FI127281B - Method of separating metals - Google Patents

Abstract

In a method for separating metals from metal borate precipitates obtained from recovering metals from solutions, solid metal borate precipitate is heated under reducing conditions to a temperature at which the metal is separated from the precipitate in molten state, and the melt is separated from the remaining solid material.

Description

Many processes generate liquids in which metals are in dissolved form. Such processes include a variety of processes involving the treatment of solid state metals or metallic solids. The result is often liquid substances classified as waste in which the metals are dissolved, for example in the form of a salt. In addition to purposeful treatment processes, such wastes may be generated when metals or metallic substances are otherwise in contact with liquids.

The purification of such fluids from metals, many of which (for example, heavy metals) are harmful to health or the environment, is hindered by their low concentration or other substances present in the solution. For example, ion exchange has been used in which the harmful metal cation is replaced by a harmless cation. However, such ion exchange applications are expensive.

In addition, methods are known for precipitating a dissolved metal from a solution with a suitable chemical containing an anion which forms an insoluble salt with the metal cation. Many precipitation chemicals are known in water purification, for example based on hydroxides, which precipitate metals as insoluble hydroxides. However, their functionality depends on other conditions, such as other substances in the water and, for example, pH.

From International Publication WO2014 / 076375 there is known a method for forming stable metal precipitates by means of boron compounds. In the process, hydroxide is used in the precipitation of metals together with a boron compound, for example borax or boric acid, to give a sparingly soluble borate to which the metal is bound. Precipitation occurs by first adding a sufficient amount of the boron compound to the metal-containing fluid, and then gradually increasing the pH of the liquid by adding an alkaline compound which contains hydroxide ions. Different metals can be separated

20165207 prh 11 -03-2016 by separating the precipitate obtained at a specific precipitation pH before raising the pH to the precipitation region of the following metal.

International Publication No. WO2015 / 036658 discloses a method for carrying out the precipitation of two or more metals as metal borates by utilizing the precipitation cores produced by raising the pH.

The method can be used for the final deposition of metals, especially in the case of harmful heavy metals. In this case, the metal is no longer available. However, there is also a need to recover metals from various waste solutions and other liquids so that they can be recovered as metals and possibly separated.

Wang G. et al. 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, p. 45-51 separation of iron from naturally occurring ludwigite ore by carbon reduction. Iron and boron are present in different minerals in the ore, and the purpose is to increase the boron content by separating the iron in the form of a magnetite mineral.

The process yields a molten metal iron fraction and a boron rich slag that can be used as a raw material for the boron industry.

It is an object of the invention to provide a process for recovering metals insoluble in the metal form starting from borate precipitation so that they can be recovered directly therefrom without formation of chemical intermediates. Another object of the invention is to enable the recycling of metals. To accomplish this purpose, the method according to the invention is essentially characterized in what is set forth in the characterizing part of the appended claim 1.

In the process, one or more metals are separated from the borate precipitate by heating under reducing conditions. For example, carbon, which is a known reducing agent, can be used as the reducing agent.

The heating process can be used to separate one metal or two or more metals from a borate precipitate. After several

20165207 prh 11 -03-2016 metals are separated from the precipitate, they can be separated by utilizing their different melting temperatures so that each metal is recovered molten at its characteristic melting temperature by increasing the precipitation temperature. It is noteworthy that in the separation process, as metal borates, the melting points of the metals at which they can be separated are lower than those of the corresponding pure metals.

When heated, metals are sensitive to oxides due to oxygen in the borates. To prevent the oxidation of the precipitated metals, a reducing agent is used which oxidizes instead of the metals.

After separation of the metals, the temperature is raised to 1300 degrees, whereupon the precipitate solidifies into slag. The slag is insoluble and can be disposed of as permanent waste.

In practice, the process may be carried out by placing the borate precipitate in a suitable crucible or the like in a temperature controlled oven and raising the temperature of the oven. Preferably, the furnace temperature can be increased by a predetermined temperature time program, i.e. the temperature increases as desired over time. In particular, the temperature may be held at a certain value for a longer period of time if at that temperature a particular metal is molten.

On an industrial scale, it is a temperature-controlled melting furnace in which the precipitate is placed. The bottom of the furnace has a discharge connection for molten removal of one or more distinctive metals.

The borate precipitate may be derived from the recovery of metals from solutions utilizing, for example, the aforementioned WO2014 / 076375 and

WO2015 / 036658. In particular, the metal may be separated from the solution form by precipitating the metal as borate under conditions favorable to the precipitation, which include the correct pH and the presence of precipitating cores. In practice, the precipitation is carried out by adding boron compound to the solution, raising the pH of the solution to a region favorable for precipitation, and separating the resulting borate precipitate from the solution. The process can also be used to precipitate two or more metals. As the precipitation nucleus, a hydroxide precipitate of a metal, especially iron, formed at a higher pH, which results in the precipitation of a metal precipitated at a higher pH as a hydroxide as borate, may be used as the precipitation step. Precipitating cores can also be introduced externally, for example in the form of already precipitated borates.

