The application of lithium currently extends to diverse industries, including the rechargeable battery, glass, ceramic, alloy, lubricant, and pharmaceutical industries. The lithium rechargeable battery has recently been receiving attention as a main power source for hybrid and electric cars, and the market for lithium rechargeable batteries for cars is expected to continue growing to approximately one-hundred times the conventional compact battery markets for cell phones and notebooks.
In addition, a global movement towards more stringent environmental regulations is likely to expand the application of lithium to not only the hybrid and electric car industries, but to the electrical, chemical and energy fields as well. Thus, a dramatic increase of both domestic and foreign demand for lithium is expected.
Some notable main sources for the lithium could be brine containing lithium produced in nature, and a lithium bearing solution supplied from minerals possessing lithium. Such lithium bearing solution, however, contains a substantial amount of impurities, including magnesium, boron and calcium. The extraction of the impurities in advance is considered to be a critical process in order to obtain high purity lithium necessary for preparing a lithium rechargeable battery.
Conventionally, after absorbed on a boron-selective ion exchange resin containing a N-methylglucamine functional group, the boron ions included in a lithium bearing solution are extracted by washing with an acid solution for desorption. The magnesium and calcium ions included in the lithium bearing solution are extracted by adding alkali and precipitating in the form of magnesium hydroxide and calcium hydroxide.
Such method, however, is not suitable for extracting the boron from the lithium bearing solution, because a relatively expensive ion-exchange resin and usage of a variety of chemicals (e.g., substantial amounts of an acid and a base) in the management of the boron-extracting process are required. Further, the loss of lithium is likely to be substantial because the addition of excessive alkali elevates the pH of the lithium bearing solution, which, in turn, causes a negative charge to be built up on the surface of the precipitated magnesium hydroxide and calcium hydroxide, and thus the absorption of positive lithium ions. As a result, the extraction of lithium along with the impurities cannot be avoided.
U.S. Pat. No. 5,219,550 describes a method of eliminating impurities by extracting magnesium and calcium from the brine after the extraction of boron in an organic phase by mixing an organic solvent with lithium bearing brine at a volume ratio from 1:1 to 5:1. This complicated process, however, has some drawbacks, namely environmental pollution caused by using the organic solvent and a substantial loss of lithium due to the uncontrolled pH.
Further, as one of the most economical lithium supplying sources, the concentration of lithium contained in the brine ranges from approximately 0.3 to 1.5 g/L, and lithium contained in the brine is usually extracted in the form of lithium carbonate having a solubility of about 13 g/L. Even assuming that lithium contained in the brine is completely converted to lithium carbonate, the concentration of lithium carbonate in the brine is limited to 1.59 to 7.95 g/L (the molecular weight of Li2CO3 is 74, and the atomic weight of Li is 7. If the concentration of lithium is multiplied by 5.3 (74÷14≈5.3), the concentration of lithium carbonate can be estimated). Since most of the lithium carbonate concentration is lower than the solubility of lithium carbonate, the extracted lithium carbonate re-dissolves, and thus there is a problem of the lithium extraction yield being extremely low.
Traditionally, in order to extract lithium carbonate from lithium contained in brine, the brine pumped from the natural salt lake was stored in evaporation ponds and subsequently naturally evaporated outdoors over a long period of time, for instance about one year, to concentrate the lithium by several tenfold. Then, the impurities such as magnesium, calcium, boron were precipitated in order to be removed, and the method required an amount greater than the solubility of lithium carbonate to precipitate.
For instance, Chinese Patent Pub. No. 1,626,443 describes a method of extracting lithium, wherein brine is evaporated and concentrated under solar heat, and the concentrate is subject to repeated electro-dialysis in order to obtain brine containing concentrated lithium with a low amount of magnesium.
Meanwhile, a method of electrolysis is widely utilized to prepare lithium. This method constructs three chambers including acidic, basic, and alkaline chambers by placing a bipolar membrane, an anion exchange membrane, and a cation exchange membrane between an anode and a cathode. A lithium chloride aqueous solution supplied to the basic chamber allows the extraction of hydrochloric acid from the acidic chamber, and lithium hydroxide from the alkaline chamber. Such method, however, involves the addition of lithium chloride which is known to be a highly deliquescent substance, and thus undesirably requires the prevention of moistures absorption during its storage, transfer as well as handling. The task is considered to be very troublesome to lower the productivity and incur unnecessary expenses. Furthermore, a great amount of noxious, and corrosive chlorine gas is produced from the positive electrode. The installment of facilities to collect this chlorine gas for detoxification increases the manufacturing costs. In addition to such noxious gas production, the complex structure of using three chambers further separates the distance between the electrodes, causing higher resistance and power consumption necessary for electrolysis. In order to solve the aforementioned drawbacks, Japanese Patent No. 3,093,421 discloses the utilization of a lithium chloride as an electrolytic bath but a lithium carbonate as an electrolytic material to suppress the production of the chlorine gas. The complete prevention of chlorine production, however, is not achievable because lithium chloride is still used as an electrolytic bath, and thus the corrosion still interferes with preparation of highly purified lithium.