Field of the Invention

[0001] This invention relates to the recovery of metals such as scandium and certain other rare earth elements from residues generated during production of T1O2 using the sulphate process.

Background to the Invention

[0002] The production of T1O2 using the sulphate process results in huge volumes of sulphate waste containing up to 25 v/v% free acid and 45 grams per litre ferrous iron among other metal ions. The waste is typically neutralised us ing slaked lime or limestone before landfilling or disposal by other means. With in the sulphate waste are valuable and/or strategic metals such as vanadium, scandium, zirconium and niobium, which could be recovered for establishing a circular economy, while improving the sustainability and environmental creden tials of the T1O2 industry. As one of the most valuable of the present metals, scandium has been the focus of most research, and due to its low concentration (up to 20 ppm), established processes utilise solvent extraction or ion exchange for its recovery. Solvent extraction and ion exchange processes are very ex pensive to operate for such low concentrations and huge volumes of waste, therefore a concentration enhancement step, or alternative technologies are re quired for making a scandium recovery process greener and sustainable. Scan dium is normally soluble below pH 4. It has now been found that scandium and other rare earth metals can be selectively co-precipitated with calcium sulphate by addition of calcium ions to a rare earth element-containing sulphate solution, thereby permitting precipitation below pH 4 and even where the pH is below 0. This is important where sulphate waste can be generated at a pH below 0, typi cally about pH -0.5 for a material having >23% free H2SO4.

[0003] US2253590 discloses the use of calcium sulphate at temperatures of

90°C and above to remove “objectionable rare earth impurities” which cause discoloration from residual acid liquor in titanium dioxide pigment manufacture. RU2716693C1 discloses processing titanium dioxide production waste and mentions adding milk of lime to neutralise and produce gypsum for building ma terials. It explains that a disadvantage of this is that some rare-earth metals such as scandium are “irretrievably lost”. To overcome this, it proposes the use of an extractant containing di(2-ethylhexyl) phosphoric acid and tributyl phos phate. No information is disclosed on the efficacy of this. CN102275969B dis closes a method of processing titanium dioxide production residue by adding calcium chloride, thereby forming calcium sulphate. The document is silent con cerning rare earth metals.

Summary of the Invention

[0004] According to the invention, a process for recovery from acidic sul phate waste from titanium dioxide production of at least one rare earth metal from the group consisting of scandium, cerium, lanthanum, neodymium, prase odymium and yttrium, comprises treating the sulphate waste with at least one calcium compound selected from calcium chloride, calcium hydroxide, calcium carbonate, calcium chlorate, calcium nitrate and calcium hypochlorite at a tem perature below 30°C thereby selectively co-precipitating calcium sulphate and rare earth metals from solution, filtering the precipitate from the solution, and extracting the rare earth metals from the filtered precipitate.

[0005] Other rare earth elements - dysprosium, erbium, europium, gadolini um, holmium, lutetium, samarium, terbium and thulium - remain in solution for recovery through pH adjustment at later processing stages.

[0006] The precipitation step is preferably carried out at low temperatures, suitably 20°C. The sulphate waste solution preferably contains at least 10 v/v% free acid. The precipitated calcium sulphate/rare earth element residue is fil tered and washed in water, preferably cold water. For effective extraction of scandium, an intermediate leaching step using sodium carbonate may be car ried out first. The solid calcium sulphate is converted by the sodium carbonate solution into solid calcium carbonate. The scandium remains at this stage in the solids and sodium sulphate solution is filtered off. Dilute sulphuric acid is then preferably used for re-dissolving the rare earth elements from the calcium car- bonate, leaving a calcium sulphate residue. The acid level is controlled so that the scandium remains in solution at this stage. Conventional methods for pre cipitation and/or purification can then be employed for obtaining a pure scandi um product. Other rare earth elements present in the residue can be re dissolved and selectively precipitated using conventional methods and rea gents.

Brief Description of the Figures

[0007] Figure 1 shows the fraction of scandium precipitated when different amounts of calcium or barium ions are introduced to a scandium containing sul phate solution at 20°C and stirred for 30 minutes; and

[0008] Figure 2 is a flow chart illustrating the steps in the process described hereinafter in the Examples.

