GB2578655A

GB2578655A - Metals recovery

Info

Publication number: GB2578655A
Application number: GB1902462.9A
Authority: GB
Inventors: Grimley Andrew; Makanyire Terence
Original assignee: Grimley Smith Associates Env Ltd
Current assignee: Grimley Smith Associates Env Ltd
Priority date: 2018-11-01
Filing date: 2019-02-22
Publication date: 2020-05-20
Anticipated expiration: 2039-02-22
Also published as: GB201817904D0; GB201902462D0; GB2578655B

Abstract

A process for the recovery of scandium and other rare earth metals from titanium dioxide production residues comprises treating the residue with an aqueous solution of sodium carbonate and at least one other anion such as chloride or sulphate to leach scandium and other metals into solution, and then selectively precipitating the scandium and other metals from the solution. Vanadium and niobium, if present, can also be recovered.

Description

METALS RECOVERY

Field of the Invention

This invention relates to the recovery of metals such as vanadium, scandium and other rare earth elements, as well as niobium, from residues from the production of pigment grade titanium dioxide.

Background to the Invention

The production of pigment grade TiO2 is from titanium-rich minerals such as ilmenite or rutile, extracting by using chlorine (the chloride process) or sulphuric acid (the sulphate process). The residues from these processes, either to as a neutralised filter cake or as an untreated acidic residue known as sluice, contain vanadium, niobium, scandium and other rare earth elements. These metals, when separated and purified, have a very high industrial value. They are used extensively in rapidly growing industries such as the manufacture of mobile phone components, catalysts, HSLA steel, and in energy conversion and storage devices.

There are currently no established technologies for recovery of any metal values from the TiO2 process wastes, the key deterrent being the complexity of a metal recovery process, which typically requires a non-selective acid leach followed by selective precipitation stages and further purification. The presence of significant amounts of iron and aluminium in the cake makes selective precip-itation processes particularly difficult as both are well known for being difficult to filter. US 5407650 discloses a process for purifying an acidic technical-grade iron chloride solution formed from cyclone dust from the production of TiO2 by the chloride process. Undesired metal ions, including niobium and vanadium, are separated out by selective adjustment of pH to form a metal hydroxide pre-cipitate while avoiding formation of iron hydroxide. The hydroxides are filtered off and are then considered suitable for landfill The present invention seeks to improve the extraction of these elements, thereby to make recovery more economically viable.

It has been found (Makanyire, Terence (2016) Reclamation of metal val- ues from TiO2 production waste residues. PhD thesis, University of Leeds, abstract at http://etheses.whiterose.ac.uk/17108/) that adding NaNO3 as an oxi- -2 -dant to alkali leaching systems significantly improved the extraction of scandium and vanadium from a selectively precipitated concentrate. However, when applied to neutralised filter cake (ready for landfill and that already landfilled), this approach can still leave over 50% of the scandium present in the solid residue u n recovered.

It has now surprisingly been found that the efficacy of extraction of scandium and other rare earth elements, as well as zirconium, can be substantially improved by treating the residues with an aqueous solution of sodium carbonate and readily available anions and that the extraction mechanism is not depend-ent on oxidation.

Summary of the Invention

According to the invention, a process for the recovery of metals from titanium dioxide production residues, comprising treating the residue with an aqueous solution of sodium carbonate and at least one other anion to leach scandi- um and other metals into solution, and then selectively precipitating the scandi-um and other metals from the solution.

The additional anion is preferably a sulphate ion, a chloride ion, or a nitrate, chlorate, nitrite, sulphite or chlorite ion, or combinations of these, most suitably in the form of sodium salts in the solution. Calcium chloride, calcium carbonate have been found not to be suitable sources of anions, and hydroxide ions are also ineffective.

The leaching step is preferably carried out at an elevated temperature, preferably at least 60°C and more suitably 90°C. The aqueous solution preferably contains 50 to 250 g/dm3 of sodium carbonate. The concentration of source of additional anions, for example the sodium salt, is suitably 10 to 200 g/dm3.

