Wastewater treatment in sulphate-route TiO2 production

By Albert Li
Published: Friday, 29 September 2017

Large volumes of wastewater represent both a cost and environmental burden for China’s titanium dioxide industry, but new membrane filtration technology could help with water conservation and yield a suite of valuable by-products, Albert Li, IM Correspondent, finds.

Yidu Zhang, deputy general manager of Ningbo Xinfu Titanium Dioxide Co., is something of an expert in the treatment of wastewater from titanium dioxide (TiO2) processing.

Based in Ningbo, Zhejiang province, in eastern China, the company is a high-tech business specialising in the production of high-purity TiO2, with capacity of around 100,000 tpa.

Speaking at the China TiO2 Industry Green Manufacturing and Application Forum in Longkou, Shandong province, earlier this year, Yidu discussed the challenges facing companies that produce TiO2 using sulphuric acid – known as sulphate-route processing.

This process involves digesting a TiO2 feedstock (usually rutile or ilmenite) in strong sulphuric acid which converts the titanium components into titanyl sulphate and the iron into sulphates. This is then clarified to give pure TiO2.

Sulphate-route TiO2 production consumes large quantities of water. As technology and equipment have developed, water consumption has been reduced considerably, but still amounts to between 50 and 80 cubic metres of water per tonne of TiO2 produced.

As a result, most TiO2 facilities typically have large volumes of wastewater to treat.

As sulphuric acid functions only as a carrier in TiO2 processing, once it has digested the feedstock and removed the impurities, it becomes an impurity itself and needs to be removed. 

Yidu explained that using current technologies around 15-20% of the sulphuric acid can be recycled via acid blending or reusing it as an inducing liquid. 

The problem with reusing this material is that it tends to reduce processing system operating rates, consumes more energy and creates a ferrous monohydrate by-product that is even more difficult to dispose of than the original wastewater. 

Neutralising this highly corrosive effluent is a heavy cost burden on China’s TiO2 industry, especially since tighter environmental regulations have made it imperative for companies to responsibly dispose of their waste. 

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Producing High purity TiO2 requires the addition of corrosive chemicals 

which then need to be removed from the resulting wastewater. Madaise, via Flickr

Membrane technology

A new membrane treatment technique for TiO2 wastewater has been in development for the last few years in China. 

It involves mixing the wastewater with uncontaminated neutral water and then passing it through a micro filter, which separates out the TiO2 particles and recycles the filtered water back to the production plant. 

Membrane filtration makes it easier to purify the resulting acid wastewater and there is also potential to enrich the separated impurities into valuable by-products.

Ningbo Xinfu is the first Chinese TiO2 producer to adopt this technique and the company has plans to construct a facility capable of handling 1.2m cubic metres of wastewater per year. The project is due to be completed by mid-2019. 

This membrane technology could also be potentially applied to sewage treatment, where materials used to neutralise effluent include carbide slag, lime and limestone powder.

One of the main benefits of new membrane filtration technology in this area is that it can remove the carbide slag impurities from the effluent, meaning that the residue will mostly consist of limestone and lime, which forms gypsum of the right quality to be used by the cement industry.

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Ningbo Xinfu Titanium Dioxide Co. ships its TiO2 in 25kg bags 
Alibaba.com 

Cleaning up China’s TiO2 industry

According to Ningbo Xinfu’s Yidu, China’s TiO2 industry is making substantial progress in reducing its pollution, but still has a long way to go to change perceptions of the industry.

At present, there are few economic incentives for Chinese companies to improve water conservation, but Yidu 

believes that if it becomes more lucrative to sell recycled by-products from TiO2 processing, then cleaner wastewater will be a positive side effect.

He hopes that new membrane technology will help make it easier for companies like his to develop their by-products for secondary material markets. 

Yidu also believes that there is potential to recycle heat given off by rotary kilns during the TiO2 production process, converting it into steam which can be used to power the processing plants in place of coal, which will help factories meet tougher emissions standards.

TiO2 waste acid treatment methods in China

The following treatment, disposal and recycling methods are currently the most common ways of handling sulphuric acid wastewater from TiO2 processing in China:

  • Based on the industrial characteristics of the processing location, such as sulphur-phosphor-titanium, sulphur-ammonia-titanium, or sulphur-iron-titanium, chains are used to neutralise the acid
  • Sold directly to paper mills, printing and dyeing mills, and small chemical factories
  • Used to make a water-purifying agent, polyferric sulphate (PFS)
  • Sent to industrial wastewater treatment facilities to be neutralised and for gypsum to be extracted for use in cement factories, or be sent to landfill
  • After condensing the waste acid, the residue can be reused in TiO2 processing.