Detailed Description of the Illustrated Embodiment

[0009] Production of TiC>2 by the sulphate process results in toxic acidic resi dues, which are made less hazardous by neutralisation with limestone or slaked lime before disposal at landfill sites. If not recovered, scandium and other rare earth elements (“REE”) follow the neutralised ions to form a filter cake for land fill.

[0010] Although generally soluble below pH 4, scandium can co-precipitate with calcium sulphate when a source of calcium is added to waste sulphate liq uor below pH 0. This is carried out at room temperature, with up to 5 w/v% cal cium ions. A scandium-rich precipitate is then filtered and leached, first in a so dium carbonate solution and then in dilute sulphuric acid to form a scandium- rich solution, leaving behind a calcium sulphate residue.

[0011] The invention is illustrated by the following Examples:

Example 1

[0012] A 100 ml sample of residual acid liquor from the production of T1O2 by the sulphate process (hereinafter referred to as “TiC>2 sulphate waste”) is mixed with 13 g CaCl2 in a beaker. The mixture is stirred for 30 minutes and filtered. Over 97 % of the scandium in initial sulphate solution co-precipitates with calci- um sulphate. The filtered precipitate is then washed using water and leached in hot dilute sulphuric acid at 70°C to achieve an overall recovery of over 95 %.

Example 2

[0013] A 100 ml sample of T1O2 sulphate waste is mixed with 18.5 g CaCC>3 in a beaker. The mixture is then stirred for 30 minutes and filtered. Over 55 % of the scandium in initial sulphate solution co-precipitates with calcium sulphate. The precipitate is then washed with cold water and leached in hot dilute sul phuric acid to achieve an overall recovery of over 52 %.

Example 3 [0014] A 100 ml sample of T1O2 sulphate waste is mixed with 15 g Ca(OH)2 in a beaker. The mixture is then stirred for 30 minutes and filtered. Over 80 % of the scandium in initial sulphate solution will have co-precipitated with calcium sulphate. The precipitate is then washed with water and leached in dilute sul phuric acid at 70°C to achieve an overall recovery of over 75 %. Example 4

[0015] A 100 ml sample of T1O2 sulphate waste is mixed with 15 g CaS04 in a beaker. The mixture is then stirred for 30 minutes and filtered. Over 15 % of the scandium in initial sulphate solution will have adsorbed to the calcium sul phate. The precipitate is then washed with cold water and leached in hot dilute sulphuric acid to achieve an overall recovery of about 10 %.

Example 5

[0016] To a litre of T1O2 sulphate waste at room temperature, 60g of calcium chloride powder were added for scandium and rare earth metals recovery. The mixture was stirred for a total of 60 minutes at room temperature before filtering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the con centrations of Scandium (Sc); Cerium (Ce); Lanthanum (La); Neodymium (Nd); praseodymium (Pr) and Yttrium (Y) dropped by between 65% and 99%, while concentration of other rare earths (Dy; Er; Eu; Gd; Ho; Lu; Sm; Tb and Tm) re mains unchanged. Example 6

[0017] To a litre of T1O2 sulphate waste at room temperature, 38g of calcium hydroxide powder were added for scandium and rare earth metals recovery. The mixture was stirred for a total of 60 minutes at room temperature before fil tering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the amount of Scandium (Sc); Cerium (Ce); Lanthanum (La); Neodymium (Nd); praseodymium (Pr) and Yttrium (Y) dropped by between 55% and 96%, while concentration of other rare earths (Dy; Er; Eu; Gd; Ho; Lu; Sm; Tb and Tm) re duce by a maximum of 2%.

Example 7

[0018] To a litre of T1O2 sulphate waste at room temperature, 50g of calcium carbonate powder were added for scandium and rare earth metals recovery. The mixture was stirred for a total of 60 minutes at room temperature before fil tering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the amount of Scandium (Sc); Cerium (Ce); Lanthanum (La); Neodymium (Nd); praseodymium (Pr) and Yttrium (Y) dropped by between 40% and 80%, while concentration of other rare earths (Dy; Er; Eu; Gd; Ho; Lu; Sm; Tb and Tm) re duce by a maximum of 10%.

Example 8

[0019] To a litre of T1O2 sulphate waste at room temperature, 1800g of calci um chloride powder were added for scandium and rare earth metals recovery. The mixture was stirred for a total of 60 minutes at room temperature before fil tering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the concentrations of Scandium (Sc); Cerium (Ce); Lanthanum (La); Neodymi um (Nd); praseodymium (Pr) and Yttrium (Y) dropped by 100%, while concen tration of other rare earths (Dy; Er; Eu; Gd; Ho; Lu; Sm; Tb and Tm) remains unchanged.

