HK1156084A - Method of making high purity lithium hydroxide and hydrochloric acid - Google Patents

Abstract

The present invention relates to a process for producing high purity lithium hydroxide monohydrate, comprising following steps: concentrating a lithium containing brine; purifying the brine to remove or to reduce the concentrations of ions other than lithium; adjusting the pH of the brine to about 10.5 to 11 to further remove cations other than lithium, if necessary; neutralizing the brine with acid; purifying the brine to reduce the total concentration of calcium and magnesium to less than 150 ppb via ion exchange; electrolyzing the brine to generate a lithium hydroxide solution containing less than 150 ppb total calcium and magnesium, with chlorine and hydrogen gas as byproducts; producing hydrochloric acid via combustion of the chlorine gas with excess hydrogen and subsequent scrubbing of the resultant gas stream with purified water, if elected to do so; and concentrating and crystallizing the lithium hydroxide solution to produce lithium hydroxide monohydrate crystals.

Description

Process for preparing high purity lithium hydroxide and hydrochloric acid

According to 35u.s.c. § 119(e), the present application claims benefit of united states provisional patent application No. 61/125,011 filed 4/22, 2008, which is incorporated herein by reference in its entirety for all purposes.

Technical Field

The present invention relates to a process for producing high purity lithium products, particularly lithium hydroxide monohydrate for commercial use, especially in batteries.

Background

Lithium hydroxide monohydrate (LiOHH)₂O) can be produced by slaked lime (Ca (OH)₂) And lithium carbonate (Li)₂CO₃) In waterCausticizing reaction to produce. The hydrated lime can be mixed with water (H)₂O) combined calcium oxide (CaO). This process will produce an approximately 3% aqueous solution of lithium hydroxide which is then concentrated to a saturated solution and crystallized by standard commercial procedures. The reaction is shown below:

CaO+H₂O＝Ca(OH)₂+ Heat

Li₂CO₃+Ca(OH)₂2LiOH (aq) + CaCO₃

2LiOH (aqueous solution) -2LiOH H₂O (lithium hydroxide monohydrate)

The lithium source may be brine-based or ore-based. As a starting material, lithium carbonate may be derived from natural or synthetic sources. Finally, the purity of the final product is affected by the quality of the starting materials lithium carbonate, lime and the quality of the water used to form the aqueous solution.

Lithium hydroxide monohydrate is increasingly being used in a variety of battery applications. Battery applications typically require very low levels of impurities, particularly sodium, calcium, and chloride. When a calcium-based compound such as lime is used as the base, it is difficult to obtain a lithium hydroxide product with a low calcium level unless one or more purification steps are performed. These additional purification steps increase the time and cost of manufacturing the desired lithium hydroxide product.

In addition, natural brines typically contain only very small amounts of lithium, although natural "concentrated" brines are occasionally found to contain about 0.5% more lithium. However, these natural brines also contain high concentrations of magnesium or other metals, which makes lithium recovery uneconomical. Thus, the production of lithium hydroxide monohydrate from natural brines is a very difficult task, not least for economic reasons of working with lithium in very low concentrations in nature; furthermore, it is difficult to separate lithium compounds from closely related chemicals in useful concentrations, the lithium salts of which are often contaminated with, for example, sodium salts. In the production process, it is difficult to obtain lithium hydroxide monohydrate particularly pure by a conventional method using a calcium-containing compound such as slaked lime. However, the demand for lithium is rapidly increasing and new methods for producing high purity lithium products, particularly lithium hydroxide monohydrate, are needed.

In addition, U.S. patent No.7,157,065B2 discloses a method and apparatus for producing low sodium lithium carbonate and lithium chloride at about 6.0 wt% lithium from brine concentrate. A process and apparatus for the direct recovery of technical grade lithium chloride from concentrated brine is also disclosed.

The recovery of lithium compounds from natural brines and/or the production of lithium products therefrom is described in the prior art literature.

U.S. patent No.4,036,713 describes a process for producing high purity lithium hydroxide from brines, natural or other lithium-containing sources, and other alkali and alkaline earth metals, primarily halides. The lithium source is primarily concentrated by precipitation to a lithium content of about 2-7% to separate most of the alkali metals and alkaline earth metals except lithium; the concentrated brine is then increased to a pH of from about 10.5 to about 11.5, preferably using the product of the process, to substantially precipitate lithium hydroxide from the remaining magnesium contaminants, and then adding lithium carbonate to remove the calcium contaminants to provide a purified brine; the purified brine is then electrolyzed as an anolyte in a cell containing a cation permselective membrane and the anolyte is separated from the catholyte, which is water or an aqueous lithium hydroxide solution, wherein lithium ions migrate through the membrane to form a substantially pure aqueous lithium hydroxide solution at the cathode from which a highly pure lithium crystalline compound such as lithium hydroxide monohydrate or lithium carbonate product is separated.

Kirk-Othmer encyclopedia of chemical technology, second edition, Redbook, pp 438-467, discusses the bittern of great salt lake, Utah, USA and attempts to recover various valuable chemical products from the bittern so far. Of particular interest is the fact that the composition of the source brine varies widely, not only by its location in the lake, but also from year to year. This reference describes values obtained from different processes for the reduction of valuable lithium from these brines, including: evaporation-crystallization-thermal decomposition; ion exchange; complexing lithium aluminum; and solvent extraction. All of these previously proposed methods appear to be complex and expensive and do not provide a sufficiently high purity product for most commercial applications.

U.S. patent No.2,004,018 describes a prior art process for separating lithium salts from mixtures of lithium salts with other alkali and alkaline earth metals, wherein the mixed salts are initially converted to sulfate salts, which are then treated with aluminum sulfate to remove most of the potassium in precipitated form. A controlled amount of soluble carbonate is added to the solution to remove the magnesium and calcium carbonate and then precipitate and separate the lithium carbonate from the other alkali metal carbonate solution. However, Rosett et al prefer to work with chlorides obtained by treating mixed salts with hydrochloric acid. The resulting solution is concentrated by boiling to boiling point, cooled, and the maximum amount of precipitated mixed alkali chloride, with lithium chloride remaining in solution. The solution can be further concentrated to boiling point, cooled and lithium chloride precipitated as monohydrate.

