WO2014076375A1

WO2014076375A1 - Method for recovery of metals

Info

Publication number: WO2014076375A1
Application number: PCT/FI2013/051079
Authority: WO
Inventors: Vesa Rissanen
Original assignee: GLOBAL ECOPROCESS SERVICES Oy
Current assignee: GLOBAL ECOPROCESS SERVICES Oy
Priority date: 2012-11-16
Filing date: 2013-11-18
Publication date: 2014-05-22
Anticipated expiration: 2015-05-16
Also published as: FI124262B; FI20120381A7

Abstract

In a method for recovering metals from a liquid in which they are present in dissolved form, at least one metal is separated by adding a hydroxide together with a boron compound to the liquid, wherein a permanent precipitate is obtained.

Description

METHOD FOR RECOVERY OF METALS

The invention relates to a method for recovery of metals as presented in the preamble of the appended claim 1.

Liquids are produced in many processes, metals being present in dissolved form in the liquids. Such processes include various types of processes, in which metals in solid form or metal-containing solids are treated. A result is often substances in liquid state which are classified as waste and in which the metals are present in dissolved form, for example in the form of a salt. Such waste may be formed not only in treatment processes carried out on purpose but also when metals or metal-containing substances come into contact with liquids in another way. The purification¹ of such liquids from metals, many of which (for example heavy metals) are harmful for the health or for the environment, is complicated by their low content or other substances present in the solution. For example, ion exchange has been used, wherein a harmful metal cation is replaced with a harmless cation. However, such ion exchange applications are expensive.

Furthermore, methods are known, in which dissolved metal is precipitated from the solution by using a suitable chemical that contains an anion which forms an insoluble salt with said metal cation. In water purification, many chemical precipitants are known which are based on, for example, hydroxides that precipitate metals in the form of insoluble hydroxides. However, their functionality is dependent on other conditions, such as other substances contained in water, and for example the pH. For example, US patent 5,443,619 discloses a method, in which calcium hydroxide or calcium oxide is used as a precipitant chemical for precipitating metals which are precipitable at various pH values, one after the other, and for recovering valuable metals. A corresponding consecutive precipitation method is disclosed in US patent application publication No. US- 201 /233139, where acid mine drainage is treated. A problem with the precipitation methods is that they, in a way, produce new waste in the form of precipitates or dissolved salts which are difficult to separate or for which there is no further use.

It is an aim of the present invention to eliminate the above mentioned draw- backs and to present a method for recovery of metals in dissolved form, which method does not have the above mentioned drawbacks and which can be implemented in a simple and inexpensive way. To achieve this aim, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim .

The invention is characterized in that hydroxide is used together with a boron compound for the precipitation of metals. The boron compound can be a suitable hydroxo compound of boron or a compound that contains boron in the form of an oxoanion. An example of the former is boron acids (oxyacids, particularly boric acid H3BO3. An example of the latter is borate salts, particularly borax. Boric acid H3BO3 is the most common acid of boron and it is an inexpensive precipitation chemical which is capable of forming poorly soluble precipitates with metal hydroxides. Borax, in turn, is a commonly found form of boron acting in the same way. Later changes in the conditions, such as the pH, cannot affect the precipitate, because the metal hydroxides form very permanent precipitates with boron compounds, held together by OH groups. Certain boron compounds, in which the boron is bound to oxygen, tend to form chains or be cross-linked, thanks to the hydrogen bonds formed by the hydroxy groups. The precipitate is a borate, to which the metal to be sepa- rated is bound.

The method can be used for recovering metals in order to bind the metals which are present in soluble form (ions) into a permanently solid form, for disposal or further utilization of metals. The solution, from which the dis- solved metal is recovered by utilizing precipitation by adding a boron compound, can be an aqueous solution, in which the metal has been dissolved as an ion in some conditions. It can be effluent from the processing of metal or metal-containing material, water that contains entrained metals accumulated in other conditions, or process water produced on purpose in processes of dissolving metals, for the recovery of the metal from the process water. In the case of a liquid containing two or more dissolved metals, it is possible to aim at the recovery of one specific metal by adjusting the pH conditions to be favourable for the borate precipitation (boratization) of this metal, and to leave the other metal(s) in the solution, or to precipitate two or more metals as a mixed borate. Even in this case, after the precipitation, some metals may remain dissolved in the liquid, depending on the initial quantities of the different metals.

