FIELD OF INVENTION
The invention relates to an improved process for the purification of lithium carbonate.
BACKGROUND OF THE INVENTION
Lithium carbonate (Li2CO3) is produced commercially from three sources: (1) extraction from mineral sources such as spodumene; (2) lithium-containing brines; or (3) from sea water.
There are a number of commercial applications of lithium carbonate including: as an ditive in aluminum molten salt electrolysis and in enamels and glasses. In its purer forms, (for example, having 99.1 wt % Li2CO3) Li2CO3 is used to control manic depression, in the production of electronic grade crystals of lithium niobate, tantalate and fluoride. High purity lithium carbonate is also required in the emerging technologies of lithium batteries. (There are two major classes of rechargeable lithium batteries, those using lithium ion and thin film polymer electrolyte-lithium metal.)
In the case of the lithium ion battery, purified lithium carbonate is required for the cathode. In the case of thin film batteries using polymer electrolytes, lithium metal is obtained by chlorinating lithium carbonate to form lithium chloride and subsequent electrolysis to metallic lithium. The key to obtaining lithium of the grade required for lithium batteries is to use purified lithium chloride and carrying out electrolysis in the virtual absence of air and humidity to minimize lithium's rapid reactions with these substances.
Electrolytic production of lithium metal is practiced commercially using an eutectic melt of LiCl and KCl (45 and 55 wt %, respectively) at 450° C. under anhydrous conditions. During electrolysis, lithium metal produced typically at a steel cathode rises to the surface of the melt due to its significantly lower density (0.5 g/ml relative to 1.5 g/ml for the melt). At the anode, chlorine gas is evolved. In some cell designs, there is a diaphragm between the anode and cathode to prevent or at least partially prevent recombination of chlorine and lithium. In other cell designs, as described in U.S. Pat. Nos. 4,617,098 and 4,734,055, a diaphragm is avoided by using the so-called “gas-lift” effect which reduces the contact time between the lithium metal and the chlorine gas, thus reducing their rate of recombination. It is also believed that the molten salt provides a protective covering over the surface of the lithium.
As mentioned above, the key to obtaining high purity lithium metal is to minimize impurities such as sodium, calcium and magnesium in the lithium chloride feed to the electrolyser. There are, however, other impurities such as carbonate, sulfate and borate, which, while not significantly affecting the purity of the lithium metal produced, they do affect the performance of the electrochemical cell, by increasing the consumption of the carbon anodes by the oxidation of these species at the anode, resulting in the production of carbon dioxide and by decreasing the current efficiency of the metal production. This effect is well known in molten salt electrolysis, though poorly understood. They are also known to adversely affect the current efficiency of both lithium and magnesium cells, though the mechanism is not known.
Published accounts of the commercial production of lithium chloride describe the reaction of hydrochloric acid with lithium carbonate in an aqueous solution. Impurities such as sulfate are removed by addition of barium chloride and filtration. Lithium chloride is recovered by evaporation and crystallization. In these processes, some impurities are removed by a bleed of the liquor during the evaporation and crystallization. Lithium chloride is extremely hygroscopic, leading to difficulties in the drying step with corrosion and with increased energy requirements. The major difficulties with these processes are the large energy demand, theoretically 30×103 kJ/kg, and the need for corrosion resistant materials and inability to use directly (i.e., without pretreatment) lithium carbonate from sources other than minerals, such as lithium carbonate from brines, since they often contain significant concentration of borates which are deleterious for the operation of the electrochemical cells.
Alternative methods have been described for the production of lithium chloride, including our co-pending patent application for the direct chlorination of lithium carbonate by chlorine at 300-650° C. in molten lithium chloride. There are equally numerous patents describing the recovery of lithium chloride from brines including U.S. Pat. Nos. 5,219,550, 4,980,136 and 4,243,392 but these do not appear to have yet achieved commercial practicability.
In U.S. Pat. Nos. 4,271,131, 4,243,392, 4,261,960 and 4,474,834, Brown, et al. teach processes whereby lithium chloride is concentrated to 40% by weight and then heated to over 200° C. to render the boron insoluble as boron oxide. Isopropanol extracts lithium chloride, leaving a residue of boron oxides and other insoluble materials. Purified lithium chloride is recovered by evaporation of isopropanol and crystallization. These processes involve a calcination step which is costly, both in terms of capital and operating costs due to the materials of construction. Additionally, yields are reduced, further increasing operating costs.
In an improved version of the above process, fatty acid alcohols such as iso-octyl alcohol dissolved in kerosene are used to extract boron as boric acid from lithium brine. The boron-free aqueous brine is then evaporated at 105-115° C. under a vacuum of 70-90 mm Hg absolute pressure to give crystals of lithium chloride. The majority of the calcium and magnesium remain in solution so purer lithium chloride is recovered by filtration or by centrifugation to give 99% pure lithium chloride. Additional washing with low molecular weight alcohol gives greater than 99%. When combined with extraction with isopropanol, 99.9% pure LiCl is obtained, as described in U.S. Pat. No. 4,274,834.
U.S. Pat. No. 5,219,550 describes a process for producing low boron lithium carbonate. Lithium chloride-rich brine is contacted with a fatty alcohol dissolved in kerosene to extract boron. Magnesium and calcium are removed by precipitation and liquid-solid separation. The brine is then treated with sodium carbonate to precipitate lithium carbonate and sodium chloride brine. Lithium carbonate produced by this process has a purity of 99.57%. Boron content is reduced to 1 ppm from 500 ppm, with calcium levels at 80 ppm and magnesium at 20 ppm. This grade of lithium carbonate contains levels of magnesium and calcium in excess of that required for production of battery-grade lithium.