The boron compound to be used may be any suitable boron hydroxy compound or boron oxoanion compound. The foregoing is exemplified by boric acids (oxygenates), in particular boric acid H3BO3. An example of the latter is borate salts, especially alkali metal borates such as borax.

Boric acid H3BO3 is the most common boron acid and inexpensive coagulant that can form sparingly soluble precipitates with metal hydroxides. Borax (Na-borate), on the other hand, is a common form of boron that functions in the same way. Subsequent changes in conditions, such as changes in pH, also have no effect on the precipitation, since the metal hydroxides, together with the boron compounds, form very stable precipitates which are held together by OH groups. Certain boron compounds in which boron is attached to oxygen have a tendency to chain or crosslink due to hydrogen bonds formed by hydroxy groups. The precipitate is borate to which the metal to be separated is bound.

The recovered one or more metals that were initially dissolved may be derived from industrial process waste water and wastewater, mining storage basins, or fly ash or contaminated land slurried to liquefy the metals.

The metal borates obtained by the recovery process described above are considered to be stable because they are practically insoluble. This is the case if the borates are at normal temperatures and ambient conditions, generally the environmental conditions prevailing at their disposal sites. It has now been found that by bringing the metal borates under controlled conditions, at a sufficiently high temperature and under the action of a reducing agent, the metals can be removed from the borates.

Thus, according to a preferred embodiment, the process is biphasic, in which, in the first step, one or more metals are recovered as a borate precipitate, and in a 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 limited to where the metal borate originates or how it is formed.

20165207 prh 11 -03- 2016

The process is particularly well suited for the recovery of iron, nickel, copper, molybdenum, chromium, cadmium and zinc, but the invention can also be applied to other metals and also to semi-metals such as antimony and arsenic.

The invention is also suitable for separating only one desired metal from a borate precipitate or for separating two or more metals, particularly so that the metals can be separated at the same time.

The invention is suitable, for example, for the extraction of nickel as a metal, starting with oxy-dic ores containing nickel. Oxidic nickel deposits are located below ground level, typically in the following order: red limonite, yellow limonite and weathered saprolite as garnishite and serpentine. There is a transition layer between yellow limonite and saprolite rauma.

In particular, the process described in Finnish Patent No. FI-124419 and International Publication No. WO2014 / 118439 can be used in which the starting material is an oxide-based ore, in particular laterite ore, from which nickel and possibly also cobalt are recovered. The laterite ore used as starting material preferably comprises limonite.

Also in this case, the process is biphasic, in which, in the first step, one or more metals (especially Ni and Co) are recovered as the 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 process, the nickel and optionally the cobalt are separated by a hydrometallurgical method as disclosed in the above publications, which avoids the use of acidic solutions. The ore starting material, for example limonite, is first ground to a fine grit (preferably less than 1 mm grain size), then slurried in water, and the resulting slurry is stirred vigorously for a sufficient time to begin to reach pH

20165207 prh 11 -03-2016 over 7. The aqueous metal solution is separated from the slurry solids and its pH is raised to a range suitable for nickel precipitation in the presence of a boron compound which provides nickel precipitation as borate. Borax (disodium tetraborate) is preferably used, while at the same time it is possible to raise the pH to a suitable precipitation range above 9, for example to precipitate nickel to a preferred pH of about 9.3.

In the second step, the nickel and optionally cobalt are separated as metals by heating the borate precipitate under reducing conditions as described above. The process can also be used to separate nickel and cobalt by separating them based on different melting temperatures according to the above principle.

In the following, the invention will be described in more detail by describing experiments performed on borate15 precipitation.

Example

The metal precipitate was prepared from a sample supplied by a mining company, a batch of concentrate from the slag concentrate before flotation. The precipitate was prepared as described in Finnish Patent Application No. 20135921 and International Publication No. WO2015 / 036658 by 'Precipitation of metals in a sample with boric acid and sodium hydroxide as a method for treating metals'. The sample contained several metal components, e.g. sodium, magnesium, aluminum, silicon, potassium, calcium, iron, copper, zinc, lead, barium, antimony and arsenic. For the experiments, the precipitate was prepared by grinding it to homogeneity.

The literature on metal borates is limited. According to information from some sources, for example, zinc borate crystallizes into an optically usable lens blank at about 1550 ° C, so the starting point is a temperature of 1500 ° C.

A smaller crucible was used in the assay, which was dosed with about 2/3 of a volume of homogenized pellet. My experimental furnace was such that the melting event could not be observed as a function of time, but was observed when

20165207 prh 11 -03-2016 the set end temperature was reached and the test batch was allowed to dwell there for 10 minutes. The oven temperature was increased slowly at 2 ° C / min.

