WO2014118436A1

WO2014118436A1 - Method for recovery of metals from oxidic ores

Info

Publication number: WO2014118436A1
Application number: PCT/FI2014/050073
Authority: WO
Inventors: Vesa Rissanen; Vesa KAINULAINEN
Original assignee: Global Ecoprocess Services Oy
Priority date: 2013-01-29
Filing date: 2014-01-29
Publication date: 2014-08-07
Also published as: FI20130030A; FI124419B

Abstract

The invention relates to a method for recovering metals from oxidic ores by hydrometallurgical methods. In the method, the oxidic ore is contacted with water under conditions in which the metal is released from the ore in water, and the metal is recovered from the solution thus obtained by precipitating with a boron compound.

Description

METHOD FOR RECOVERY OF METALS FROM OXIDIC ORES

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

In many processes, liquids are produced, in which metals are present in dissolved form. Such processes include various types of processes for the treatment of metals in solid state or solids that contain metals. They often result in liquid substances 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 otherwise come into contact with liquids.

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 by 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 the dissolved metal is precipitated from the solution by using a suitable chemical which contains an anion that 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 effectiveness is dependent on other conditions, such as other substances contained in the 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 precipitating 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- 2011/233139, in which 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 of the kind which are difficult to separate or for which there is no further use.

Conventionally, oxidic ores have been processed by pyrometallurgical or hydrometallurgical methods. The pyrometallurgical methods involve the disadvantage of high energy consumption, and they are normally more profitable when the ore has a high metal content.

In hydrometallurgical methods, the metal is extracted from the ore by means of an acidic solution, normally a sulphuric acid solution, after which the metals present in dissolved form are recovered by precipitation. For example, for the processing of laterite ores that contain nickel and cobalt, the HPAL (high pressure acid leaching) process has been developed, in which the metals are extracted with a sulphuric acid solution under high pressure (about 35 to 55 bar) and at a high temperature (about 245 to 270°C). The method involves high energy and equipment costs. Alternative methods have also been developed, such as the so-called atmospheric leach process, which is described in patent EP 1228257 (and corresponding US patents 6680035 and 6261527), and the so-called resin-in-pulp method, which is described in US patent 6350420. Both methods apply sulphuric acid for extracting nickel and cobalt from ore.

The processing of sulphuric acid causes problems which are typical of acidic waters: environmental risks, the need of neutralization of residual liquors, and the risk of corrosion of pipe systems and reservoirs, which increases equipment costs.

It is an aim of the invention to present a hydrometallurgical method for recovering metals from oxidic ores, whereby the treatment of acidic solutions can be avoided. 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 1.

In the method, the oxidic ore is contacted with water under conditions in which the metal is released from the ore in water, and from the solution thus obtained, the metal is recovered by precipitation with a boron compound. The boron compound can be a suitable hydroxo compound of boron or a compound that contains boron in the form of an oxoanion. In particular, the boron compound may be boric acid or borax. The method is applicable for recovering divalent metals from basic oxidic ores which are of such quality that when the ore is slurried in water, the pH of the slurry will be already alkaline, and no addition of a base (such as sodium hydroxide) will be needed for precipitating the metal with the boron compound. The method is useful for recovering particularly nickel and possibly also cobalt from laterite ores. About three fourths of the nickel deposits in the earth's crust are present in oxidic ores (laterites), and the rest in sulphide ores. A laterite ore which is particularly useful for the method is limonite. Advantageously, borax (disodium tetraborate) is used, which makes it possible to bring the pH to a range suitable for precipitation.

In a way typical of hydrometallurgical methods, the method is also particularly suitable for the treatment of low-grade metal ores and, for example, laterite ores with a low nickel content, as well as laterite ores with a high content of other rock, can be utilized in the method.

In the following, the invention will be presented in more detail by describing different steps of the method.

In the recovery of metals from oxidic ores according to the method, a boron compound is utilized.

The method is suitable for the separation of metals particularly from laterites. A metal of particular interest in view of the method is nickel, because three fourths of the nickel deposits in the crust consist of ultra alkaline oxidic later- ites. Three types of such ore deposits are known: limonite, silicate and transition ores. The transition ores are intermediate forms between the two preceding types. An object of particular interest is limonite.

Limonite contains 0.8 to 1.5% nickel, and it has a high iron content, typically higher than 40%. As a result, the formula of limonite is typically presented in the form FeO(OH) nH₂0. Limonite has a very uniform mineralogical structure and is therefore suitable for hydrometallurgical processing. Originally, limo- nite was defined as a single mineral, but at present, it is considered a mixture of hydrated iron oxides. The primary mineral of limonite is goethite, in which nickel is dispersed in iron.

