium oxalate is dissolved in the presence of said oxalic acid, passing said solution containing recirculated sodium hydrogen oxalate through said ion exchange column to increase concentration of oxalic acid in said effluent.

13. A process of claim 1, wherein said impurities include in addition to Al, metals selected from the group consisting of Ca, Fe, Si, V, Pb and As, all of said metals being removed from said solution and retained on said bed of resin.

14. A process of claim 1, wherein any insolubles are separated from said solution before passing through said column.

15. A process of claim 1, wherein said solution of sodium oxalate is filtered to remove any insolubles before passing through said column.

16. A process of claim 1, wherein said effluent solution is concentrated to remove from said effluent sodium hydrogen oxalate, oxalic acid or mixtures thereof in solid form.

17. A process of claim 13, wherein said bed of resin is regenerated as needed to exchange hydrogen ions for said sodium ions and other metal ions.