The experiments were performed sequentially with the final heating temperature 5 being lowered by 50 ° C at a time.

It was found that at a final temperature of> = 1350 ° C, the precipitated charge upon thawing begins to flush vigorously, and as a result of the effervescence, "climbs" out of the crucible. It was interpreted as probably due to strong oxidation, which is evident since borate is chemically composed of three oxygen atoms attached to one boron atom. In experiments with a final temperature of 1350 ° C or more, no observations were made besides effervescence except that all the metals had melted and that the slag formed after cooling was insoluble.

In a test at a final temperature of 1300 ° C, the melted precipitate remained in the crucible. Examination of the melt showed that all the metals present were melted. After cooling, the crucible and its molten structures were slit on a diamond blade, cast into an epoxy resin and made into a cross section20, so that the metal particles in the material could be reliably investigated. Samples were examined by scanning electron microscope (SEM) and analyzed by energy dispersive elemental analyzer (EDS). Prior to microscopic examination, the samples were further charcoalized to render the examination surfaces electrically conductive.

In the SEM cross-section, three phases were distinguished: ceramic oxide (AIO), iron-rich granules, and other metals in the form of crystals. The ED study revealed that phase 1, the dark matrix, consisted mainly of aluminum and silicate oxides and that more than one third (34.93%) of the elemental oxygen was present. In Phase 2, several metals were found, also very oxidizing. In phase 3 predominantly iron granules were present, but also to some extent aluminum and silicon.

It was found that the melting points of borate metals are lower than the melting points of the elemental metals. It was also found that by melting alone, the metals do not deposit, which would be a prerequisite for the melt separation, but that the high amount of oxygen present20165207 prh 11 -03-2016 causes oxidation, which results in phases instead of being deposited in a molten disorder.

The experiments were continued with carbon reduction. The purpose of the carbon added to the metal precipitate is to reduce the metal-based components back to metal, e.g.

Fe 2 O 3 + 3C -> 2Fe (meth) + 3CO.

In this experiment, the metal precipitate was treated in two different modes:

1. metal precipitate not heat-treated (reference); and

2. a metal precipitate which was heat treated with carbon at 1300 ° C

The metal precipitate was 2 parts by volume and carbon 1 part by volume.

After the experiment it was found that in the reference sample (appendix 1) the particles were not very firmly attached to each other and therefore no good cross-sectional carbon could be produced from the sample. However, the elements and the presence of some compounds, such as borates, could be identified.

The heat-treated sample (Appendix 2) was found to be sintered and the cross-sectional surface smooth. The slag material consisted of silicate and borate crystal types of aluminum, sodium, magnesium, potassium, calcium and iron. No chromium, nickel, barium, arsenic or antimony found in the sample were found in the reference areas. It can only be explained by their presence as borate25 salt crystals. After the heat treatment, several metals were observed as metals. EDS analysis does not directly determine the crystal structure of the material. The X-ray diffraction (XRD) method can be used to support the crystal structure, but its difficulty is that in the case of a multicomponent, the number of components also affects their identifiability. The XRD method was not tested in this study. There is a tendency in the structure of the phases to arrange in a queuing order, so it is expected that if the sample batch were larger, deposits would occur.

Thus, the main objectives of the study were identified. Separation of metals by melting from the mixed metal borate precipitate is possible by reduction, melting temperatures are lower than those of the elemental metals, and slag from the precipitate residues is insoluble.

It has been found that when the borate precipitate is heated to above 1200 ° C, the remaining solid material, the slag, becomes completely insoluble.

The slag remaining from the separation of metals can therefore be classified as inert waste and is easy to dispose of. Slag is also useful as a building material, for example in infra construction, because it is not only insoluble but also impermeable to water and therefore a very good sealing material.

Claims (8)

Claim: 20165207 prh 29 -05- 2017 1. A method for separating metals from metal borate precipitates originating from solutions from metal recovery, characterized by heating a Solid metal borate precipitate under reducing conditions to a temperature at which the metal differs as a melt from the precipitate, and to separate the melt from the remaining solid material. Process according to claim 1, characterized in that the method 10 separates two or more different metals from the same borate precipitate by separating the metals from one another at temperatures specific to them as the temperature is raised. Method according to claim 1 or 2, characterized by using koi 15 as a reducing agent. Process according to any of the preceding claims, characterized by raising the temperature to 1300 ° C. 20 Process according to any of the preceding claims, characterized in that the final temperature of the heating is lower than 1350 ° C. Process according to any of the preceding claims, characterized in grinding the borate precipitate and mixing it with reducing material before Heating. Process according to any of the preceding claims, characterized in that it comprises - precipitation of one or more metals as a borate precipitate from liquid phase under conditions favorable to precipitation, and - separating said one or more metals from the borate precipitate by heating under reducing conditions. Process according to any of the preceding claims, characterized by depositing slag obtained after the separation of the melt, in the form of waste, or using it as a building material.