U.S. patent No.2,726,138 relates to a process for preparing the high purity lithium chloride by first concentrating a crude aqueous solution containing about 2% of the total lithium, sodium, and potassium chlorides, evaporating at elevated temperature, concentrating to about 40-44% lithium chloride, then cooling to 25-50 c, and precipitating the sodium and potassium chlorides with more soluble lithium chloride remaining in solution. The resulting solution is then extracted with an inert organic solvent to obtain lithium chloride.

Us patent No.3,523,751 relates to a method of precipitating lithium chloride from a lithium carbonate solution by adding sodium carbonate. It further discloses incidentally that lithium hydroxide solution is easily carbonated to precipitate lithium carbonate. It was also noted that the lithium chloride solution reacted with sodium carbonate, resulting in precipitation of lithium carbonate.

U.S. patent No.3,597,340 relates to a process for recovering lithium hydroxide monohydrate from a chloride brine solution containing both lithium chloride and sodium chloride by electrolyzing the brine in a diaphragm cell maintaining separation between the anolyte and catholyte solutions; the diaphragm is a conventional asbestos fiber mat.

Us patent No.3,652,202 describes a process for the preparation of alkali metal carbonates from an alkali metal carbonate hydroxide solution electrolyte by electrolysis of alkali metal chlorides in an electrolytic cell in contact with a carbonated electrolyte containing attapulgite clay, whereafter the alkali metal carbonate crystallizes the salt from the treated electrolyte.

U.S. patent No.3,268,289 describes concentrating large salt lake brine by solar evaporation and a method for increasing the ratio of lithium chloride to magnesium chloride in the concentrated brine. The resulting brine may be treated in various ways, such as to remove magnesium from the electrolytic cell, or to oxidize magnesium to magnesium oxide.

U.S. patent No.3,755,533 describes a method of separating lithium salts from other metal salts by complexation with monomeric (monomelic) or polymeric organic chelating agents.

The above processes for obtaining lithium from natural brines or mixtures of alkali and alkaline earth metal salts all have certain difficulties or expensive separation costs and generally do not provide lithium products of sufficient purity for certain industrial applications.

Object of the Invention

It is therefore an object of the present invention to provide a relatively simple and economical process for recovering valuable lithium in the form of high purity lithium compounds which are also readily converted to other high purity lithium compounds.

It is another object of the present invention to provide an improved electrolytic process for concentrating valuable lithium which is characterized by high efficiency and prolonged operation due to the absence of interfering cations.

It is a particular object of the present invention to produce a high purity aqueous lithium hydroxide solution from which valuable products, namely crystalline lithium hydroxide monohydrate and lithium carbonate, can be readily isolated.

These and other objects of the present invention will be illustrated hereinafter by the following methods.

Importantly, when the calcium and magnesium levels in sodium brines have been reduced to the conventional base ppb level range, it has proven extremely difficult to reduce the calcium and magnesium levels in lithium-containing brines to such levels, and it is not believed that they have not been reduced to levels of 150ppb or less (combined), which is a significant advantage of the present invention. Therefore, lithium brines having combined levels of less than 150ppb, preferably less than 50ppb of each component, and methods of obtaining the lithium brines, are important objects of the present invention.

Disclosure of Invention

The present invention relates to a process for producing high purity lithium products, particularly lithium hydroxide monohydrate. The process is suitable for all lithium containing brines, but natural brines are preferred. Lithium-containing ores can also be used as a source to provide lithium-containing brines produced therefrom.

The source of brine used may contain various impurities, i.e., ions other than lithium, such as magnesium, calcium, sodium, potassium, and the like. Prior to ion exchange purification, the respective impurities are preferably removed or reduced by suitable methods known in the art for removing or reducing these impurities.

After removal or reduction of these impurities, the brine, with or without impurities removed, is concentrated to a certain lithium content. Preferably, the brine is concentrated to a lithium content of about 2-7% by weight of lithium chloride, preferably 2.8-6.0% by weight, or to about 12-44% by weight, more preferably 17-36% by weight, such that a majority of the sodium and potassium precipitates from the solution.

The pH of the concentrated brine is then adjusted to about 10.5-11.5, preferably about 11, to precipitate divalent or trivalent ions, such as iron, magnesium and calcium. This can also be done, for example, by adding lithium hydroxide and lithium carbonate in amounts corresponding to the iron, calcium and magnesium contents. The pH is preferably adjusted by adding a base, preferably a lithium containing base such as lithium hydroxide and lithium carbonate, which is preferably the product recovered by the process. As a result of the pH adjustment, a significant amount of iron, calcium and magnesium is removed from the concentrated and pH adjusted brine.

Calcium and magnesium, as well as other divalent and trivalent ions, may then be further removed by ion exchange, with the end result that the brine contains less than 150ppb of combined calcium and magnesium.

The purer brine is then electrolyzed to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150 ppb. A semi-permeable membrane that selectively passes cations is employed in the electrolysis process, wherein lithium ions migrate through the membrane to form a substantially pure aqueous solution of lithium hydroxide in the catholyte, from which a highly pure lithium crystalline compound product, such as lithium hydroxide monohydrate or lithium carbonate, can be formed.

A particularly preferred process of the invention relates to a process for producing lithium hydroxide monohydrate crystals by purifying a brine containing lithium and, optionally, sodium and potassium to reduce the total calcium and magnesium concentration to less than 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150ppb, with chlorine and hydrogen as by-products; the lithium hydroxide solution is concentrated and crystallized to produce lithium hydroxide monohydrate crystals.

Another preferred process of the invention relates to a process for producing hydrochloric acid by purifying a brine containing lithium and sodium and optionally potassium to reduce the total calcium and magnesium concentration to less than 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150ppb, with chlorine and hydrogen as by-products; hydrochloric acid is produced by combustion of excess hydrogen with chlorine.

Another preferred process of the invention relates to a process for the simultaneous production of lithium hydroxide monohydrate and hydrochloric acid by purifying a brine containing lithium and, optionally, sodium and potassium to reduce the total calcium and magnesium concentration to less than 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150ppb, with chlorine and hydrogen as by-products; concentrating and crystallizing the lithium hydroxide solution to produce lithium hydroxide monohydrate crystals; hydrochloric acid is produced by combustion of excess hydrogen with chlorine.

