Lithium carbonate is widely used as an additive for heat resistant glass, optical glass and the like, a ceramic material, a material for a semiconductor laser, lubricating grease, a material for a lithium ion battery, and the like.
In recent years, particularly, a lithium ion battery finds increasing applications for compact batteries for a mobile telephone and a notebook computer, and also is promised as a battery for an electric vehicle. Accordingly, strategic resource management of lithium is becoming important for addressing the future increase of the number of electric vehicles.
The major resources of lithium include lithium ores and lithium-containing brines.
Known lithium ores include spodumene (LiAlSi2O6), petalite (LiAlSi4O10) and lepidolite (K(Li,Al)3Si4O10(OH,F)2), which are yielded in pegmatite deposits or the like. In the case of ore type resource, the concentrated ores may have a content of approximately from 3 to 6% by mass in terms of Li2O.
On the other hand, in the case of brine type resource, salt lake brines are the most important. In the orogenic zones, for example, the Andes, water soluble components including sodium chloride, which are eluted from the surrounding marine rocks, flow with stream water into the mountain top lakes formed through rapid uplift, and concentrated over a long period of time, and thus salts are deposited and accumulated to form salt lakes.
The salt lakes accumulate inside saturated brines, which are referred to as salt lake brines. The salt lake brine contains sodium chloride derived from marine water as a major component, and also contains cationic components, such as potassium, lithium, magnesium and calcium, and anionic components, such as chlorine, bromine, sulfuric acid and boric acid. The composition varies depending on the mineral species and the volcanic activity around the salt lakes, in addition to the influence of the marine water components.
Among the salt lake brines, one that has a high lithium concentration becomes a target of development as a lithium resource. As for the salt lakes that are currently operated for lithium recovery, there are reports that the salt lake brine of Atacama Salt Lake, Chile, has a lithium concentration of 2 g/L, the salt lake brine of Hombre Muerto Salt Lake, Argentina, has a lithium concentration of 0.5 g/L, and the salt lake brine of Silver Lake, the U.S.A., has a lithium concentration of 0.3 g/L (see, for example, Non-patent Document 1).
The method for recovering lithium from the brines mainly includes a concentrating step by solar evaporation, an impurity removing step by addition of chemicals, and a carbonation step by addition of sodium carbonate.
In the evaporation concentrating step, lithium is concentrated from the aforementioned lithium concentration to a high concentration that is required for the carbonation step. For the salt lake brine of Atacama Salt Lake, Chile, the lithium concentration is increased through solar evaporation concentration to approximately 60 g/L over a period exceeding one year.
A brine mainly containing chlorides has a high solubility for lithium chloride, and the lithium concentration can be increased to a high concentration around 60 g/L. In the case where a brine contains a large amount of sulfate ion, however, lithium sulfate (Li2SO4.H2O) is deposited in the process of evaporation concentration. Thus, the lithium concentration can be increased up to only approximately 6 g/L, and lithium is lost as lithium sulfate.
Furthermore, while a salt lake brine contains various ionic components as described above, magnesium is deposited as magnesium carbonate through the carbonation step and may be mixed in lithium carbonate as a final product, thereby decreasing the purity thereof, and therefore, magnesium is necessarily removed before the carbonation step. The method of removing magnesium currently employed includes a method of adding calcium hydroxide for removing magnesium in the form of magnesium hydroxide, and a method of adding sodium carbonate for removing magnesium in the form of magnesium carbonate (see, for example, Patent Document 1).
In the carbonation step, sodium carbonate is added to the brine having a high lithium concentration prepared in the concentrating and chemical-adding steps, thereby depositing lithium carbonate (see, for example, Non-patent Document 2). In this step, a large amount of sodium carbonate is consumed, and it is said that the cost of sodium carbonate occupies the major proportion of the production cost of lithium carbonate. In the lithium production from the salt lake brine of Atacama Salt Lake, Chile, the brine is concentrated by solar evaporation to a brine having a high lithium concentration within the salt lake, and the concentrated brine is then transported with tank trucks to the plants in the coastal area, at which the concentrated brine is subjected to the carbonation step by using inexpensive sodium carbonate available as natural soda ash.
In the production of lithium carbonate from brines, as described above, lithium carbonate is produced through the solar evaporation and concentrating, the removal of impurities by adding chemicals, and the carbonation with sodium carbonate, but it is the current situation that the production process is limited only to a few examples represented by Atacama Salt Lake, Chile, and for addressing the growing demand of lithium in the future, it is necessary to develop much salt lake brine resources.
In the production of lithium carbonate described above, however, a brine that has a large content of interfering components, particularly magnesium and sulfate ion, cannot be applied to the production method that is currently employed in Atacama Salt Lake, Chile.
Specifically, brains in Uyuni Salt Lake, Bolivia, Qinghai Salt Lake, China, and the like have a high magnesium content, and the Mg/Li concentration ratio is from 19 to 62 (the Mg/Li concentration ratio is 6 in Atacama Salt Lake, Chile, and is 1 in Hombre Muerto Salt Lake, Argentina). Accordingly, not only chemicals, i.e., calcium hydroxide and sodium carbonate, are required in large amounts for removing magnesium, but also a large amount of sludges of magnesium hydroxide and magnesium carbonate are formed, and the concentrated brine is trapped with the sludges, which prevents recovery of the concentrated brine containing lithium.
Furthermore, there are often salt lake brines having a high sulfate ion concentration, and for example, the SO4/Li concentration ratio is 24 in Uyuni Salt Lake, Bolivia, and 138 in Qinghai Salt Lake, China (the SO4/Li concentration ratio is 11 in Atacama Salt Lake, Chile), in which it is the current situation that the lithium concentration can be increased only to 6 g/L in the evaporation cocentrating step, and thus a concentrated brine suitable for the carbonation step, which is generally applied to a high concentration region of approximately 60 g/L or more, cannot be obtained.
Moreover, sodium carbonate, which is necessary in the carbonation step, is available as relatively inexpensive natural soda ash by large scale transshipment in the coastal area, but most of salt lakes are located in inland highlands, at which sodium carbonate is difficultly available in many cases.
Under the circumstances, for addressing the growing demand of lithium resources in the future, there are demands of an efficient lithium recovery technique from brines containing large amounts of interfering components, such as magnesium and sulfate ion, and a technique relating to a carbonation step that uses no sodium carbonate.
The present inventors have worked around the demands, and propose a method for producing lithium carbonate without the use of sodium carbonate, in which ammonia and carbon dioxide gas are mixed with an aqueous solution containing lithium chloride to perform carbonation reaction, and the solid matter thus formed is recovered through solid-liquid separation (Japanese Patent Application No. 2010-266077).    Patent Document 1: U.S. Pat. No. 5,993,759    Non-Patent Document 1: GSJ Chishitsu News No. 670, pages 22 to 26, “Lithium Resources”    Non-Patent Document 2: GSJ Chishitsu News No. 670, pages 49 to 52, “Production of Lithium from Salar de Atacama, Chile, and Use of Lithium Compounds”