GB2280897A

GB2280897A - Extraction of zirconium values from zircon

Info

Publication number: GB2280897A
Application number: GB9316504A
Authority: GB
Inventors: Frank Pitts
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-08-09
Filing date: 1993-08-09
Publication date: 1995-02-15
Anticipated expiration: 2013-08-09
Also published as: GB2280897B; GB9316504D0

Abstract

Zircon is reacted with sodium hydroxide at elevated temperature in the presence of metallic zinc and the cooled reaction product is lixiviated with water, filtered and washed to separate solid hydrated zirconium oxide. Optionally, magnetic separation is applied to remove oxidic iron particles. Optionally, the wash liquid is an aqueous alkaline solution.

Description

SPECIFICATION E)CIRACTION OF ZIRCONIUM VALUES FROM ZIRCON Technical Field This invention relates to a process for the extraction of zirconium values in acidsoluble form from the mineral zircon (zirconium silicate).

Background of the Invention Several methods for processing zircon sand to produce compounds of zirconium are used commercially. The present invention relates to one such method, namely, that in which zircon sand is caused to react at elevated temperature with sodium hydroxide. The reaction is depicted by the following equation: +3HO ZrO2-SiO2 + 6NaOH = Na20*ZrO2 + (Na20)2 + 2 (zircon sand) - Equation 1.

although it is known to vary the proportions somewhat from the stoichiometric values deriving from the equation in the light of the relative costs of zircon and sodium hydroxide. Thus, when the cost of zircon is low it is customary to use a higher than stoichiometric ratio of zircon to sodium hydroxide although this leads to some loss of zirconium values in the form of unreacted zircon. On the other hand, when the cost of zircon is high it is preferred to reduce this ratio below the stoichiometric proportion which ensures substantially complete reaction of the zircon.

The reaction is normally carried out in mild steel or cast steel reaction vessels at a temperature of the order of 650"C to 7500C and the reaction product, when cool, is lixiviated with water which hydrolyses the sodium zirconate to form a hydrated zirconium oxide. The latter is separated by filtration from the alkaline sodium silicate solution and is washed with water. The moist filter cake is hydrated zirconium oxide containing 40 to 45% ZrO2 and also minor amounts of sodium, silica and iron. Such a product is known as a "washed zirconium frit." In the present patent application, this product will be referred to as "washed zirconium frit;" the filtrate from the filtration operation will be referred to as "frit wash liquor" and is a strongly alkaline solution of sodium silicate containing about 12% of total NaOH.The washed zirconium frit contains an amount of iron which gradually builds up to about 1% (as Fe2O3) of the ZrO2content as the reaction vessel is repeatedly used to produce succesive batches of the zircon/sodium hydroxide reaction product ("zirconium frit"). This iron content almost all derives from the steel reaction vessel and is present to a minor extent as particles of iron in oxidic form but it is mostly present in association with the hydrated zirconia, adsorbed thereon or in chemical combination therewith.

For some industrial purposes it is required to have a zirconium-containing product which is soluble in a mineral acid; such is the case in the coating of titanium dioxide pigments with zirconium. Whereas the hydrated zirconium oxide contained in the "washed frit" is readily soluble in sulphuric acid solutions, such as 30% sulphuric acid, and "washed zirconium frit" would, therefore, provide an economically viable source of zirconium for this purpose were it not that the iron content is too high for such purposes. Since no simple method is known for removing the iron, or any significant proportion of it, from washed frit it has been necessary to apply further, and costly, processing steps to provide an acid -soluble zirconium- containing product of sufficiently low iron content.Preferably the iron content should be less than 0.4% (as Fe203) of the acid soluble ZrO2.

It is an objective of this invention to provide a method of making a "washed zirconium frit" of low iron content.

Statement of the Invention According to the present invention there is provided an improved method of making a washed zirconium frit in which zircon is caused to react with sodium hydroxide at elevated temperature in the presence of metallic zinc and the reaction product when cool is submitted to magnetic separation to removed particles of iron in oxidic form and is then lixiviated with water and the solids are separated from the aqueous liquor by filtration and washing to provide a washed zirconium frit in moist solid form and a liquor containing sodium silicate, excess sodium hydroxide and a water-soluble zinc compound. In one embodiment of the invention the solids fraction when filtered is washed with an aqueous strongly alkaline solution and subsequently with water.In another embodiment of the invention the aqueous wash solution is a strongly alkaline solution of sodium silicate.

Specific embodiments of the invention will now be described by way of examples: Examples In each of the following examples the frit was prepared using the same proportions of zircon sand and sodium hydroxide. The sodium hydroxide was used in the form of a 73% weight for weight aqueous solution in an amount 15% in excess of the weight corresponding to the stoichiometric amount indicated in Equation 1.