Yet another preferred embodiment of the present invention relates to a process for producing lithium hydroxide monohydrate crystals by concentrating a lithium and sodium and optionally potassium containing brine to precipitate sodium and optionally potassium from the brine; optionally, purifying the brine to remove or reduce the concentration of boron, magnesium, calcium, sulfate, and any residual sodium or potassium; adjusting the pH of the brine to 10.5-11 to further remove any cations other than lithium; further purifying the brine by ion exchange to reduce the total calcium and magnesium concentration to less than 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150ppb, with chlorine and hydrogen as byproducts; and concentrating and crystallizing the lithium hydroxide solution to obtain lithium hydroxide monohydrate crystals.

In a preferred embodiment, the lithium hydroxide solution obtained from the process is converted into a high purity lithium product having a combined calcium and magnesium content of less than 150ppb, more preferably high purity lithium carbonate.

In a particularly preferred embodiment, the lithium hydroxide monohydrate crystals are centrifuged and then recovered. The centrifuged or otherwise recovered crystals may optionally be dried and the dried material then encapsulated.

Preferably, the brine is concentrated to a lithium concentration of about 2-7%, preferably 2-6.5%, more preferably 2.8-6.0% by weight prior to electrolysis.

In another preferred embodiment, the lithium-containing brine is concentrated by solar evaporation.

Alternatively, the boron content of the brine may be reduced by, for example, organic extraction methods or ion exchange.

Preferably, the magnesium is reduced by adding or reacting controllably with lime or slaked lime, preferably lime. Preferably, the calcium is reduced by adding oxalic acid to form a calcium oxalate precipitate. Calcium and magnesium may also be removed by ion exchange or by combining these ions in lithium brines in any manner known in the art.

Alternatively, the sulfate may be reduced by, for example, adding barium to form a barium sulfate precipitate.

Sodium can be reduced by fractional crystallization or other means if desired or necessary.

For electrolysis, the electrodes are preferably made of a highly corrosion resistant material. In a particularly preferred embodiment, the electrodes are coated with titanium and nickel. In another preferred embodiment, in the electrolysis step, the electrochemical cells are arranged in a "pseudo zero gap" configuration. Particularly preferably, in the electrolysis step, a monopolar membrane cell is used, for example an Ineos ChlorFMl 500 monopolar membrane.

In a preferred embodiment, the cathode side electrode is a lantern blade design to promote turbulence and gas release during hydrolysis.

One preferred process of the invention involves producing hydrochloric acid by (a) concentrating a brine containing lithium and optionally potassium and sodium to precipitate sodium and optionally potassium from the brine; purifying the brine to remove or reduce the concentration of boron, and if necessary, magnesium, calcium, sulfate, and any residual sodium or potassium; adjusting the pH of the brine to about 10.5-11 to further remove any cations other than lithium; further purifying the brine by ion exchange to reduce the total calcium and magnesium concentration to below 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of less than 150ppb, with chlorine and hydrogen as by-products; hydrochloric acid is produced by combustion of excess hydrogen with chlorine. Any of the embodiments may be incorporated into the present process as desired, for example to reduce unwanted ions such as calcium and magnesium.

The invention also relates to lithium hydroxide monohydrate containing Ca and Mg in a combined total concentration of less than 150ppb, preferably in a combined total concentration of less than 50ppb, more preferably in a combined total concentration of less than 15 ppb.

Another aspect of the invention relates to an aqueous lithium hydroxide solution containing Ca and Mg in a total concentration of less than 150ppb, preferably in a total concentration of less than 50ppb, more preferably in a combined total concentration of less than 15 ppb.

It is also an aspect of the present invention to incorporate the product or other product of manufacture, such as a battery, into the aforementioned lithium hydroxide monohydrate and/or lithium hydroxide aqueous solution.

Drawings

There is shown a flow chart of a preferred method of the present invention.

Detailed Description

The present invention relates generally to a process for producing lithium hydroxide monohydrate, hydrochloric acid, or both by purifying a brine containing lithium and, optionally, sodium and potassium to reduce the total calcium and magnesium concentration to less than 150 ppb; electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium content of less than 150ppb, with chlorine and hydrogen as by-products; then at least one of the following steps is carried out: concentrating the lithium hydroxide solution to crystallize lithium hydroxide monohydrate crystals; or alternatively hydrochloric acid is produced by combustion of chlorine with excess hydrogen.

In a preferred embodiment, the process of the present invention for producing lithium hydroxide monohydrate and hydrochloric acid typically comprises the steps of: concentrating the lithium-containing brine by, for example, solar evaporation or heating; preferably, any boron impurities that may be contained in the brine are reduced, if desired, by, for example, organic extraction processes or ion exchange processes; if so, carrying out controllable reaction with lime and/or hydrated lime to generate magnesium hydroxide precipitate to reduce the content of magnesium; if desired, any calcium is initially reduced, for example by treatment with oxalic acid to form a calcium oxalate precipitate. If desired, the sulfate may be reduced by treatment with barium, for example. The level of sodium in the brine can be reduced by, for example, fractional crystallization. Importantly, the levels of Ca and Mg are reduced to below 150ppb (combined total), more preferably below 50ppb (combined total), most preferably below 15ppb (combined total), by ion exchange alone or in combination with other methods, for example by precipitation as described above.

The resulting purified aqueous lithium-containing solution, which contains less than 150ppb Ca and Mg (combined total), is electrochemically separated into a lithium hydroxide solution, with chlorine and hydrogen being produced as by-products. Optionally, the water is electrochemically treated and the water is separated to produce a hydrogen gas stream. Optionally, chlorine and hydrogen are dried.

Hydrochloric acid is then produced by combustion of chlorine with excess hydrogen, followed by washing of the resulting gas stream with purified water.

The lithium hydroxide solution is then concentrated or otherwise modified to produce lithium hydroxide monohydrate crystals, which are sufficiently pure by, for example, vacuum cooling or evaporation to yield a lithium hydroxide monohydrate product for battery applications, e.g., containing less than 150ppb Ca and Mg (combined total), preferably less than 50ppb total, and most preferably less than 15ppb (combined total).

The crystals are centrifuged and optionally washed to increase purity, but this is not essential.

Alternatively, the crystals are dried, preferably after washing, to give pure monohydrate crystals, and the dried material is then encapsulated.

