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
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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
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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.