The residue is preferably washed in water and more preferably then dried before treatment with the aqueous solution. Drying may not be necessary if adequate stirring is provided to break down small clumps of cake that are pre- sent even after leaching when wet cake is used. Drying followed by grinding al-lows the filter cake to be powdered before leaching. Washing is for removal of residual calcium chloride. -3 -

While the process of the invention is especially effective in the extraction of scandium from the residues, analysis of the filtrates from selective scandium extraction also shows that lanthanides, actinides, zirconium and any pentavalent vanadium, when present, are co-extracted. The leaching system can there-fore be used for the recovery of lanthanides and actinides from mineral wastes, as well as removal of zirconium from niobium precipitates. While the addition of an oxidant (for example hydrogen peroxide) in the Na2CO3 leaching stage was found not to be significant in the extraction of scandium, it does enable co-extraction of pentavalent vanadium with the scandium.

Calcium carbonate, sodium bicarbonate and sodium hydroxide have also been tested in place of sodium carbonate, but little or no scandium extraction occurred.

It has also been found that scandium and rare earth metals can be precipitated from the leach liquor by treatment with hydroxide ions at a temperature of at least 30°C, and preferably at least 70°C. For example, the hydroxide ions may be provided by adding 1-5% NaOH to the heated liquor.

Vanadium can be leached from the residue by the use of sodium hydroxide, and the liquor resulting from scandium extraction is ideal for re-leaching the vanadium-containing residue, as vanadium will dissolve quantitatively due to the added NaOH. Treatment of the titanium dioxide production residues with sodi-um hydroxide before the scandium extraction process dramatically reduces the yield of scandium, so the scandium extraction is carried out before the vanadium recovery step.

Brief Description of the Figures

Figure 1 is a flowchart illustrating the steps in the process of recovering metals from neutralised cake and landfill cake; and Figure 2 is a flowchart illustrating the steps in the process described hereinafter in Examples 3 to 5.

Detailed Description of the Invention

The existing processes for extraction of TiO2 result in acidic residues, and the waste metal chlorides removed from chlorination of the ore, for example, are usually made less hazardous by neutralising with lime and filtered to -4 -form a stable metal hydroxide filter cake and calcium chloride filtrate (with magnesium and manganese chlorides, if present). Residual CaCl2 after filtration is detrimental to extraction of vanadium by alkali leaching because a significant amount of vanadium forms the insoluble calcium vanadate. The easiest and perhaps most economical way of removing residual calcium from the filter cake is by water washing -CaCl2 is very soluble in water. After washing in water, the solids fraction is separated from the calcium-rich liquor by filtration. This is the first stage illustrated in the process flowchart of Figure 1.

Although generally not soluble in alkali, scandium can be leached from the filter cake by a Na2CO3 -anion system in accordance with the invention. In Figure 1, the anions are provided by NaCI, by way of example. The leaching process is controlled by surface reaction, so temperature is important. A sharp decrease in extraction efficiency can be observed down to about 60°C, below which no extraction occurs. Raising the leaching temperature above 90°C does not have any significant impact on extraction efficiency.

A scandium-rich filtrate is then separated from the leach mix by filtration and scandium is purified from the leach liquor by selective precipitation. This is achieved by heating the liquor to a temperature of at least 30°C, and preferably in excess of 70°C, and adding 1-5% NaOH while stirring. Any leached vanadi-um or aluminium remain in solution, while Sc and REE precipitate. Separation of the scandium-rich filtrate leaves a metal hydroxide residue from which vanadium can then be extracted.

