Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing single metal
Reexamination Certificate
2000-03-20
2001-09-11
Gorgos, Kathryn L. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing single metal
C204S220000, C204S221000, C204S250000, C205S528000
Reexamination Certificate
active
06287448
ABSTRACT:
The present invention relates to an improved process for the electrochemical production of metallic lithium from aqueous lithium salt solutions which makes, inter alia, simplified recycling of lithium possible.
The invention also describes an electrolysis cell for implementing this process and describes the principle of a production plant.
Lithium is an important basic inorganic chemical and has a number of applications. Thus, it is used for preparing organolithium compounds, as alloying addition to aluminum or magnesium and for lithium batteries. Lithium is produced industrially by melt electrolysis of a eutectic mixture of lithium chloride and potassium chloride at from 400 to 460° C. (Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release).
This process has a high energy consumption (28-32 kWh/kg of Li). In addition, the process has the serious disadvantage that only anhydrous lithium chloride can be used. The lithium chloride available primarily as an aqueous solution therefore has to be converted into the anhydrous solid in an energy-intensive process. Since lithium chloride is hygroscopic, drying and handling is particularly difficult.
U.S. Pat. No. 4,156,635 and J. F. Cooper et al., Proc. Electrochem. So. 1995, 95-11, 280-290 describe a process for the electrochemical production of lithium from an aqueous lithium salt solution using a lithium amalgam electrode. In this process, a lithium salt solution, in particular a lithium hydroxide solution, is electrolyzed using an amalgam cathode. Electrolysis forms lithium amalgam which is made the anode in a second electrolysis cell. Lithium cathode and amalgam anode are separated by means of boron nitride seals. In this second electrolysis cell, a 2 cm salt melt of 2 alkali metal iodides serves as electrolyte (preferably LiI and CsI or LiI and KI), while lithium metal is produced at the cathode. The current density is from 1 to 4 kA/m
2
, without mass transfer limitation occurring. In the recovery of lithium from the amalgam by this method, a current yield of only 81-87% is achieved. A particularly serious problem is that the lithium obtained is contaminated with mercury, since the mercury can diffuse through the electrolyte.
EP-B 0497410 describes a process for altering the concentration of lithium in a liquid metal from the group aluminum, copper, zinc, tin and lead by electrochemical means. Here, use is made of an electrochemical cell comprising the liquid metal and an electrically conductive material which can absorb lithium. Between these two there is a dry electrolyte which can conduct Li ions and other alkali metal ions. A DC voltage is then applied so that lithium ions and other ions of main group I migrate from the liquid metal through the dry electrolyte and are absorbed by the electrical conductor. The liquid metal is made the anode, and the conductive material on the other side of the dry electrolyte is made the cathode. The following dry electrolytes are used: &bgr;-Al
2
O
3
, &bgr;″-Al
2
O
3
, mixture of Na
2
O and Al
2
O
3
, NASICON and bismuth or a bismuth alloy.
GB-B 1,155,927 describes a process in which sodium metal can be isolated from sodium amalgam by electrochemical means using a solid sodium ion conductor, e.g. &bgr;-Al
2
O
3
, with amalgam as anode and sodium as cathode. However, when applied to lithium, the process described in GB-B 1,155,927 does not lead to the results described there in respect of lithium conversion, product purity and current density. Furthermore, the system described is unstable over the course of a few days if the temperature range claimed is adhered to.
It is an object of the present invention to find an improved process for the electrochemical production of lithium from aqueous solutions of at least one lithium salt via lithium amalgam which allows more energetically favorable production of lithium than does the melt electrolysis used hitherto.
For this purpose, the process described in U.S. Pat. No. 4,156,635 and J. F. Cooper et al., Proc. Electrochem. Soc. 1995, 95-11, 280-290, is to be altered so that the problems described above are eliminated and the process can be carried out on an industrial scale. This also makes it necessary to achieve a higher current yield than that reported in U.S. Pat. No. 4,156,635 and J. F. Cooper et al., Proc. Electrochem. Soc. 1995, 95-11, 280-290. The process described in GB-B 1,155,927 should therefore be decisively improved for the recovery of lithium from amalgam.
In such an improved process, the following essential requirements have to be met:
The process should start from the lithium salt solutions which are customarily used on an industrial scale and are obtained, for example, by dissolving lithium carbonate in aqueous hydrochloric acid solution. It should also be possible to make use of Li salt solutions which are obtained as waste streams, for example in the synthesis of organolithium compounds. The lithium metal has to be obtained primarily in such a purity that further process steps are unnecessary. This requires a heavy metal content of less than 1 ppm in the lithium. The process should be able to be implemented on an industrial scale and therefore has to make sufficiently high current densities and space-time yields possible.
The present invention accordingly provides a process for producing metallic lithium starting from an aqueous solution of at least one lithium salt, which comprises the following steps:
(I) Production of a lithium amalgam from an aqueous solution of at least one lithium salt; and
(II) Electrolysis using an anode comprising the lithium amalgam, a solid electrolyte which conducts lithium ions, and liquid lithium as cathode, wherein the lithium amalgam as anode is kept in motion.
The term “lithium amalgam” refers to a solution of lithium in mercury which is liquid at the reaction temperature.
The novel process can be implemented in a manner analogous to an integrated chloralkali electrolysis process by the amalgam method, as exists at present.
Furthermore, the present invention provides a process in which lithium waste, e.g. that from batteries and reaction solutions, can be re-used or used as starting materials for producing the aqueous lithium salt solutions used according to the present invention. For example, organolithium reactions produce appreciable amounts of lithium halides in the form of aqueous solutions. Likewise, aqueous solutions of various lithium salts, e.g. lithium halides, lithium sulfate, lithium sulfonates or lithium salts of organic acids, can be recovered, e.g. leached out, from lithium ion batteries. A further possibility for the recovery of such lithium salt solutions is the acid digestion of the electrolytes and electrodes used in batteries, for example by means of hydrochloric acid or sulfuric acid. In a preferred embodiment, the lithium waste is converted, for example, into an aqueous lithium chloride solution by means of hydrochloric acid.
In the first step of the coupled process of the present invention, the aqueous Li salt solution is electrolyzed in a chloralkali amalgam cell. This forms chlorine at the anode, if lithium chloride solutions are used. The chlorine is, in a manner typical of such a process, conducted away, purified and passed to customary uses. The process proceeds analogously to the isolation of chlorine from sodium chloride by the amalgam process (Ullmann's Encyclopedia of Industrial Chemistry, 6
th
Edition, 1998 Electronic Release). In the case of lithium sulfates, oxygen is given off at the anode. The electrolysis solution then has to be maintained at a pH in the range from 2 to 4 by means of Li salts which provide base.
The cathode process converts the lithium into reduced metallic form in the liquid amalgam. Mercury or amalgam flows along the bottom of the electrolysis cell as cathode. A lithium chloride solution containing from 220 to 350 g/l of lithium chloride flows over the mercury. The chlorine formed at the anode and the depleted lithium chloride solution (160-210 g/l) are discharged from the cell. The lithiu
Guth Josef
Huber Gunther
Putter Hermann
Schierle-Arndt Kerstin
Schlafer Dieter
BASF - Aktiengesellschaft
Gorgos Kathryn L.
Keil & Weinkauf
Nicolas Wesley A.
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