An alternative process is described by Brown, et al., in U.S. Pat. Nos. 4,036,713 and 4,207,297. These patents describe transformation of impure Li2CO3 into LiOH and precipitation of calcium carbonate by treatment with CO2. The process concentrates brines, either natural or otherwise, containing lithium and other alkali and alkaline metal halides to 2-7% of lithium content. Most of the alkali and alkaline earth compounds are removed by precipitation at a pH between 10.5-11.5. The pH is modified with recycled LiOH, which removes the remaining magnesium, and by Li2CO3 and/or CO2, which produces calcium carbonate as a precipitate.
The purified brine is electrolyzed in the anolyte of an electrochemical cell divided by a cation exchange membrane, the catholyte being LiOH. In the process, lithium ions migrate through the membrane to form highly pure lithium hydroxide which can be recovered as LiOH H2O or as Li2CO3.
Brown, et al. describe the purification of technical grade Li2CO3 by first slurrying Li2CO3 in an aqueous solution and caustifying with hydrated lime (Ca(OH)2). Impurities including calcium carbonate precipitate out and a lithium hydroxide solution is either fed to an evaporator to give pure solid LiOH.H2O as a solid or to a carbonation reactor to which CO2 and Li2CO3 are added to preferentially precipitate calcium carbonate, which is then separated by filtration or a similar method. The purer LiOH can then be reacted with CO2 to give Li2CO3. The dilute solution is returned to the caustification reactor. The concentration of Ca2+ is still around 50-60 ppm. The So4 2− concentration is approximately 100 ppm and thus the purity would not meet the specifications of lithium carbonate to be used to produce battery grade lithium metal.
As can be gleaned from the prior art described above, a significant research and development effort has been invested in the search for economic means of commercially exploiting lithium-containing brines and to produce lithium salts such as chloride and carbonate of sufficient purity to produce high-purity lithium metal.
One method of obtaining pure lithium carbonate is extraction from mineral sources such as spodumene or lithium aluminum silicate ore (LiAlSi2O6). Usually recovered from open pit mines, spodumene is exploited commercially because of its relatively high lithium content and ease of processing. After ore decrepitation, the a-spodumene concentrate (of 5-7% Li2O content) is transformed into P-spodumene by heating to over 1100° C.
This treatment facilitates extraction of the spodumene into sulfuric acid at 250° C. to give lithium sulfate. After filtration to remove solids, the solution is treated with sodium hydroxide and sodium carbonate to form sodium sulfate (Glauber salt) and precipitate lithium carbonate, recovered by filtration; lithium sulfate solution to be recycled. Glauber salt is precipitated by cooling.
An alternative to this commercial process is described by Archambault in U.S. Pat. Nos. 3,112,170 and 3,112,171, practiced by Quebec Lithium Corporation and further improved by Olivier, et al. (U.S. Pat. No. 4,124,684). In this process, β-spodumene is treated directly with a 15% excess of sodium carbonate at around 215° C. and 310 psig. The lithium is transformed into insoluble lithium carbonate. Once cooled to around 20-30° C. and the slurry is fed to a carbonation reaction treatment with carbon dioxide under pressure to form lithium bicarbonate, solubilizing the lithium content. The solution is then filtered to remove solids such as aluminum silicate and iron and magnesium salts. The liquor is then fed to a precipitation reactor under atmospheric pressure at 80-90° C., liberating carbon dioxide and precipitating lithium carbonate. The lithium carbonate is then recovered by filtration, and the liquor is recycled back to the extraction process. The purity of the material, once dried, is approximately 99%, but is insufficient for battery grade lithium metal production or for pharmaceutical grade lithium carbonate. In particular, calcium levels are too high.
The commercial production for battery grade lithium requires spodumene-derived lithium carbonate to obtain the desired lithium purity and even then requires further purification during the transformation of lithium carbonate to lithium chloride. The alternative source of lithium values are brines which produce lithium carbonate at a lower cost but at a lower purity than mineral sources. To produce lithium chloride of high purity, the carbonate is first transformed into lithium hydroxide before chlorination to give battery grade lithium chloride, a comparatively expensive process.
Prior patents involving the production of high purity lithium carbonate and lithium purification include Japanese Application #1-152226 (1989) and U.S. Pat. Nos. 3,344,049, 4,988,417 and 4,842,254.
Prior art patents which deal with bicarbonation processes include U.S. Pat. Nos. 2,390,095, 2,630,371 (both for magnesium) and 4,124,684, 3,112,170 and 3,112,171 for lithium.
SUMMARY OF THE INVENTION
The present invention addresses these and other problems in the prior art by providing, in one aspect, a process in which an impure feed of Li2CO3 is mixed with an aqueous solution and reacted with CO2, a preferably under pressure, to produce dissolved aqueous LiHCO3. Insoluble impurities such as iron, magnesium and calcium are removed by physical means such as filtration or centrifugation. Soluble divalent or trivalent ions such as magnesium, calcium and iron are absorbed by selective ion exchange or other similar methods. Carbon dioxide is then completely or partially removed by raising the solution temperature or otherwise and pure Li2CO3 precipitates. Preferably, at least a part of the solution is returned to the bicarbonation reaction zone to enhance the economics of the process. Undesirable impurities remain in solution. The unrecycled solution can be neutralized to give technical grade lithium carbonate (i.e., having maximum impurity levels ppm of: Na(25), Ca(20), Mg(5), Fe(0.5), K(5), SO4 2−(25) and B(2)).
Bicarbonation can be carried out with an excess of CO2 up to about 10 times the stoichiometric requirement. Excess CO2 can be separated and recycled to enhance process economics.
In another aspect of the present invention, bicarbonation can be carried out in a series of reactors. Similarly, Li2CO3 precipitation can be in a series of reactors operating at increasingly higher temperatures close to the boiling point of water.