Consequently, the above described separation method can be applied for separating laterite nickel by slurrying it in water, whereby a ready alkaline solution will be produced. Iron will also precipitate in hydroxide form, but it will not interfere with the separation if the precipitation is stirred first. Thus, a requirement is strong agitation of the laterite slurry, in order to dissolve the nickel from the solid material in the solution. Depending on the deposit, the raw material may also contain minerals other than limonite which remain as solids in the separation. In a laboratory test with chemicals, the behaviour of the Ni ion was measured in an initial concentration of 500 ppm when the iron precipitated, which occurred quickly. Thus, the Ni content fell to an almost zero level. When this precipitate was later stirred with a magnet stirrer, the Ni content increased to its initial level of 500 mg/liter again. Precipitating iron will readily absorb other ions, such as heavy metals, onto its surface.

In practice, the invention is implemented by making an aqueous slurry of laterite first. For example in limonite [(Fe, Ni) O (OH)], the pH of the aqueous slurry is approximately neutral, wherein the nickel adsorbed to the iron hydroxide is released in soluble form in water. The slurry is strongly agitated. The supernatant (clear solution) that contains nickel is led to be precipitated to nickel borate in the above described way. All the other precipitated material of the slurry, including the unchanged mineral material (such as silicate minerals) from the ore, can be easily kept separate, for example at the bot- tom of a precipitation tank or basin. If necessary, any method of Iiquid/solids separation can be used, for example one based on weight. Thus, the invention is also applicable for such laterite ores which contain other rock in addition to limonite. When a boron compound is added to the separated liquid, the nickel can be bound as a borate. What is important is to keep the pH value alkaline in the best precipitation range of nickel, advantageously higher than 9. This can be done by applying borax (disodium tetraborate) as the precipitating boron compound, borax being as such capable of buffering the separated liquid (supernatant) to a pH value exceeding 9; or by increasing the pH to a value exceeding 9 separately by adding an alkaline compound (for example, NaOH) and precipitating the nickel with a boron compound, in which case also boric acid can be applied. In the latter case, a sufficient amount of the boron compound is first added to the liquid, and the pH of the liquid is then increased by adding the alkaline compound.

If the aqueous suspension that contains laterite remains alkaline, which is usually the case with these ultra basic laterites, the process can be described qualitatively by a reaction formula, in which boric acid is used as an example of the boron compound:

NiO(OH) + 2H₃B0₃ → N1B2O3 + 4H₂0

On the other hand, if the aqueous slurry that contains laterite becomes acidic, for example because of sulphur in the ore, the process can be described by a reaction formula, in which borax is used as an example of the boron compound:

NiO(SO₄) + Na₂(B₄O₇) 10H₂O→ NiO(B₄O₇) 10H₂O + Na₂SO₄

As above, the borate formed in this way is also a tough ligand which can be easily filtered from the water.

In some cases, depending on the quality of the limonite, it may be necessary to adjust the pH of the alkaline slurry to a slightly acidic range in order to cause release of NiO from goethite more easily. Thus, when the pH increases, the first precipitating nickel hydroxides act as precipitation nuclei for further precipitation in a way known in hydrometallurgy. Precipitation nuclei can be added, if needed. When borax is added to the liquid, the pH can be increased to a range favourable for the hydroxide precipitation of nickel, and the nickel can be simultaneously bound in the way presented in the last reaction formula. Consequently, in the invention, the nickel can be brought in the liquid to a form that can be precipitated by a boron compound, by strongly agitating the aqueous slurry of the ore and by separating the solids-free and nickel-containing water (supernatant) as soon as possible after the slurrying. The sepa- ration of nickel from iron, i.e. the elimination of the iron hydroxide precipitate from the supernatant, can be improved by some efficient method of separation of the supernatant, by which the light iron hydroxide entrained in the supernatant can be removed before the treatment with the boron compound. The method can be implemented in a continuous manner so that fine-grained laterite ore is continuously slurried in water under agitation, and supernatant is continuously removed from the agitated suspension and is possibly subjected to further treatment for removing iron hydroxide mechanically, after which the supernatant is led to treatment with a boron compound. From the borate precipitate obtained, nickel can be separated by pyrometal- lurgical methods.

The invention will be illustrated with the following example which should not be interpreted to limit the invention.