Two or more metals are precipitated by first adding a sufficient quantity of a boron compound to the liquid that contains the metals, and then gradually increasing the pH of the liquid by adding an alkaline compound that brings hydroxide ions. The metals are precipitated in the form of borates in the order of formation of their hydroxide precipitates as a function of the pH value. In this way, a given metal / given metals can be left in the liquid by first precipi- tating the metal/metals precipitating at a lower pH in the form of a borate / mixed borate, and by isolating the precipitate from the liquid. The remaining liquid can be processed in another way or the metal/metals can be precipitated from it separately by continuing the pH increase by adding an alkaline compound to a range in which this metal is / these metals are precipitated in the form of borates. Alternatively, all the metals can be precipitated as a mixed borate by increasing the pH by adding an alkaline compound all the way to the range of borate precipitation of the last metal, after which the mixed borate precipitate that contains all the metals to be precipitated with borate is separated from the liquid.

When valuable metals, for example nickel, copper or cobalt, are separated from solutions, the metal can later be recovered from the borate compound produced in the precipitation. By the precipitation method carried out with a boron compound, it is possible to replace even sulphide precipitation which is used in the mining industry for the precipitation for further processing of metal sulphate obtained from ores by leaching. Hydrogen sulphide, which is difficult to handle and is normally used, can be replaced with boron compounds which are easier and safer to handle. In the following, a reaction equation is presented as an example, describing the precipitation of metals dissolved as sulphates by means of hydroxide and boric acid. MeS0₄ + 2NaOH + 4H₃B0₃ -> [Me(OH)₂ + Na₂(B₄07)^*5H₂0] + H₂S0₄

The substances that form a permanent precipitate are marked in square brackets. The solution remaining after the precipitate contains sulphuric acid which has many uses. It can be, for example, recirculated for dissolving met- als in acidic conditions. Sulphuric acid can be separated from a solution by means of heat, whereby it evaporates. The heat can be obtained from an exothermic reaction, or if reaction heat alone is not sufficient, the solution can be heated. Underpressure can be utilized in the recovery. The equation is not to be understood as the only possibility, because borates are a very versatile group of salts and they can be present, for example, in various ratios of boron and oxygen and in various quantities of bound water; in addition, there are various ways of notation of these compounds. What is essential is that the metal to be removed is bound to the borate.

"Me" can be a divalent metal (a metal that forms a divalent metal ion), for example Cu, Fe, Ni, Co, Mn, Mg, or Zn.

In particular, when Me is Cu, Ni or Co, the method can be used for recover- ing dissolved valuable metals from process waters and/or effluents of the mining industry in order to separate the metal from the precipitate later on.

Instead of boric acid, borax can be used for precipitation. Similarly, the hydroxide can also be other than sodium hydroxide, because the only important thing is to introduce hydroxide ions to the solution. The alkaline compound can be, for example, calcium hydroxide, if an addition of sodium to the liquid is to be avoided. Furthermore, calcium is capable of precipitating sulphate in the form of calcium sulphate (gypsum precipitate), which is useful if the liquid contains metals in the form of sulphates and the sulphate is to be removed from the liquid. Calcium hydroxide can be used, if the metal to be removed from the liquid by the precipitation method is, for example, sodium itself, in which case the method is particularly suitable for the treatment of sodium-containing effluents and process waters. The sodium can be disposed with the precipitate, or it can be recovered from the precipitate by a separation method.

The precipitation can be performed in containers or larger reservoirs (for example various waste tanks and storage reservoirs), into which the hydroxide and the boron compound is introduced. For separating the precipitate from the remaining solution (supernatant), it is possible to apply normal mechanical separation methods.

If various metals are to be separated by the method, it is possible to utilize the precipitation of various metals in the form of hydroxides at different pH values, as is previously known. When a specific pH value at which a metal precipitates has been achieved with the hydroxide, the resulting precipitate can be separated, and after possible intermediate purification operations, more hydroxide is added to the remaining solution again, to achieve the precipitation point (pH value) of the next metal. The difference to prior methods is naturally that a boron compound is also used for the precipitation. Alterna- tively, two or more metals can be precipitated at a time, in the same precipitate, and the metals can be separated from each other in further processing steps. The pH of the liquids to be processed is usually on the acidic side, i.e. lower than 7, before the addition of the alkaline compound. The initial pH value can also be influenced by the boron compound.

The method can also be used for recovering other metals, for example other divalent or monovalent, or also trivalent metals in such a way that they will not remain in the solution. It can be used for binding, for example, sodium sulphate from water. The monovalent metal to be recovered can be any alkali metal, that is, sodium, potassium, lithium, or cesium.

The invention can also be used for cleaning contaminated solid material, for example by suspending solid material in water, in which the soluble metal salts present in the solid material are thus dissolved, and they can be removed by adding said chemical precipitants. The metals can then be removed from the solution as presented above. Alternatively, the chemical precipitants can be directly admixed to the solid material, and the borate precipitate formed of the metal that was in soluble form in the solid material can be left in the solid material, because no more metal is dissolved. Examples

In the following, three examples will be presented of the possibilities of applying the invention. In the first one, chromium was to be isolated and separated, because most of it was heavy metal hexavalent chromium.