Of course, the starting brine will vary in its ionic content depending on the source, and the process can be modified accordingly. For example, prior to ion exchange purification, it is often necessary to purify the brine to remove or reduce unwanted ion concentrates, such as Ca, Mg, B, Fe, Na, sulfates, and the like. Such removal methods are known in the art, and other methods may be developed and used. In a preferred embodiment, a method of operation of the invention is to use brines containing lithium and often also other alkali and alkaline earth metals as ionized halide salts. The brine is first concentrated by any suitable means to a lithium concentration of about 2-7% by weight, resulting in the precipitation of a major portion of the sodium and potassium from the brine as halides that are insoluble in the lithium halide solution, which has a concentration of about 12% to about 44% as lithium chloride. On the other hand, when the electrolytic brine is close to lithium chloride saturation, i.e., about 44% (7.1% lithium), such concentrated brines are not commonly used because of the increased tendency of chloride ions to migrate through the membrane. Therefore, it is most practical to use brines containing about 2-5% lithium or about 12% to about 30% lithium chloride as the anolyte for best results and efficiency.

After separation of the sodium and potassium salts, the brine is adjusted to a pH of about 10.5 to about 11.5, preferably about 11, and lithium carbonate is added to precipitate any residual calcium and/or magnesium and any iron to reduce or eliminate the presence of these ions. The pH can be adjusted in any suitable manner, but is preferably accomplished by the addition of lithium hydroxide and lithium carbonate, both of which are readily available from the product of the process, as described in detail below. Lithium hydroxide and lithium carbonate are added in amounts corresponding to the contents of iron, calcium and magnesium in a metered manner, with the result that insoluble iron and magnesium hydroxides and calcium carbonate are formed, which cations can be removed substantially completely.

Substantially all of the cations other than lithium in the resulting brine are removed or largely removed, to meet desired limits, and the brine is then preferably neutralized, preferably with hydrochloric acid or other suitable inorganic or organic acid, and treated with an ion exchange resin to further reduce the levels of calcium and magnesium. The purer brine is then subjected to electrolysis to produce a lithium hydroxide solution containing less than 150ppb total Ca and Mg, which can be evaporated or heated to produce lithium hydroxide monohydrate crystals of the same purity, which can be used, for example, in battery applications.

The product of the process of the present invention, which contains a substantially pure aqueous solution of lithium hydroxide having a total amount of Ca and Mg of less than 150ppb, more preferably less than 50ppb, and most preferably less than 15ppb, can be readily converted to other high purity lithium product solutions for commercial use or precipitated to give a monohydrate salt. For example, the solution may be treated with carbon dioxide, preferably precipitated as high purity lithium carbonate. Alternatively, the aqueous lithium hydroxide solution may be partially or completely evaporated to produce high purity lithium hydroxide monohydrate.

A particularly preferred operation is to partially evaporate the solution to crystallize high purity lithium hydroxide monohydrate, recycle the remaining solution with freshly prepared brine for disposal (with a blanket), since the crystallized lithium hydroxide monohydrate produced in this manner is of higher purity than produced by other processes. The lithium product produced in this manner is of high purity and contains a maximum residual chlorine of 0.05%, more typically a chlorine content of 0.01%. This is important in many applications, such as the use of lithium hydroxide in greases, where the content of chloride ions is required to be as small as possible because of its potential corrosive properties. In addition, if chlorine is not excluded, it is very difficult to produce high-purity lithium hydroxide by recrystallization using a commonly used industrial monopolar membrane in a battery.

The reason why it is necessary to minimize the concentration of cations other than lithium in the brine used for electrolysis in the process of the present invention is to ensure the production of lithium hydroxide of high purity, and also because specific cations such as calcium, magnesium and iron have a tendency to precipitate as insoluble calcium hydroxide, magnesium and iron in the selective cation permeable membrane. Of course, such precipitation is highly undesirable because it not only reduces the efficiency of lithium ion passage through the membrane, but also greatly reduces the useful life of the electrolytic membrane, thus potentially affecting the continuous operating life of the cell and increasing manufacturing costs.

The process of the invention can be carried out in any natural or synthetic lithium brine. The starting brine will typically contain one or more of the following impurities: magnesium, calcium, boron, rubidium, and the like, typically in soluble form and often as salts of the corresponding chlorides. It will be appreciated that the procedure required to remove these impurities will vary depending on whether or not the impurities are present. Thus, if no impurities are present, or if the level is sufficient for the particular application for which the end product is intended, then no removal step is required for those impurities.

These removal steps can be performed by methods known or achievable in the art.

After the necessary removal steps, there will still be a certain amount of residual impurities, and therefore subsequent removal steps, which may be the same or different from the above-described removal steps, need to be applied.

The method of the invention can be widely applied to all lithium-containing brine solutions. Suitable brines naturally occurring in nature are well or mine ground water, as well as surface water of the ocean or lake, such as the natural brines found in nevada, argentina, and chile. The brine can also be synthetically prepared by reacting hydrochloric acid with lithium mineral to produce a brine containing lithium chloride. Hydrochloric acid used for this purpose can be obtained by reacting hydrogen and chlorine as by-products in the electrolysis step of the present invention. Typically, such brines contain very low concentrations of lithium, usually 50-500ppb, or less, although brines containing up to 0.5% lithium are also found. In theory, the process of the invention can be carried out with any concentration of brine from very low to saturated, and it is clear that low lithium containing brines are economically less viable due to the scale of equipment and time required. For this reason, it is desirable to concentrate the natural dilute brine in a first step until the lithium concentration is increased to at least about 0.04% to about 1%, and preferably at least about 0.1%.

Although several methods of evaporation have been indicated due to the difficulty of chemically separating the mixed salt components typically present in brines, lithium-containing dilute brines may also be concentrated by any suitable method. When evaporation is carried out in any known manner, the brine is preferably simply stored in a pond, allowing concentration by solar evaporation for a period of time. This solar evaporation readily separates sodium chloride and potassium chloride, which are partially less soluble than lithium chloride. Furthermore, due to the absorption of carbon dioxide in the air, a certain amount of magnesium is also removed from the basic brine in the form of magnesium carbonate.