Various amounts of metallic zinc in granular form were added to the reactants contained in a mild steel crucible. This was placed in a furnace and the temperature gradually raised over about an hour from about ambient temperature to 730"C, held at that temperature for five minutes and the crucible allowed to cool. The friable reaction product (zirconium frit) was removed from the crucible and the magnetic particles were removed with a horseshoe hand magnet. A portion of the zirconium frit was lixiviated with four times its weight of water, filtered and washed either with water or with an aqueous alkaline solution followed by washing with water.The moist solids comprising the filter cake are "washed zirconium frit." All the preparations of zirconium frit were made in the same mild steel crucible which had previously been used to make a large number of batches of zirconium frit without addition of zinc. The samples of washed zirconium frit were tested by dissolution in 30% sulphuric acid and analysed for zirconium (as Zero2), iron (as Fe203), zinc (as Zn), soluble silicon (as SiO2) and acidinsolubles. The figures reported in the examples are as percentages of the acid soluble ZrO2.

Example 1 This example shows the effect of the addition of metallic zinc in the zirconium frit - making process on the iron content of the washed zirconium frit. The experiments were carried out as described above and the figures quoted for the Foe203 content are averages of several similar experiments.

ZINC Added* Average Fe203 content of washed frit** 0 0.62 4 0.55 10 0.43 17 0.36 22 0.35 * As % of ZrO2 in the reactants.

* * As % of acid-soluble ZrO2 in the washed zirconium frit.

This example shows the beneficial effect of carrying out the frit-making reaction in presence of metallic zinc on the iron content of the washed frit.

Example 2 This example shows the effect of washing the zirconium frit with water and with various alkaline solutions followed by washing with water.

ZINC Wash Acid Added* Liquid SiO2** Foe203** Zn** Insolubles** 2 3 10 Water 4.9 0.38 0.3 1.8 10 Alkaline sodium silicate+ 4.8 0.38 0.4 1.3 10 10% NaOH 4.0 0.33 0.3 1.4 17 Water 9.3 0.36 3.7 3.1 17 Alkaline sodium silicate+ 4.5 0.34 3.1 1.0 22 Water 9.7 0.35 5.3 1.3 22 5% NaOH 5.1 0.32 3.0 2.9 22 10% NaOH 5.2 0.34 2.3 2.0 22 Alkaline sodium silicate+ 5.6 0.35 1.4 1.4 * As % of ZrO2 in the reactants.

* * As % of acid-soluble ZrO2 in the washed zirconium frit.

These results show that the wash liquor used has no significant effect on the iron content of the washed frit. With an addition of 10% metallic zinc the residual zinc content in the washed frit after washing with water was low (0.3% of ZrO2) and an alkaline wash had no effect on the zinc or silica levels.

At higher rates of metallic zinc (17% and 22%), however, washing with water led to high levels of zinc and also of silica in the washed frit. In these cases, an alkaline wash proved beneficial in reducing both the zinc and the silica levels to acceptable values. As the alkalinity of the wash solution was increased the residual zinc level was progressively reduced.

All percentage values referenced in the description are percentages by weight.

Claims

1. A process for the extraction of zirconium values from zircon which comprises causing zircon to react with sodium hydroxide at elevated temperature in the presence of metallic zinc and the reaction product, when cool, is lixiviated with water, filtered and washed to separate the solid hydrated zirconium oxide (washed zirconium frit) from the aqueous solution of sodium silicate and sodium hydroxide.

2. A process according to Claim 1 in which the reaction product is submitted to magnetic separation before lixiviation with water to remove particles of iron in oxidic form.

3. A process according to Claim 1 in which the slurry resulting from the lixiviation of the reactions product is submitted to magnetic separation to remove particles of iron in oxidic form.

4. A process according to Claim 1 or Claim 2 or Claim 3 in which the solid hydrated zirconium oxide after filtration is washed with an aqueous alkaline solution and then with water.

5. A process according to Claim 4 in which the aqueous alkaline solution is an aqueous solution of sodium hydroxide.

6. A process according to Claim 4 in wich the aqueous alkaline solution is an aqueous solution of alkaline sodium silicate.

7. A process according to Claim 6 in which the aqueous alkaline solution is the filtrate resulting from the filtration of the slurry formed by aqueous lixiviation of the product of the reaction of zircon with sodium hydroxide at elevated temperature in the absence of metallic zinc.

8. A process for the extraction of zirconium values substantially as described in any one of the foregoing Examples.

9. A hydrate zirconium oxide made by any one of the preceding claims.

10. A method of extracting zirconium oxide from zircon sand comprising: reacting zircon sand with sodium hydroxide to give sodium zirconate; and hydrolysing the reaction product to form a precipitate comprising zirconium oxide; characterized in that the reaction between the zircon sand and the sodium hydroxide takes place in the presence of metallic zinc to inhibit contamination of the zirconium oxide.

11. A method according to claim 10 in which the reaction between the zircon sand and the sodium hydroxide takes place in the presence of iron, the presence of metallic zinc inhibiting contamination of the zirconium oxide with iron.

12. A method according to claim 10 or 11 further comprising subjecting the reaction product to magnetic separation to remove magnetic contaminants before hydrolysis.