Further, in the second example, heavy metal copper and nickel were to be separated from the rest of the concentrate, as a separate precipitate.

In the third example, the aim was to provide pure water to be discharged in the nature, and to isolate metal residues and thereby also sulphate in a precipitated bottom product (precipitate) that is easily storable, Each example describes how the desired end result can be achieved with a correctly adjusted pH value and the metals can be separated alone and/or separately.

Example 1

Chromium-containing effluent is present in a secondary flow as part of a neutralization process by an industrial enterprise whose business is surface plating. The effluent is a concentrate that also contains traces of other dissolved metals, but their total content remains lower than 0.1 % by weight. Thus, in practice, the concentrate only contains chromium in two different oxidation levels, namely hexavalent and trivalent chromium, dissolved as sulphates in sulphuric acid. In the initial situation, the pH of the solution was 1.4. To this chromium-containing solution, borax was added in a molar ratio of :2 (metal: borax), after which the pH was adjusted with sodium hydroxide to the value of 7.8. After slight stirring, the solution was allowed to settle. After the settling, a slightly greenish clear liquid supernatant and a ligand-like bottom precipitate had separated in the concentrate. The separated bottom product consisted of 99.99 % pure chromium borate. The supernatant contained the other metals (Ni, Mg, Mn) of the original solution, because they did not yet precipitate in this pH range.

Example 2 An industrial secondary flow consisted of effluent containing 85.0 wt-% of copper, 13.5 wt-% of nickel, and 1.5 wt-% of magnesium, the pH being 1.0. Boric acid was added to the solution at a metal:molar ratio of 1 :2, and the pH was increased with sodium hydroxide to the value of 9.2. The solution was mixed and allowed to settle. After the settling, 100 % copper-nickel mixed borate was separated as a bottom product. Magnesium was still dissolved in the supernatant, because its precipitation had not yet started.

In the preceding examples 1 and 2, it is possible to continue the precipitation of the metals remaining in the supernatant by the same method, i.e. by rais- ing the pH to their precipitation range.

Example 3

A secondary flow relating to disposal in mining industry had been purified of all transition metals but it still included a considerable content of alkali metal sodium bound in sulphate and a minor content of lime from preceding process steps. Because of the salt content, the water could not be discharged in natural water system, in which it causes eutrophication. The water contained 70.6 wt-% of Na and 29.4 wt-% of Ca, the pH being 7.0. By adding boric acid at a molar ratio of metal: boric acid of :2, the pH was first reduced to a value of about 4. Then, by adjusting the pH with 10% milk of lime, Ca(OH)₂, to the value of 9.3, pure water as supernatant and a combination of gypsum and a mixed borate of sodium and calcium as bottom product was separated in settling. Because boric acid does not contain sodium, it is an advantageous boron compound for the removal of sodium. W

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The invention is not limited to the above presented example, but it can be modified within the scope of the inventive idea presented in the claims, and also be applied in uses which have not been mentioned above.

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Claims

Claims:

1. A method for recovering metals from a liquid in which they are present in dissolved form, wherein at least one metal is separated from the liquid by precipitation, by adding a hydroxide to the liquid, characterized in that the hydroxide is used together with a boron compound for the precipitation.

2. The method according to claim 1 , characterized in that the boron compound is added to the liquid first, after which one or more metals are precipi- tated in the form of a borate by increasing the pH by adding an alkaline compound to the liquid.

3. The method according to claim 2, characterized in that two or more metals are precipitated by adding an alkaline compound to the liquid in such a way that the metals are precipitated in the form of borates in different pH ranges as the pH is increased.

4. The method according to claim , 2 or 3, characterized in that the precipitate obtained is separated from the liquid and is subjected to final disposal.

5. The method according to claim 1 , 2 or 3, characterized in that the precipitate obtained is separated from the liquid, and the metal is separated from this precipitate and recovered.

6. The method according to any of the preceding claims, characterized in that the metal is Cu, Fe, Ni, Co, Mn, Mg, or Zn.

7 The method according to any of the claims 1 to 5, characterized in that the metal is Na and the hydroxide used is a hydroxide other than sodium hydroxide, advantageously calcium hydroxide.

8. The method according to any of the preceding claims, characterized in that the boron compound used for precipitation is boric acid or borax.

9. The method according to any of the preceding claims, characterized in that it is used for separating two or more metals, for example in such a way that they are separated in different precipitates.

10. The method according to any of the preceding claims, characterized in that it is used for recovering metals from effluent or process water.