Technical field The application relates to a method for the separation of metals. Background

Liquids are produced in many processes, metals being present in dissolved state in the liquids. Such processes include various types of processes in which metals in solid state 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 purification 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.

International publication WO 2014/076375 discloses a method for forming permanent precipitates of metals by means of boron compounds. In the method, metals are precipitated by using hydroxide in combination with a boron compound, such as borax or boric acid, resulting in poorly soluble borate to which the metal is bound. For the precipitation, a sufficient amount of the boron compound is first added to the liquid which contains metals, and the pH of the liquid is then gradually increased by adding an alkaline compound which introduces hydroxide ions. Different metals can be separated from each other by separating the precipitate obtained at a given precipitating pH before the pH is increased to the precipitating range of the next metal.

International publication WO 2015/036658 discloses a method for precipitating two or more metals in the form of metal borates by utilizing precipitation nuclei formed as a result of increasing the pH.

The method can be applied for the final disposal of metals in the form of a permanent precipitate, particularly in the case of harmful heavy metals. In this case, the metal is no longer usable. However, a need also exists to recover metals from various waste solutions and other liquids in such a way that they are recovered in metal form and possible even separated from each other. In the article "New Separation Method of Boron and Iron from Ludwigite Based on Carbon Bearing Pellet Reduction and Melting Technology", ISIJ International, Vol. 52 (2012), No. 1 , pp. 45-51 , Wang G. et al disclose the separation of iron from natural Ludwigite ore by reduction with carbon. In the ore, iron and boron are present in different minerals, and the aim is to increase the boron content by separating the iron present in the form of magnetite mineral in the ore. The method yields metallic iron fraction melt and slag rich in boron, which can be used as raw material in the boron industry. Brief description of the invention

It is an aim of the invention to present a method for recovering metals in insoluble metallic form, starting from borate precipitates in such a way that they can be recovered directly from the precipitate without forming chemical intermediate products. It is also an aim of the invention to make recycling of metals possible. To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1 .

In the method, one or more metals are separated from a borate precipitate by heating under reducing conditions. As the reducing agent, for example coal can be used, which is a common reducing agent.

The heating method can be used for separating one metal or for separating two or more metals from the borate precipitate. When several metals are separated from the precipitate, they can be separated by utilizing their different melting temperatures in such a way that each metal is recovered in molten state at a temperature specific for it, by increasing the temperature of the precipitate. It is noteworthy that in the method of separating in the form of metal borates, the melting temperatures of the metals which can be separated are lower than those for the respective pure metals.

When the metals are heated, they easily form oxides due to the oxygen contained in the borates. To prevent oxidation of the metals separated from the precipitate, a reducing agent is used, to be oxidized instead of the metals.

After the separation of the metals, the temperature is raised further to 1300 degrees, at which residual precipitates are solidified into slag. The slag is insoluble and it can be disposed of as permanent waste. Detailed description

