China is a major producer of titanium dioxide, with an annual production of about 3 million tons of titanium dioxide, of which sulfuric acid method titanium dioxide accounts for 95% of the total output. In the production process of sulfuric acid method, in order to wash ferrous ions, a variety of waste acid and waste water will be produced, mainly including first wash waste acid (25% sulfuric acid), second wash water (5% sulfuric acid), and third wash waste water.
A large amount of titanium gypsum industrial sludge is produced after the first wash waste acid and the second wash water are neutralized by quicklime. The main component of the sludge is CaSO4﹒2H2O, which is usually 70~80% in grade, acidic or neutral, because it contains a lot of iron ,which called red gypsum or yellow gypsum. Due to the high iron content, the strength of calcined into hemihydrate gypsum is low, or even non-strength, which cannot be used in the construction field, resulting in low comprehensive utilization rate of titanium gypsum.
At present, the vast majority of titanium dioxide enterprises in China adopt the method of storage or landfill for disposal. The accumulated amount of landfill over the years is nearly 100 million tons. In addition to occupying a large amount of land resources, it also needs to pay a high environmental construction cost of the storage yard.If the treatment is not good, it will cause water, air and soil pollution. There is a large amount of titanium dioxide in the three washing wastewater. Direct discharge not only causes economic loss of titanium dioxide, but also wastes a lot of water resources.
Aiming at the bottleneck that cannot be implemented with the existing technology,JIUWU HI-TECH relies on mature and complete membrane integration process technology and responds to the national environmental pollution "source control" policy and takes inorganic membrane as the main membrane process to treat waste acid and wastewater, mainly including recovery of metatitanic acid or titanium dioxide, reuse of reclaimed water and waste water, purification of waste acid as gypsum, etc. It not only enables the reuse of titanium dioxide resources and water resources, but also solves the problem of red gypsum stacking.
3. Process description
3.1 Second washing water of sulfuric acid process (low acid wastewater)
The second washing water (low acid wastewater, containing 5% acid and 5000ppm ferrous) is used to recover metatitanic acid by inorganic membrane. After heat exchange, ferrous ion is removed by acid resistant nanofiltration membrane. The permeate of nanofiltration is neutralized with lime to obtain dihydrate gypsum.
α or β building gypsum is produced by JIUWU HI-TECH owned special crystal transformation process, and the reclaimed water after neutralization can be reused more than 60% through membrane integration device.
3.2 Three washing water of sulfuric acid process (post treatment water of chlorination process)
The third washing water is the wastewater after coating (titanium dioxide: 200-500ppm, conductivity: 6000-10000 μ s / cm). The inorganic membrane is used to recover titanium dioxide. After heat exchange, the produced water is concentrated by reverse osmosis membrane, and the recovery rate is more than 90%. The concentrated solution is treated by nanofiltration membrane to separate sodium sulfate from sodium chloride. Sodium chloride can be used as raw material of chlor alkali, and sodium sulfate can be used as neutralizing water of calcium chloride (in the process of chlorination) .A large amount of calcium chloride is produced after neutralization of chlorinated slag water with gypsum.
4. Process advantage
1. The turbidity of the produced water is stable below 0.5 NTU, the conductivity is less than 10 μ s / cm, and the water quality is stable.
2. Compared with the traditional process, a titanium dioxide enterprise with a budget of 100,000 tons can recover more than 1 million dollars of titanium dioxide every year by using the membrane process and save more than three hundred thousand dollars in water resources.
3.The construction investment and project transformation are in line with the existing process conditions. Traditional industrial disposal methods, such as landfill, are used as cement additives.
4.The service life of inorganic membrane is more than 5 years, which is not affected by acid, alkali, high temperature and high pressure.
5.The membrane process has the advantages of simple operation, short technical process and few control points, which can realize DCS automatic control and unattended.
Fig 1. On-site photo of ceramic membrane recycling titanium dioxide project
Fig2.On-site photo of ceramic membrane recycling titanium dioxide project