When the dilute brine has a lithium concentration of about 0.04-1%, or preferably at least about 0.1%, the pH of the brine is optionally adjusted to about 10.5-11.5, preferably about 11, to aid in the removal of cationic impurities, i.e., cations other than lithium, preferably magnesium, if this element is present in large amounts. This may be done by adding any suitable alkaline material, such as lime, sodium carbonate or sodium hydroxide, for low cost. The brine is then further concentrated by solar concentration, typically containing about 0.5-1% lithium (i.e., about 3.1-6.2% lithium chloride). Since carbon dioxide drawn in from the air can reduce the pH to about 9, it can be readjusted to 10.5-11.5 by adding lime, calcium hydroxide or sodium carbonate to reduce the residual magnesium and calcium in the solution to about 0.1%.

The brine is then further concentrated by any suitable means, such as solar evaporation or more rapidly by underwater combustion according to techniques known in the art. During this process the brine may reabsorb carbon dioxide from the atmosphere, thus possibly reducing the pH to about 9 again. Thus, the volume of brine is reduced, with a lithium concentration of about 2-7%, i.e., about 12-14% lithium chloride. The concentration of lithium chloride is easily calculated by multiplying the lithium concentration by a factor of 6.1. Sodium chloride and potassium chloride have less solubility in brine than lithium chloride, and therefore substantially all of the sodium and potassium are removed when the lithium concentration exceeds about 40%. The lithium content is about 7.1% or about 44% when saturated in aqueous solution at room temperature. Therefore, this is a practical upper limit for brine concentration where lithium chloride does not precipitate with contaminants. As noted above, since large amounts of sodium and potassium remain in solution when the lithium concentration reaches about 35%, this is a practical lower limit for the evaporative concentration step of the process unless sodium and potassium cations are removed by recrystallization of the hydroxide to obtain high purity lithium.

Since the concentrated and purified brine is further purified by electrolysis, any interfering cations are better removed. In a preferred embodiment, the electrolyzed brine is diluted to a lithium content of about 2-5% (about 12-30% lithium chloride) as necessary to limit chloride ion migration during electrolysis, at which concentration the electrolysis efficiency is actually improved. Of course, such dilution is not necessary, and if the lithium concentration exceeds 5%, the concentration step is not performed. By further adjusting the pH of the brine to about 10.5-11.5, preferably about 11, substantially all interfering ions, typically primarily calcium and magnesium, and possibly iron, are removed. It can be carried out by adding any suitable alkali metal, but in order to obtain an optimum separation free of contaminants, it is preferred to add metered amounts of lithium hydroxide and lithium carbonate. In this way substantially all of the interfering cations such as magnesium hydroxide, calcium carbonate or iron hydroxide are removed. Lithium hydroxide and lithium carbonate to achieve this are readily obtained from the products of the present invention, as described in more detail below.

As noted above, the brine to be electrolyzed should be substantially free of interfering cations and, in fact, tolerable despite small amounts of alkali metal ions such as sodium and potassium, so long as no more than 5% by weight remains in the recrystallization. Cations such as iron, calcium and magnesium precipitate in the cation permeable membrane and can severely interfere with electrolysis, and therefore the cations must be reduced to very low levels. The total content of these ions should preferably not exceed about 0.004%, although concentrations above their solubility limit in the catholyte may also be tolerated. Such higher concentrations may be used if desired, but at the expense of the operable life of the cell membrane. The concentration of electrolyzed anions in the brine, other than chloride ions, should not exceed about 5%.

The catholyte may be composed of any suitable material containing sufficient ions to generate an electric current. Although water alone may satisfy the aforementioned limitations, it is preferred to employ a product resulting from ionization, i.e., lithium hydroxide. The starting concentration of lithium hydroxide may vary from just sufficient for battery operation to a saturated concentration at ambient conditions. However, since lithium hydroxide precipitation in the cell is in principle undesirable, it is highly necessary to avoid precipitation of the hydroxide in the membrane, saturation should be avoided. Furthermore, since no available cation selective membrane is perfect and able to pass certain anions, the higher the concentration of hydroxide ions in the catholyte, the more these ions migrate through the membrane to the anolyte, which is undesirable because these ions react with chloride ions to produce oxychloride, thereby reducing the efficiency of production of chlorine as a by-product, and reducing the current efficiency of the cell as a whole.

Although the process described herein is highly efficient, it is preferred to have a cycle in which the spent lithium chloride solution is replenished with freshly prepared purified lithium brine. The recycled brine is treated to remove any chlorine oxides that may have formed using methods known to those skilled in the art. The method thus maintains high efficiency and maximizes the use of valuable lithium.

Any useful semi-transparent electrolyte membrane that selectively passes cations and inhibits anions can be used in the present method. Such membranes are well known to those skilled in the art of electrolysis. Suitable commercial electrolytic membranes include the Nafion series available from E.I. DuPont de Nemours & Co. This selectively permeable membrane is placed between the anolyte brine to be electrolyzed and the catholyte as described above, maintaining physical separation of the two solutions.

About 100amps/ft in the electrolytic process²To about 300amps/ft²Through the membrane to the catholyte. Preferably, the current range is 150amps/ft²To 250amps/ft². Preferably, the calcium and magnesium concentrations should be maintained at a combined Ca and Mg concentration of between < 20 and < 30ppb, depending on the current density, to avoid membrane fouling.

During electrolysis, among several chemicals, chloride ions in the anolyte migrate to the anode and are discharged to produce chlorine gas, which can be recovered as a by-product and used to produce hydrochloric acid, as described below, or by other methods. The hydroxide ions in the catholyte are attracted to the anode but are substantially prevented from entering the anolyte by the impermeability of the membrane to such anions. The lithium ions entering the catholyte combine with hydroxide ions generated from water in the catholyte, thereby releasing hydrogen ions which are discharged at the cathode to form hydrogen gas, which can also be collected as a by-product for reaction with, for example, chlorine to produce HCl. Alternatively, hydrogen may be used as a heat source for generating energy.

