Minerals containing lithium are mostly spodumene, petalite and lepidolite. There may also be lithium in the hypolimnion of brine lakes, but the ratio of lithium to magnesium in it is decisive for industrial production. Likewise there is also lithium in seawater. The greatest users of lithium at present are the glass and ceramics industry, and the accumulator and battery industry. The share of the latter is growing constantly, since lithium accumulators play a significant role for instance in the development of the electric cars. Some of the lithium is used as lithium carbonate or is at least a commercial intermediate product.
In lithium recovery, lithium mineral is concentrated, after which concentrate processing usually comprises a change in the crystal structure at high temperature, pressure leaching, carbon dioxide treatment and the filtration and purification of the lithium bicarbonate LiHCO3 that is generated. Purification can take place either on the liquid-liquid extraction principle or by ion exchange. U.S. Pat. No. 6,048,507 describes a method in which the purification of lithium bicarbonate takes place by carbon dioxide treatment and ion exchange. The purpose of ion exchange is to remove divalent metal ions, such as calcium, magnesium, iron and aluminium ions, from a lithium-containing solution. After this, the pure lithium bicarbonate is crystallised, so that pure lithium carbonate Li2CO3 is generated.
Ion exchange is done typically with selective cation exchange resins, in which the ion exchange group is for example iminodiacetic acid (IDA) or aminophosphonic acid (APA). The resins concerned are manufactured for example by Rohm & Haas under the trade name Amberlite IRC 748 (IDA) and Amberlite IRC 747 (APA). The resins are selective for multivalent metal ions and are used for the removal of calcium and magnesium etc. from concentrated NaCl-salt solution in the chlor-alkali industry. The ion exchange groups of the resin are weak organic acids. The resins are especially selective for heavy metal ions (Cu2+, Pb2+, Ni2+). In the column process, the solution to be purified is run through the column and the purified solution is collected from the solution exiting the column. When the resin is no longer able to produce pure solution, the metals bound to the resin are eluted with an acid solution, and the resin is converted to the acid form. An excess of acid has to be used in relation to the ion exchange groups. In acid form, the ion exchange group is undissociated in aqueous solution and is unable to bind metal ions; instead it has to be neutralised before the following purification cycle.
Selective cation exchange resins are generally used in metals recovery for instance from wastewaters and pickling solutions and the metals to be recovered are usually the above-mentioned heavy metals such as copper, nickel and lead. In this case the regeneration of resins generally takes place in accordance with the following sequence;
Washing waterElutionacid solution (e.g. HCl, H2SO4, 1-2 mol/l)WashingwaterNeutralisationalkali solution (e.g. NaOH, 1 mol/l)Washingwater
The washes with water displace the previous solution from the resin column between the acid and alkali stages.
US patent publication 6,048,507 describes the purification of lithium bicarbonate solution by ion exchange, in which metallic impurities, particularly divalent ones, are bound to the resin used. When the resin is saturated for example with regard to calcium, it is regenerated. Regeneration consists of firstly washing with water and subsequently treatment with hydrochloric acid to remove the calcium ions from the resin. When the calcium ions and other metal ions have been removed from the resin, it is washed again with water. Lithium hydroxide solution is used for regeneration with an alkali before the following purification cycle. Both the lithium hydroxide solution and the hydrochloric acid solution used can be utilised according to the text of the patent a number of times before they need to be replaced.
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