Example

Laterite from Australia was used, having the following technical specification: % moisture 18.9

Al (%) 1.164

Cr (%) 0.289

Fe (%) 7.99

Mg (%) 2.08

Mn (%) 0.122

Ni (%) 1.09

Si (%) 35.1

Description:

"Low-iron silica-rich oxidised Nickel laterite containing goethite, silicified goethite, quartz and clay silicate minerals with minor hematite and chromite" In the direction from the ground surface deeper below, oxidic nickel deposits are typically found in the following order: in the form of red limonite, in the form of yellow limonite, and in the form of garnierite and serpertine in weathered saprolite. A transition layer is found between the yellow limonite and the weathered saprolite layer. The location of the laterite used was in the border area between the yellow limonite and the transition layer, and the analysis correlated well with that presumed for this deposit.

The batch obtained was very non-homogeneous in grain size; therefore, the minerals were ground to uniformly fine grains. The size of the grains corresponded to fine sand, 0.6 to 1.2 mm. The largest grains consisted of silicate. The grain size of the actual material to be slurried can be easily made smaller than 1 mm, typically 0.6 to 0.7 mm, by screening off the larger silicate-based grains or by using rock that is purer in terms of laterite.

For a test, a 500 g batch of the grains was separated and slurried in 1000 ml of water. After this, the suspension was introduced in a stirrer which was switched on, and efficient stirring was continued for 1 hour. The stirring time for the test arrangement was determined to find a time in which the iron oxide in the laterite could be sufficiently disintegrated for removing the nickel. Some of the iron was present in the form of oxide and some in the form of Fe³⁺ hydroxide, because the pH of the suspension was 7.84. According to literature, nickel starts to precipitate at the pH of 7.8, and the precipitation is most intensive at the pH of 9.3. Thus, most of the adsorbed nickel was still in the form of Ni²⁺ ions.

After the agitation, the supernatant was immediately separated. The pH of the supernatant had now increased to the value of 7.98, so that the precipitation of nickel had started. This was the first evidence that stirring really dissolves nickel in the aqueous phase. The nickel content of the supernatant was 0.25% about 15 min after the separation.

The supernatant was allowed to settle overnight. The measurements were taken again about 12 h later, when the pH was 7.82 and the Ni content was 0.10%. It was suspected that light iron precipitate of hydroxide type, mixed in the supernatant, would start to re-adsorb nickel in time; to find out this, the supernatant was strongly agitated again. After the agitation, a Ni content of

0.25% was measured again; in other words, the earlier result was restored. Now, when the pH of the supernatant was adjusted with borax to 9.3, a precipitate was obtained that contained nickel and iron and which, being a poorly soluble precipitate, no longer had any tendency of re-adsorption.

In this particular case, where the valuable metal is nickel whose isoelectric point of primary precipitation is at pH 9.3, it is sensible to use sodium borate,

1. e. borax, as the precipitating chemical, because its aqueous solution also has a pH of 9.3.

Compared with other methods for separating laterite nickel on the basis on acid dissolution, the present method has the advantage of not only considerable cost savings but also the capacity of substantially more efficient recovery and the capacity of utilizing ore bodies of clearly lower grade. Particularly in comparison with dissolution with sulphuric acid, an advantage may also be the fact that even if the ore body had a high silicate content, no metallurgical disadvantage is caused even if the iron were precipitated in the form of jaro- site, because the sulphate content is in any case negligible.

The invention is not restricted to the examples presented above, but it can be modified within the scope of the inventive idea presented in the claims. What is essential is that the metal can be bound to borate and it will not re-dissolve, for example because of pH changes, and it is possible to use any suitable hydroxo compound of boron or a compound that contains boron in an oxoanion. The invention can also be used for precipitating cobalt together with nickel, or separately. The invention can also be applied in uses not mentioned above.

Claims

Claims:

1. A method for recovering metals for oxidic ores by hydrometallurgical methods, characterized in that the oxidic ore is contacted with water under conditions in which the metal is released from the ore in water, and the metal is recovered from the solution thus obtained by precipitating with a boron compound.

2. The method according to claim 1 , characterized in that the oxidic ore is made to a slurry in water, the liquid is separated from the solids of the slurry, and the precipitate obtained is separated from the liquid.

3. The method according to claim 1 or 2, characterized in that the oxidic ore is basic ore, particularly laterite ore, for recovering nickel and possibly cobalt.

4. The method according to claim 3, characterized in that the laterite ore comprises limonite.

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

6. The method according to claim 5, characterized in that borax is used for the precipitation, and is simultaneously used to increase the pH of the solution to the precipitation point of nickel.

7. The method according to any of the preceding claims, characterized in that the mixture of ore and water is strongly agitated.

8. The method according to claim 7, characterized in that water is separated from the slurry simultaneously when the mixture of ore and water is agitated.