In practice, the method can be implemented by placing the borate precipitate, contained in a suitable melting pot or corresponding container, into a furnace whose temperature is adjustable, and by raising the temperature of the furnace. The temperature of the furnace can advantageously be raised by a predetermined temperature/time program; in other words, the temperature increases as a function of time as desired. In particular, the temperature can be maintained at a given value for a longer time if the separation of a given metal in molten state takes place at this temperature. In industrial scale, a temperature-controlled melting furnace is feasible, the precipitate being placed in it. An outlet is provided at the bottom of the furnace for discharging one or more separating metals in molten state. The borate precipitate may come from the recovery of metals by utilizing, for example, the principle presented in the above-mentioned documents WO 2014/076375 and WO 2015/036658. In particular, the metal can be separated from a solution form by precipitating the metal as a borate under conditions favourable for precipitation, including appropriate pH and the presence of precipitation nuclei. In practice, the precipitation is carried out by adding a boron compound to the solution, by increasing the pH of the solution to a range favourable for precipitation, and by separating the resulting borate precipitate from the solution. The method can also be used for precipitating two or more metals. As precipitation nuclei, it is possible to use a hydroxide precipitate of a metal formed at a lower pH, particularly of iron, which makes the metal that precipitates as a hydroxide at a higher pH, precipitate in the form of a borate. Precipitation nuclei may also be entered from the outside, for example in the form of borates already precipitated. The boron compound to be used may be a suitable hydroxo compound of boron, or a compound that contains boron as an oxo-anion. An example of the former is acids of boron (oxoacids), particularly boric acid H3BO3. An example of the latter is borate salts, particularly alkali metal borates, such as borax. Boric acid H3BO3 is the most common acid of boron and an inexpensive precipitation chemical which is capable of forming poorly soluble precipitates with metal hydroxides. Borax (Na borate), in turn, is a commonly occurring form of boron acting in the same way. Later changes in the conditions, such as changes in the pH, cannot affect the precipitate either, because the metal hydroxides form very permanent precipitates with boron compounds, which precipitates are held together by OH groups. Certain boron compounds, in which the boron is bound to oxygen, tend to form chains or networks enabled by the hydrogen bonds formed by the hydroxy groups. The precipitate is a borate, to which the metal to be separated is bound. This is different from mineral borate. About 230 different ore-based mineral borates are known in the art. Most of these are based on sodium, calcium or magnesium, and they do not present transition metals. Most important mineral borates being excavated include sassolite (H3BO3); the sodium borates borax (Na2B ₄ O7- 10H2O), kernite (Na2B ₄ 0y4H2); the calcium borites colemanite (Ca ₂ BeOii -5H20), inyoite (Ca ₂ BeOii - 13H20), and priceite (Ca ₄ Bi ₀ Oi ₉ -7H ₂ O); the sodium calcium borates ulexite (NaCaBsOg-ehbO) and probertite (NaCaB509-3H20); the magnesium borates szaibelyite (ascarite; Mg2B20 H20), inderite or kurnacovite (Mg2B60i 15H20; monoclinic or triclinic, and pinnoite (MgB20 ₄ -3H20); the magnesium calsium borate hydroborasite (CaMgBeOn -ei-bO); the boron silicates datolite (Ca ₂ B2Si209- H20) and ludwigite (MgFeBOs); and the double salt of magnesium chloride borasite (Mg3B70i3CI). Excavation is mostly carried out for boric acid and borax (disodium tetraborate decahydrate). The above mentioned do not include transition metals, except for ludwigite which contains iron but no other transition metals. The borate compounds present in mineral borate ore bodies differ from borates obtained by precipitation substantially in a number of ways. Mineral borates consist of several cations and anions, they are very large molecules, or they have become facilities of large dimensions with respect to their cationic or anionic properties, such as boron silicates, boracites, etc. In mineral deposits, they are typically low in content in ore bodies, and except for boric acid and borax, they are typically excavated in the form of ores secondary to other main minerals and often in the form of weathered ores. Borate precipitates obtained from solutions from metal recovery, for example metal borates obtained by the method described in Finnish patent Fl 124419 and international publication WO 2014/118439, are not mineral. The content of metal borates utilized in them is tens or even several thousands of times greater than in minerals. In practice, they always contain more than one metal borate, and their molecular structure is substantially simpler than that of borate minerals, because their ionic bond is formed by a metal cation and a borate anion only. When a multiborate precipitate is melted at its specific temperatures, one metal is separate at a time, whereby the anion is decomposed into amorphous boron slag together with other slag substances, such as reduced CaC03. This process differs from the melting process of mineral iron borate in that the temperature has to be set suitable for each metal separately, and due to the high content, the boron in this case substantially decreases the melting temperature of each metal. Moreover, the amount of slag produced per produced metal unit remains substantially lower, because no mineral impurities are involved in the melting.

Typically, such a metal borate obtained from the solution, having the general formula ΜβίΒβΟζΗβ), has the form of a triborate, for example zinc borate ΖηίΒβΟζΗβ), which may also be written in the form ZnB30 ₄ (OH)3. Here, the hydrogen bonds between the OH groups make more and more triborate groups link together to form long chains, binding them in this way. The method can be performed as a batch process but as a continuous process as well, whereby precipitation nuclei from the preceding feed are present and used as catalysts. This makes the dwell times shorter.

One or more dissolved and recovered metals may come from waste solutions and effluents from industrial processes, storage basins of mining industry, or fly ash or contaminated soil which has been suspended to bring the metals into a liquid phase.

Metal borates obtained by the above described recovery method are considered permanent, because they are practically insoluble. This is the case if the borates are at normal temperatures and in a normal environment, generally under ambient conditions which prevail at their disposal sites. Now, it has been found that by bringing the metal borates to controlled conditions, to a sufficient temperature and by subjecting them to a reducing agent, the metals can be released from the borates. In an advantageous embodiment, the method includes two steps, whereby in the first step, one or more metals are recovered as a borate precipitate, and in the second step, the metal is separated from the borate precipitate at a controlled temperature in the presence of a reducing agent. However, the invention is not necessarily limited to the source of the metal borate or the way in which it has been formed.

The method is particularly suitable for the recovery of iron, nickel, copper, molybdenum, chrome, cadmium and zinc, but the invention can also be used for other metals as well as semi-metals, such as antimony and arsenic.