Example 9

[0020] Repeating Example 5 at 50°C, 70°C and 90°C resulted in reduction of amount of scandium precipitated, with a maximum of 32% precipitated at 50°C, 28% at 70°C and 12% at 90°C. The other rare earth metals follow a different trend wherein the fraction precipitated increased with temperature to a maxi mum of 100% at 90°C. Using calcium hydroxide (similar to Example 6) and cal cium carbonate (similar to Example 7) instead of calcium chloride at 50°C, 70°C and 90°C yielded similar trends to Example 8.

[0021] The following Examples are included for comparison and are not with in the scope of the invention:

Comparative Example 1

[0022] A 100 ml sample of T1O2 sulphate waste is mixed with 15 g BaCl2 in a beaker. The mixture is then stirred for 30 minutes and filtered. No scandium co precipitates with barium sulphate.

Comparative Example 2

[0023] To a litre of T1O2 sulphate waste at 90°C containing up to 30 mg/L scandium as an oxide and up to 50mg/L rare earth metals calculated as mixed oxides, 0.45g of calcium sulphate hemihydrate (CaSC>4.1/2H20) were added for scandium and rare earth metals recovery. The residual acid liquor originates from a hydrolysis reaction used for selectively precipitating Ti02 pigment after digestion of T1O2 from typical feedstock. The free acid concentration was ap proximately 25%. The mixture was stirred for a total of 60 minutes at tempera ture before filtering and analysing the filtrate by ICP OES.

[0024] Analysis of the filtrate showed that the concentration of scandium and all rare earth metals remained unchanged, indicating that the process is not ef fective for recovering scandium or other rare earth metals.

Comparative Example 3

[0025] To a litre of T1O2 sulphate waste at 90°C, 45g of calcium sulphate hemihydrate were added for scandium and rare earth metals recovery. The mix ture was stirred for a total of 60 minutes at temperature before filtering and ana lysing the filtrate by ICP OES. Analysis of the filtrate show that the concentra tions of Scandium dropped by up to 10%, while concentration of other rare earth elements drops by up to 100%. Comparative Example 4

[0026] To a litre of T1O2 sulphate waste at 90°C, 200g of calcium sulphate hemihydrate were added for scandium and rare earth metals recovery. The mix ture was stirred for a total of 60 minutes at temperature before filtering and ana- lysing the filtrate by ICP OES. Analysis of the filtrate show that the concentra tions of Scandium dropped by up to 30 %, while concentration of other rare earth elements dropped by up to 100%.

Comparative Example 5

[0027] To a litre of T1O2 sulphate waste at 90°C, 50mg of insoluble calcium sulphate anhydrite and 400mg of calcium sulphate dihydrate were added over 30 minutes. The system was stirred for a total of 60 minutes at temperature be fore filtering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the concentration of scandium and all rare earth metals remained un changed, indicating that the process is not effective for recovering scandium or other rare earth metals at the concentrations investigated.

Comparative Example 6

[0028] To a litre of T1O2 sulphate waste at 90°C, 5g of insoluble calcium sul phate anhydrite and 40g of calcium sulphate dihydrate were added over 30 minutes. The system was stirred for a total of 60 minutes at temperature before filtering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the concentrations of Scandium dropped by up to 17 %, while concentration of other rare earth elements drops by up to 100%.

Comparative Example 7

[0029] To a litre of T1O2 sulphate waste at 90°C, 20g of insoluble calcium sulphate anhydrite and 180g of calcium sulphate dihydrate were added over 30 minutes. The system was stirred for a total of 60 minutes at temperature before filtering and analysing the filtrate by ICP OES. Analysis of the filtrate show that the concentrations of Scandium dropped by up to 33 %, while concentration of other rare earth elements drops by up to 100%. [0030] Comparative Examples 3 to 7 show that higher temperature treatment with calcium sulphate can be used to fully precipitate REE such as Cerium, Lan thanum, Neodymium, Praseodymium and Yttrium, but Scandium is only partially precipitated, with a significant proportion of that present in the waste remaining in solution. By contrast, with the process of the invention, Scandium is fully pre cipitated along with Ce, La, Nd, Pr and Y, with other REE remaining in solution.