In this process, lithium chloride in the anolyte brine is converted to lithium hydroxide in the catholyte; the conversion was almost 100% based on the lithium chloride entering the anode compartment of the cell. The electrolysis can be continued until the concentration of lithium hydroxide reaches the desired level of 14% or just below saturation. The aqueous lithium hydroxide solution has a very high purity, preferably containing no more than about 0.5% by weight of cations other than lithium, more preferably less than 0.4 wt.%, most preferably less than 0.2 wt.%. Lithium hydroxide monohydrate will preferably contain less than 0.05 wt.% of anions other than hydroxide, more preferably less than 0.04 wt.%, most preferably less than 0.02 wt.%. It is particularly noted that the chlorine content will not exceed 0.04 wt.%, more preferably below 0.03 wt.%, most preferably below 0.02 wt.%. It is noted that the process of the present invention results in lithium hydroxide monohydrate of this purity without the need for additional processing steps, although other processing steps may be employed to further purify the product if desired.

The process of the present invention provides high purity aqueous lithium hydroxide solutions that can be used or readily converted to other commercially desirable high purity lithium products. For example, an aqueous lithium hydroxide solution may be treated with carbon dioxide to precipitate high purity lithium carbonate containing no more than 0.05% chlorine, and typically about 0.01% chlorine.

Alternatively, the aqueous lithium hydroxide solution can be converted to high purity crystalline lithium hydroxide monohydrate by simple evaporative drying of the solution. Fractional crystallization, recycle and discharge (blowing) can be carried out using more elaborate crystallization techniques to obtain crystalline lithium hydroxide monohydrate of very high purity.

From the foregoing, it can be seen that a portion of the aqueous lithium hydroxide product can be converted to provide lithium carbonate and lithium hydroxide for use in earlier stages of the process to remove iron, calcium and magnesium from the concentrated brine.

It can also be seen from the foregoing that this new process for the first time provides a process for obtaining valuable lithium in the form of a high purity and direct commercial product from natural brines without further purification and with almost 100% recovery of lithium in the concentrated brine.

In addition, once the lithium hydroxide solution, the monohydrate crystals and the hydrochloric acid solution are producedThey can be used as starting materials for another production of lithium-containing compounds, in addition to being commercially available. For example by using pure compressed CO₂The gas reacts with the lithium hydroxide solution to precipitate high purity lithium carbonate, which may also be used in certain battery applications.

Alternatively, the use of lithium hydroxide solution to clean fuel gas from the combustion of fossil fuels that produce impure carbonate can also reduce greenhouse gas emissions.

Another example is the modification of a high purity lithium chloride solution using ultra pure lithium hydroxide and hydrochloric acid obtained by the process of the present invention as reactants, followed by crystallization, for the production of lithium metal requiring extremely low levels of impurities (e.g., for use in battery components).

Other examples include the production of high purity lithium fluoride and bromide and other lithium derived compounds by acid-base reaction using the lithium hydroxide solutions of the present invention to form lithium hypochlorite, which is considered a disinfectant.

Recognizing the need for high purity lithium chloride solutions, the process of the present invention utilizes ion exchange resins to effectively remove calcium and magnesium ions to a combined concentration of less than 200 ppb. These concentrations are acceptable in lithium chloride electrochemical cells and can be achieved using high capacity macroporous weakly acidic cation exchange resins with uniformly distributed particle sizes. The resin can be regenerated with hydrochloric acid and lithium hydroxide from downstream of the process to save operating costs.

The resulting purified lithium chloride solution was between 15 and 30 wt% lithium (as lithium chloride) solution using the following typical impurity analysis method:

it should be noted that great care is required in analyzing these low concentrations to avoid contamination causing falsely high readings. Commonly used analytical methods in the field of sodium chloride-alkali (sodium chloride-alkali) are not applicable.

The purified brine is then electrolyzed using an electrochemical cell. A typical electrochemical cell has three (3) major components, an anode, a permeable membrane, and a cathode. The process of the present invention will use a perfluorosulfonic acid cation exchange membrane, such as DuPont's'One member of the membrane family.

Due to the corrosive nature of the solution, particularly lithium chloride, the electrodes are preferably made of a highly corrosion resistant material. The electrodes are preferably coated titanium and nickel. Preferably the cells are arranged in a so-called "pseudo-zero gap" type configuration, such as Ineos FMOl with flat plate anodes with turbulence promoting webs on the catholyte side, i.e. to promote turbulence, also supporting the membrane from the anode surface. This arrangement is preferred over the more conventional zero-gap arrangement to avoid premature anode damage or anode coating failure due to the potentially high pH gradient region surrounding the anode.

Preferably, the cathode side electrode is a lantern blade design to promote turbulence and gas release.

The full and half reactions of the electrode are as follows:

2Cl-＝＝＞Cl₂+2 e-anodic ion reaction

2H₂O+2e-＝＝＞H₂+2 OH-cathodic ion reaction

2Cl-+2H₂O＝＝＞Cl₂+H₂+2OH^-Full ionic reaction

2LiCl+2H₂O＝＝＞2H₂O +2LiOH complete reaction

Typical operating conditions for the above-described cell are described as follows:

those skilled in the art will appreciate that these are exemplary only and not limiting, depending on variations in the procedure, the equipment used, the desired end product, and other factors.

Lithium hydroxide monohydrate can be produced by, for example, simple vacuum cooling crystallization, utilizing the latent heat of the cathode solution; designed for this purpose using standard available industrial equipment.

The lithium hydroxide monohydrate product of the present invention is of sufficient purity for battery applications as an improved result over other lithium hydroxide manufacturing processes that require additional washing or other processing steps to achieve the desired purity of the battery.

Chlorine and hydrogen produced by operation of the electrochemical cell may be dehydrated, optionally with slight compression. The chlorine and hydrogen react exothermically to produce hydrogen chloride gas. These two gases are ignited inside a suitably configured water-cooled combustion chamber through a burner nozzle. The generated hydrogen chloride gas is cooled and absorbed by water to obtain hydrochloric acid with the required concentration. The quality of the water used for absorption will determine the purity of the resulting acid. In addition, one skilled in the art may produce other chemicals from these vapors.