The invention is suitable for separating only one desired metal from the borate precipitate, as well as for separating two or more metals, particularly in such a way that the metals can simultaneously be separated from each other.

For example, the invention is suitable for separating nickel as a metal, starting from oxidic ores that contain nickel. Oxidic nickel deposits are typically found in the following order, starting from the ground surface below: red limonite, yellow limonite, and garnierite and sepertine in weathered saprolite. A transition layer is found between yellow limonite and weathered saprolite.

For example, the method described particularly in Finnish patent Fl 124419 and international publication WO 2014/1 18439 can be used, in which the starting material is oxidic alkaline ore, particularly laterite ore, from which nickel and possibly also cobalt are recovered. The laterite ore used as a starting material advantageously comprises limonite. Also in this case, the method includes two steps, whereby in the first step, one or more metals (particularly Ni and Co) are recovered as a borate precipitate, and in the second step, the metal is separated from the borate precipitate at a controlled temperature in the presence of a reducing agent. In the first step of the method, nickel and possibly cobalt are separated by a hydrometallurgical method as described in said publications, avoiding the use of acidic solutions. The ore starting material, for example limonite, is first ground, if necessary, to fine crushed stone (advantageously to a grain size of less than 1 mm), after which it is suspended in water, and the slurry obtained is strongly agitated for a sufficiently long time so that the pH begins to increase to a value higher than 7. The metal-containing aqueous solution is separated from the solids in the slurry, and its pH is increased to a range suitable for the precipitation of nickel, in the presence of a boron compound, which makes the nickel precipitate as a borate. Advantageously, borax (disodium tetraborate) is used, which can be simultaneously used to increase the pH to a suitable precipitation range above the value 9, for example, to the pH value about 9.3 which is advantageous for the precipitation of nickel.

In the second step, nickel and possibly cobalt are separated as metals by heating the borate precipitate under reducing conditions, as presented above. The method can also be used for separating nickel and cobalt from each other by separating them on the basis of different melting temperatures, according to the principle presented above.

In the following, the invention will be described in more detail by describing experiments on borate precipitate.

Examples

Preliminary tests for testing and verifying treatment with boric acid for the removal of metals and in view of the properties of the precipitate being formed

The testing arrangement was carried out on effluent water from a closed mine on site. The testing was started with laboratory tests by taking test samples of the waters to be treated. An elementary analysis was made, on the basis of which the required dosage of boric acid was calculated. Laboratory tests were taken to ensure the performance of the method. At the same time, optimal chemical feed rates as well as pH adjustment and its level changes with optimal dwell times were determined. On the basis of these, a test apparatus was built up for pilot test runs to be performed on site. The pilot test runs were performed by two methods of implementation. First, a batch pilot was run, wherein water accumulated in a discharge basin of the mine was pumped in a batch of 2.5 m ³ to be treated by applying the same method as in the preceding laboratory tests. After the test, the quality of the overflow water and the properties of the underflow precipitate were examined. The overflow water was found to be completely pure of metals. The underflow precipitate was dried, and its solubility was determined by a column percolation test. The precipitate turned out to be insoluble and also impermeable to water.

In the second pilot step, the test run apparatus was changed so that the test run could be performed in a continuous manner. The feeding of chemicals, the pH adjustment with level changes and dwell times were implemented by using two parallel containers. It was found that in the continuous run, the dwell times became shorter, because precipitation nuclei from the preceding feed were present all the time. Moreover, the precipitation nuclei already present acted as catalysts for the process so that the dosage of boric acid could be reduced. Experiments carried out on overflow water and underflow precipitate showed as good results as those obtained in earlier test steps.

The series of tests clearly showed the important role of the precipitation nuclei in the process. After their formation, the precipitation nuclei boosted the process and reduced the consumption of substances, which lowered the costs. The effect of the precipitation nuclei was also underlined in that the treated effluent contained complexing agents which tend to hamper and even inhibit the precipitation of hydroxides. As the precipitation nuclei boosted the process, the metals precipitated directly from the soluble hydroxides being just formed, into metal borate precipitate. Recovery of metals

Metal precipitate was prepared from a sample provided by a mining company, which was a batch of a dressed ore from a slag dressing plant before flotation. The precipitate was prepared by the method described in Finnish patent application 20135921 and international publication WO 2015/036658 "A method for the treatment of metals", by precipitating the metals contained in the sample into borate ligand by using boric acid and sodium hydroxide. The sample contained several metal components including sodium, magnesium, aluminium, silicon, potassium, calcium, iron, copper, zinc, lead, barium, antimony, and arsenic. For the test, the precipitate was prepared by grinding into homogeneity.

