(220i) A New Sulfuric Acid Recycle Routine in TiO2 Pigment Production

A new sulfuric acid recycle routine in TiO₂ pigment production

Liang Bin, Li Chun

Sichuan Uinversity, Chengdu 610065, P. R. China

In China, more than 99% of the titanium resources exist in the form of ilmenite located in Panzhihua area, Sichuan province. Nowadays about 60% of the world's TiO₂ pigments are manufactured by the chlorination route, in which natural or synthetic rutiles, or titanium-rich slags are used as feeding stocks. The shortage of natural rutiles has encouraged the researches to convert ilmenite into synthetic rutile. Therefore, China produces about 1.2 million tons of TiO₂ pigment per year in sulphate process, in which ilmenite is firstly leached with concentrated sulphuric acid and then the Ti-contained solution is purified and hydrolyzed. The TiO₂ is obtained by calcining the hydrolysate.

The sulphate process brings with about 8 tons of `20 wt.% spent sulphuric acid for each ton of TiO₂ production (Liang et al., 2005; Li et al., 2007d ), which results in severe environmental problems. In fact, the process can be simply described as: concentrated acid ® dissolution of ilmenite ® purification ® hydrolysis ® dilute acid. Recycling dilute acid can solved the emmision problem.

In a commercial process, the ilmenite was leached with a concentrated sulfuric acid of 98% at about 200°C. Our leaching kinetic experiments were carried out with an initial acid/ilmenite (wt) ratio of 500:15, a sulfuric acid concentration of 15.4M and temperatures from 100° to 198°C. Based on the leaching kinetics model obtained, simulation results showed that the optimal initial acid concentration was 85%. It indicated that the 20% waste acid can be usd as the digestion acid by mixing the 98% concentrate acid. The practical operation reduced about 15% emission of waste acid.

In view of recycling the waste acid, we need to further decrease the digestion acid concentration. Using a low acid concentration, however, the digestion rate is too low to practice. Milling ilmenite can activate the ore and therefore it can enhance the digestion of ilmenite in dilute acid. The dissolution of mechanically activated Panzhihua ilmenite at 100°C with a 50% sulfuric acid solution was investigated. With the mechanically activated ilmenite inside a ball mill, leaching conversion at 100°C reached 82.1%. The leaching activation energy decreased from 48.1KJ/mol to 28.4kJ/mol. (Li et al., 2008; Li and Liang, 2008) Furthermore, we investigated the simultaneously milling digestion and we found that a high leaching conversion of ~80% can be reached under an ore/acid ratio closed to the commercial operation. The initial acid concentration of 50% can reduce more than 60% of waste acid emission.

The 20% H₂SO₄ was used to directly dissolve the ilmenite, it can completely avoid the emission of waste acid. We dissolved the mechanically activated Panzhihua ilmenite in 5~20% sulfuric acid solutions and found that both titanium and iron almost dissolved at a stoichiometric ratio. However, the final dissolving degree of Ti was very low due to the hydrolysis of the dissolved titanium during the digestion process. The digestion results bring us a chance to leaching ilmenite with dilute acid and extract the iron species, while the titanium hydrolyzed to form a solid product. The residue was detected with XRD and the result showed that the product was rutile TiO₂. It means that the digestion of ilmenite with extremely dilute acid can not produce the titanium solution suitable for the hydrolysis, but it brings with a new method to produce synthetic rutile which can be used as the feedstock of the chlorination TiO₂ pigment production.

Waste sulfuric acid is a severe environmental challenge of sulphate routine TiO₂ pigment production. The solving method is reducing the initial digestion acid concentration and recycle the hydrolysis spend acid. The mechanically activation of ilmenite greatly enhances the leaching process and makes it possible to leach the ilmenite with dilute acid. It will be a sustainable process of TiO₂ pigment production.

Reference

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