Other process steps may also be added to the overall process of the invention. For example, if, for example, the ion concentration exceeds the desired range for the lithium hydroxide monohydrate product, or if, for example, the proper functioning of the electrodes is maintained, the liquid of the cell is purged from time to time as necessary. Preferred embodiments

With reference to the accompanying drawings, which disclose the process of the preferred embodiment of the invention, a lithium chloride-containing brine (1) is provided, which may be natural or prepared from, for example, ore. The brine is subjected to a primary purification step (2) to reduce the content of unwanted ions or other impurities. This can be accomplished, for example, by precipitating magnesium, boron, barium, and calcium or sodium, forming insoluble salts by those methods previously described or other methods known in the art, and adjusting the brine pH to be alkaline to precipitate unwanted ions as hydroxides. This brine is then used in a further step, using brine (3) or a brine more suitable for the present application, for a secondary purification step (4) using ion exchange such as described above. Finally, the total amount of Ca and Mg in the brine prior to electrolysis is reduced to less than 150ppb by any combination of chemical, solar evaporation or ion exchange processes.

The brine containing less than 150ppb total Ca and Mg ions is then subjected to electrolysis (5) using a cation permselective membrane to separate the anolyte from the catholyte. The lithium ions migrate through the membrane to form an aqueous catholyte solution comprising substantially pure lithium hydroxide solution.

The rectifier (21) is connected to an alternating current source (not shown) to supply direct current to the anode and cathode of the electrolytic cell (5). Preferably, cooling water is circulated through the rectifier to remove excess heat while increasing the operating efficiency of the rectifier. The battery is at 1.5kA/m²Starting up, then increasing to the operating condition of 2-3kA/m according to the production requirement². The operation is carried out at an operating voltage of 3-3.5 volts, and then the driving is carried out according to the production requirements. Over time, the cell efficiency decreases, requiring an increase in the required current density and the required voltage to meet the same production requirements.

The anolyte (14) can be reused in the process by adding HCl from an external source or from the process and can be returned to the lithium chloride feed stream (1). Preferably, the anolyte is purified (15) prior to mixing with the lithium chloride feed stream (1). In a preferred embodiment, the anolyte leaves the cell at a concentration of < 20 wt.%, more preferably < 19.5 wt.%. Due to OH^-Ions migrate across the membrane and the spent anolyte may contain chlorides and/or hydroxides. These ions are preferably neutralized by adding HCl to the recycled spent anolyte as well as to the fresh anode.

The hydrolysis produces chlorine (6) and hydrogen (7) by-products. They are then combined into a hydrochloric acid synthesis unit to produce hydrochloric acid, which is then stored (9). A chlorine absorber (10) is preferably provided which operates in emergency situations, obviously for safety reasons, and which will absorb chlorine in the event of problems in the HCl synthesis pathway.

In the preferred embodiment, the tail gas scrubber (12) receives deionized water, for example from a process stream, or directly receives hydrogen and/or chlorine gas for delivery to the HCl synthesis unit (8) to remove impurities from the vapors, such as chlorine gas that has not reacted with hydrogen in the HCl synthesis unit. The unit (12) ensures compliance with gas emission requirements.

The catholyte (13) is an aqueous lithium hydroxide solution containing calcium and magnesium as impurities at a combined concentration of less than 150 ppb. The lithium hydroxide is then separated from the catholyte by, for example, caustic concentration and/or crystallization (16), precipitated as lithium hydroxide monohydrate, and these crystals are then centrifuged, optionally drying (17) the separated lithium hydroxide monohydrate or lithium carbonate. Steam may be used in the crystal purification process. The recovered lithium hydroxide monohydrate is then stored in their final packaging as required (18).

In this preferred embodiment, the catholyte may be cooled (19), e.g. by adding cooling water, before recovering the lithium hydroxide monohydrate crystals, or the catholyte may be returned for further electrolysis (20).

The process condensate may be obtained from a steam condensate from the battery operation or from water evaporation in the crystallization operation. In order to avoid high concentrations of OH^-Ions and enhances transport of lithium ions across the membrane, process condensate is added to achieve the optimum performance level of the cell.

In an alternative embodiment, the catholyte may be used directly in other process steps (22) without recovering lithium hydroxide crystals.

After caustic concentration and/or drying (16) of the crystals, the residual solution, which may contain unrecovered lithium, is purged (24) and recycled as a caustic additive to the feed stream (1) for reprocessing to reduce any unused lithium to hydroxide. This will also help to adjust the pH of the positive electrolyte delivery vapor, which is acidic upon addition of acid, preferably hydrochloric acid generated in process (26).

All references, patents, patent applications, publications, and other citations herein are incorporated herein by reference in their entirety for the purposes of this invention.

Claims (49)

1. A method for producing lithium hydroxide monohydrate crystals comprising the steps of:

(a) concentrating a lithium and sodium and optionally potassium containing brine to precipitate sodium and optionally potassium from the brine;

(b) optionally purifying the brine to remove or reduce the concentration of boron, magnesium, calcium, sulfate, and any remaining sodium or potassium;

(c) adjusting the pH of the brine to about 10.5 to 11 to further remove lithium

Any cation other than;

(d) further purifying the brine by ion exchange to reduce the total calcium and magnesium concentration to below 150 ppb;

(e) electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of 150ppb or less, with by-product chlorine and hydrogen; and

(f) the lithium hydroxide solution is concentrated and crystallized to produce lithium hydroxide monohydrate crystals.

2. The process of claim 1, wherein the lithium hydroxide solution in (f) is converted to a high purity lithium product, preferably high purity lithium carbonate.

3. The method of claim 1, further comprising centrifuging the lithium hydroxide monohydrate crystals.

4. The method of claim 3, further comprising drying the centrifuged crystals and subsequently encapsulating the dried material.

5. The process of claim 1, wherein the brine is concentrated to a lithium concentration of about 2% to about 7% prior to electrolysis.

6. The process of claim 1, wherein the lithium-containing brine in (a) is concentrated by solar evaporation.

7. The process of claim 1, wherein the boron content of the brine in (b) is reduced by an organic extraction process or ion exchange.

8. The process of claim 1, wherein the amount of magnesium in the brine as in (b) is reduced by a controlled reaction using lime or slaked lime.

9. The process of claim 1, wherein the amount of magnesium in the brine as in (b) is reduced by using a controlled reaction of lime and slaked lime.

10. The process of claim 1, wherein the amount of calcium in the brine as in (b) is reduced by oxalic acid treatment.

11. The process of claim 1, wherein the sulfate content of the brine as in (b) is reduced by barium treatment.

12. The process of claim 1, wherein the sodium content of the brine is reduced in (b) by fractional crystallization.

13. The process of claim 1, wherein the pH of the brine is adjusted to about 11.

14. The process of claim 1, wherein the pH of the brine is adjusted by adding lithium hydroxide and lithium carbonate in stoichiometric amounts equal to the contents of iron, calcium and magnesium.