There are few reference documents on metal borates available. According to information from some sources, e.g. zinc borate is crystallized at about 1550°C into a preform usable for an optical lense, so the temperature of 1550°C was taken as a starting point.

In the test arrangement, a relatively small crucible was used, dosed with homogenized precipitate to fill a volume of about 2/3. The test furnace was such that the melting process could not be observed as a function of time, but the observations were made after the set final temperature had been reached and the test batch had dwelt in it for 10 minutes. The temperature of the furnace was increased slowly, 2°C/min.

Subsequent tests were carried out by lowering the final heating temperature by 50°C at a time.

It was found that when the final temperature was >1350°C, the precipitate batch starts to churn up strongly in connection with melting, and "goes up" out of the crucible as a result of the churning. This was interpreted to be probably due to strong oxidation, which is obvious because the borate is chemically formed of three oxygen atoms bound to a single boron atom. In the tests in which the final temperature was 1350°C or higher, no other observations were made than the churning, except that all the metals had melted and that the slag formed after the cooling was insoluble.

In the test in which the final temperature was 1300°C, the melted precipitate remained in the crucible. By examining the melt, it could be determined that all the metals present had melted. After the cooling, the crucible with the melt structure was split by a diamond wheel, cast in epoxy resin, and thin cross sections were prepared, whereby the metal particles present in the material could be examined in a reliable way. The samples were examined by scanning electron microscopy (SEM) and analyzed by energy dispersive elemental analyzer (EDS). Before the microscopy, the samples were carburized so that the surfaces to be examined became conductive. In an embodiment, the final temperature is 1300°C or higher, but lower than 1350°C.

Three phases were distinguished in the SEM cross-sectional image: ceramic oxide (AIO), grains rich in iron, and other metals present as crystals. The EDS analysis determined that phase 1 , matrix present in dark colour, consisted primarily of aluminium and silicate oxides and that more than one third (34.93 %) in the deposit consisted of elemental oxygen. In phase 2, several metals were found, these very oxidic, too. In phase 3, primarily iron grains were found, but some aluminium and silicon as well. It was found that the melting temperatures of borate metals are lower than the melting temperatures of elemental metals. It was also found that metals are not deposited merely by melting, which would be a prerequisite for separation by melting, but the high content of oxygen present causes oxidation, having the result that instead of depositing, the phases are disarrayed in the melt.

The tests were continued by carbon reduction. The purpose of carbon added into the metal precipitate is to reduce the metal based components back to metal, for example as follows: Fe203 + 3C→ 2Fe (met) + 3CO.

In this test, metal precipitate was treated in two different states:

1 . metal precipitate which was not subjected to thermal treatment (reference), and

2. metal precipitate which was subjected to thermal treatment with carbon at 1300 °C.

The content of metal precipitate was 2 volume parts and that of carbon 1 volume part. After the test, it was found that in the reference sample, the particles were not very tightly adhered to each other and thus a good thin cross section could not be made. However, it was possible to determine the elements and the presence of some compounds, such as borates.

The thermally treated sample was found to be sintered and the cross- sectional surface to be smooth. The slag material consisted of silicate and borate crystal types of aluminium, sodium, magnesium, potassium, calcium, and iron. No chromium, nickel, barium, arsenic, or antimony was detected in the examined areas of the reference sample. This can only be explained by the fact that they are present in the form of borate salt crystals. After the thermal treatment, several metals were detected in the metal form. EDS analysis does not directly disclose the crystal structure of a material. In the determination of the crystal structure, EDS can be assisted by X-ray diffraction (XRD), but this involves the problem that in the case of multicomponents, the number of components will also affect their identifiability. The XRD method was not tested in this study. In the structure of the phases, a tendency for arraying in sequences can be seen, whereby it can be assumed that layered deposits would be formed if the sample batch were larger. Thus, the essential objectives were shown by the study. The separation of metals by melting from a mixed metal borate precipitate is possible by reduction, the melting temperatures are lower than the melting temperatures of elemental metals, and the slag formed of residual precipitate is insoluble. It has been found that when borate precipitate is heated to a temperature higher than 1200°C, the remaining solid material, slag, becomes completely insoluble. Thus, the slag remaining from the separation of metals can be classified as permanent (inert) waste, and it can be easily disposed of. The slag can also be utilized as construction material, for example in infraconstruction, because it is not only insoluble but also impermeable to water and therefore a very good sealing material.

An embodiment presents the use of solid material, such as slag, obtained by the method described herein, as a building material, such as in infraconstruction, for example as a sealing material. Infraconstruction refers to the building of basic technical structrures, i.e. infrastructure, needed operation of an industrial society.