The invention is further illustrated by the following examples:

Example 1:

1. Weigh 40g as-received filter cake 2. Wash cake in 100 mL water to remove residual CaCl2 3. Filter the slurry and add the washed cake to 100 mL of 2 M Na2CO3 4. Add lOg NaCI and cover 5. Heat for 120 minutes at 90°C 6. Filter and analyse filtrates for Sc. -5 -

7. Heat the filtrates to 90°C.

8. Add 2g NaOH and leave for 30 Minutes.

9. Filter to recover the scandium-rich precipitate.

10. leach the vanadium-containing residue using filtrate from the scandium precipitation; 11. Precipitate ammonium vanadate for further purification.

Example 2:

1. Measure 300 ml of as-received acid sluice 2. Add 50 ml of 37 % HCI and cover. Heat at 70°C for 60 minutes and filter 3. Neutralise the metal chlorides solution by slowly adding the filtrates to a reactor containing Ca(OH)2 suspension 4. Wash to remove any residual calcium chloride 5. Filter the neutralised solution and add the filter cake to 200 ml of 2M Na2CO3 6. Add 20g NaCI and cover 7. Heat for 120 minutes at 90°C 8. Filter and analyse filtrates for Sc' 8. Heat the filtrates to 90°C.

9. Add 2g NaOH and leave for 30 Minutes.

10. Filter to recover the scandium-rich precipitate.

11. leach the vanadium-containing residue using filtrate from the scandium precipitation; 12. Precipitate ammonium vanadate for further purification.

Example 3:

Fresh filter cake was leached in 4M HCI to simulate acidified slurry from the chloride process' gas cyclone, but with additional calcium ions from residual CaCl2 present in filter cake. Unreacted ore, coke and silica are insoluble in HCI, and therefore were filtered off before selective precipitation. -6 -

The pH of the acidic chloride filtrate was adjusted to 0.50 using powdered CaCOs, no precipitation occurred. A suspension of CaCOs in water was then prepared and the pH 0.50 solution was added to it to achieve a final pH of 4.01, which upon filtration left a green filtrate and a dark residue. Analysis of the pre-precipitation solution and post-precipitation filtrate shows that approximately 40% of the iron content precipitated together with all metals of interest, namely V, Sc, Zr and Nb. The pH of the filtrate was 4.85, meaning that it continually increased over time as more CaCO3 dissolved. The higher pH may be the reason why so much iron co-precipitated, iron (II) hydroxide can precipitate from pH 4 to under similar conditions. Some of the iron in the as-received filter cake had converted to Fe(III) over time and would have precipitated from pH 2 or lower, this is not expected to happen if the condensate residue is slurried in HCI and processed with minimal exposure to air. The process disclosed in US5407650 is applicable here.

Once the precipitated metals were separated from the iron-rich solution by filtration, sodium chlorate and goethite seed were added to the filtrate, which was then heated for 60 minutes at 70°C. The formed precipitate was vacuum filtered and washed for analysis. In both precipitation steps, no gelatinous precipitates formed and filtration was very fast.

The first precipitate is then washed to remove residual calcium chloride before leaching using a Na2CO3 -NaCI system for scandium recovery, followed by NaOH for vanadium recovery, under similar conditions to Examples 1 and 2.

Figure 2 illustrates the processing steps required for simulating the process using filter cake.

Example 4:

Concentrated HCI was added to as-received sluice to improve filterability of the sluice and amount of vanadium and niobium dissolved (50 ml in 300 ml sluice). Unreacted TiO2, SiO2 and coke were separated from the sluice by filtra-tion to produce a solution from which selective precipitation could be carried out, as shown in Figure 2. -7 -

The solution was treated in the same way as with filtrate in Example 3, the only difference being that the targeted final pH was 2.85 instead of 4.01 used for precipitation from the filtrate solution. A greenish precipitate formed and was separated from green Fe (II) solution by filtration. The amount of co-precipitated iron is about 20 % based on ICP OES analysis.

Example 5:

A second test was done with the sluice, similar to Example 4 described above, the only difference being that the targeted pH was 1.80 instead of 2.85.