15. The process of claim 1, wherein the pH of the brine is adjusted by adding lithium hydroxide and lithium carbonate obtained from the product of the process of claim 1.

16. The process of claim 1, wherein the total concentration of calcium and magnesium in the brine is reduced to less than 150ppb by ion exchange.

17. The method of claim 1, wherein a semi-permeable membrane is applied in the electrolysis step, which selectively passes cations and inhibits passage of anions.

18. The method of claim 1, wherein in the electrolyzing step, the electrodes are made of a highly corrosion resistant material.

19. The method of claim 1, wherein in the step of electrolyzing, the electrodes are made of titanium and nickel coatings.

20. The method of claim 1, wherein in the electrolyzing step, the electrochemical cells are arranged in a "pseudo-zero gap" configuration.

21. The method according to claim 1, wherein in the electrolysis step a monopolar membrane cell is used, preferably an Ineos chlorine FMl 500 monopolar membrane.

22. The method of claim 1, wherein in the electrolyzing step, the cathode side electrode is a lantern blade design to promote turbulence and gas release.

23. A method of producing hydrochloric acid comprising the steps of:

(a) concentrating a lithium and sodium and optionally potassium containing brine to precipitate sodium and optionally potassium from the brine;

(b) optionally purifying the brine to remove or reduce the concentration of boron, magnesium, calcium, sulfate, and any remaining sodium or potassium;

(c) adjusting the pH of the brine to about 10.5 to 11 to further remove any cations other than lithium;

(d) further purifying the brine by ion exchange to reduce the total calcium and magnesium concentration to below 150 ppb;

(e) electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of 150ppb or less, with by-product chlorine and hydrogen; and

(f) hydrochloric acid is produced by combustion of the chlorine gas with excess hydrogen.

24. The process of claim 23, wherein the lithium hydroxide solution in (e) is converted to a high purity lithium product, preferably high purity lithium carbonate.

25. The method of claim 24, further comprising concentrating and crystallizing the lithium hydroxide solution to produce lithium hydroxide monohydrate crystals.

26. The method of claim 25, further comprising drying the crystals.

27. The process of claim 23, wherein the brine is concentrated to a lithium concentration of about 2% to about 7% prior to electrolysis.

28. The process of claim 23, wherein the lithium-containing brine in (a) is concentrated by solar evaporation.

29. The process of claim 23, wherein the boron content of the brine in (b) is reduced by an organic extraction process.

30. The process of claim 23, wherein the amount of magnesium in the brine as in (b) is reduced by a controlled reaction using lime or slaked lime.

31. The process of claim 23, wherein the amount of magnesium in the brine as in (b) is reduced by a controlled reaction using lime.

32. The method of claim 23, wherein the amount of calcium in the brine as in (b) is reduced by oxalic acid treatment.

33. The process of claim 23, wherein the sulfate content of the brine as in (b) is reduced by barium treatment.

34. The process of claim 23, wherein the sodium content of the brine is reduced in (b) by fractional crystallization.

35. The process of claim 23, wherein the pH of the brine is adjusted to about 11.

36. The process of claim 23, wherein the pH of the brine is adjusted by adding lithium hydroxide and lithium carbonate in stoichiometric amounts equal to the contents of iron, calcium, and magnesium.

37. The process of claim 23, wherein the pH of the brine is adjusted by adding lithium hydroxide and lithium carbonate obtained from the product of the process of claim 1.

38. The process of claim 23, wherein the total concentration of calcium and magnesium in the brine is reduced to less than 150ppb by ion exchange.

39. The method of claim 23, wherein a semi-permeable membrane is applied in the electrolysis step, which selectively passes cations and inhibits passage of anions.

40. The method of claim 23, wherein in the electrolyzing step, the electrodes are made of a highly corrosion resistant material.

41. The method of claim 23, wherein in the electrolyzing step, the electrode is made of a titanium and nickel coating.

42. The method of claim 23, wherein in the electrolyzing step, the electrochemical cells are arranged in a "pseudo-zero gap" configuration.

43. The method according to claim 23, wherein in the electrolysis step a monopolar membrane cell is used, preferably an Ineos chlorine FMl 500 monopolar membrane or other commercially available monopolar membrane cell.

44. The method of claim 23, wherein in the electrolyzing step, the cathode side electrode is a lantern blade design to promote turbulence and gas release.

45. A method for producing lithium hydroxide monohydrate crystals comprising the steps of:

(a) purifying a brine containing lithium and sodium, and optionally potassium, to reduce the total calcium and magnesium concentration to below 150 ppb;

(b) electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of 150ppb or less, with by-product chlorine and hydrogen; and

(c) the lithium hydroxide solution is concentrated and crystallized to produce lithium hydroxide monohydrate crystals.

46. A method of producing hydrochloric acid comprising the steps of:

(a) purifying a brine containing lithium and sodium, and optionally potassium, to reduce the total calcium and magnesium concentration to below 150 ppb;

(b) electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of 150ppb or less, with by-product chlorine and hydrogen; and

(c) hydrochloric acid is produced by combustion of the chlorine gas with excess hydrogen.

47. A method for producing lithium hydroxide monohydrate and hydrochloric acid comprising the steps of:

purifying a brine containing lithium and sodium, and optionally potassium, to reduce the total calcium and magnesium concentration to below 150 ppb;

(b) electrolyzing the brine to produce a lithium hydroxide solution having a total calcium and magnesium concentration of 150ppb or less, with by-product chlorine and hydrogen;

(c) concentrating and crystallizing the lithium hydroxide solution to produce lithium hydroxide monohydrate crystals; and

(d) hydrochloric acid is produced by combustion of the chlorine gas with excess hydrogen.

48. Lithium hydroxide monohydrate containing Ca and Mg in a combined total concentration of 150ppb or less, and preferably in a combined total concentration of 50ppb or less, and most preferably in a combined total concentration of 15ppb or less.

49. An aqueous lithium hydroxide solution containing Ca and Mg at a total concentration of 150ppb or less, and preferably at a total concentration of 50ppb or less, and most preferably at a combined total concentration of 15ppb or less.