The main purpose of the second test was to check the pH sensitivity of the precipitation process. Semi-quantitative analysis shows comparable iron precipitation to that of Example 4, but precipitation of all other metals was incomplete with significant losses observed for scandium.

Figure 2 also illustrates the processing steps taken for recovering V, Sc, Zr and Nb from acidic sluice. Niobium and Zirconium are selectively precipitat-ed from the acidic liquor remaining after filtration of insoluble material (unreacted ore, silica and coke). Sulphate ions are introduced to the liquor as H2SO4 or Na2SO4 and pH raised using alkali to between 1 and 1.5 (sulphate ions have been found to catalyse precipitation of the two metals). The filtrate is treated 20 with CaCO3 as in the preceding examples to precipitate the metals (V, Sc etc), leaving a filtrate containing Fe(II) ions which is treated for goethite precipitation. In each of Examples 4 and 5, as in Example 3, the precipitates are leached in a Na2CO3 -NaCI system for scandium recovery and then NaOH for vanadium recovery. Vanadium is precipitated from the final alkali leach with an NH3 salt to form insoluble ammonium vanadate. It will be appreciated that other methods of precipitating the vanadium, such as introducing calcium ions to precipitate calcium vanadate from the sodium vanadate solution, may alternatively be employed at this stage.

Figures 3 and 4 illustrate the alternative process in which the alkali leach-ing step is carried out including an oxidant to co-extract pentavalent vanadium with the scandium and other metals (i.e. the rare earth metals). In Figure 3, the -8 -oxidant is sodium chlorate, serving also as the additional anion in place of the sodium chloride used in the processes illustrated in Figures 1 and 2, while in Figure 4, the oxidant, for example hydrogen peroxide, is additional to the sodium chloride which provides the additional anion. This obviates the need for a separate alkali leaching stage of the filter cake after the alkali leaching stage for recovery of vanadium, as in Figures 1 and 2. The filter cake is passed direct to the acid leach stage for recovery of TiOaINb/Zr. -9 -

Claims

CLAIMS1. A process for the recovery of metals from titanium dioxide produc-tion residues, comprising treating the residue with an aqueous solution of sodium carbonate and at least one other anion to leach scandium and other metals into solution, and then selectively precipitating the scandium and other metals from the solution.
2. A process according to Claim 1, wherein the other metals are rare earth metals.
3. A process according to Claim 1 or 2, wherein the other anion is one or more of the following ions: sulphate, chloride, nitrate, chlorate, nitrite, sulphite and chlorite.
4. A process according to Claim 3, wherein the anion is provided in the form of a sodium salt in the solution.
5. A process according to any preceding claim, comprising treating the residue at a temperature of from 60°C to 90°C.
6. A process according to any preceding claim, wherein the aqueous solution comprises 50 to 250 g/dm3 of sodium carbonate.
7. A process according to Claim 4, wherein the concentration of the sodium salt in the aqueous solution is 10 to 200g/dm3.
8. A process according to any preceding claim, wherein the residue is washed in water before treatment with the aqueous solution.
9. A process according to any preceding claim, wherein scandium and rare earth metals are precipitated from the solution by heating the solution to at least 30°C and adding hydroxide ions.
10. A process according to Claim 9, wherein the solution is heated to at least 70°C.
11. A process according to Claim 9 or 10, wherein the hydroxide ions are provided by 1-5 w/v% NaOH.
12. A process according to Claim 9, 10 or 1 1, comprising filtering the solution to recover the precipitated scandium and rare earth metals, leaving an alkaline filtrate.
13. A process according to Claim 12, comprising subsequently further treating the residue with the alkaline filtrate to leach vanadium into solution and then precipitating the vanadium from the resulting solution.
14. A process according to any of Claims 1 to 8, wherein an oxidant is included in the aqueous solution in order to co-extract pentavalent vanadium with the scandium and other metals.
15. A process according to Claim 14, wherein the oxidant is hydrogen peroxide.