Jiuwu Hi-Tech BOT Project - Recovery of Waste Acid from Titanium Dioxide Production in Shanghai

Recently, the Shanghai titanium dioxide acidic wastewater resource utilization project, invested and constructed by Jiuwu Hi-Tech under the BOT (Build-Operate-Transfer) model, has successfully entered the critical stage of membrane system commissioning and the debugging of neutralization and black mud washing processes. As a benchmark project for acidic wastewater treatment and resource utilization in the titanium dioxide industry, this project relies on Jiuwu Hi-Tech's self-developed ultrafiltration-nanofiltration integrated membrane technology to successfully "turn waste into treasure" from acidic wastewater generated by titanium dioxide enterprises. It has helped the client save over ten million yuan in costs, providing a replicable and scalable innovative solution for the industry's green transformation, and supporting the titanium dioxide industry in breaking through environmental bottlenecks and achieving high-quality development.

In China, over 90% of titanium dioxide is produced using the sulfuric acid method, which inevitably generates large quantities of acidic wastewater (containing 2%–5% sulfuric acid). This acidic wastewater mainly comes from the washing of metatitanic acid, acid hydrolysis, cooling and washing wastewater from calcination exhaust gases, as well as acid-containing wastewater from equipment cleaning and operating areas. Currently, lime (or calcium carbide slag) is typically added to neutralize the acidic wastewater, producing acidic industrial sludge primarily composed of dihydrate gypsum, known as titanium gypsum (also called red gypsum or yellow gypsum). Based on a byproduct ratio of 7 tons of titanium gypsum (dry basis) per ton of titanium dioxide produced, the annual discharge of titanium gypsum has reached over 15 million tons (dry basis).

Due to the high impurity content in titanium gypsum, its strength after calcination into hemihydrate gypsum is very low. As a result, its application in gypsum-based building materials is very limited. Moreover, even when used as a cement retarder after treatment, the addition amount must be kept low, as excessive addition can reduce the 3d and 28d strength of cement. Consequently, the vast majority of titanium gypsum is currently disposed of in slag stockpiles or dumped directly into the environment across various regions. The cumulative stockpile over the years has reached hundreds of millions of tons, not only occupying large amounts of valuable land but also consuming enormous sums for the construction and maintenance of storage facilities.

Taking a titanium dioxide enterprise with an annual output of 50,000 tons as an example, the annual production of titanium gypsum is 350,000 tons. If the local landfill fee is 85 yuan per ton, the annual gypsum landfill cost would reach 29.75 million yuan, imposing a huge economic burden on the enterprise.

To achieve efficient recovery and recycling of acidic wastewater, Jiuwu Hi-Tech customized a fully integrated membrane process for the project, comprising "ultrafiltration – nanofiltration – neutralization – black mud washing." The designed treatment capacity is 2,500 m³/day × 340 days. Relying on three core process sections, a closed-loop resource circulation system is formed, enabling cascading recovery and efficient utilization of acidic wastewater resources. The comprehensive benefits of the process are significant, with annual economic benefits exceeding 38 million yuan.

The project employs a coupled process of multi-stage membrane separation and crystallization regulation to achieve full-component resource utilization of titanium dioxide acidic wastewater. First, specialized ultrafiltration membranes are used for primary purification, capturing and recovering suspended metatitanic acid (recovery rate ≥95%). Then, specialized nanofiltration membranes are used for secondary separation, achieving efficient separation of sulfuric acid and metal salts (recovery rates of ferrous sulfate and titanyl sulfate ≥95%). An innovative directional crystallization technology for titanium gypsum was developed to produce titanium gypsum with a particle size ≥40 μm. Combined with a high-efficiency dewatering process, the attached water content is reduced to ≤12%. Finally, a low-temperature slow calcination technology (≤180°C) is applied to convert the titanium gypsum into grade 3.0 construction gypsum that meets the GB/T 9776-2022 standard.

Taking a titanium dioxide enterprise with an annual output of 50,000 tons as an example, approximately 125 tons of metatitanic acid are recovered annually in the ultrafiltration membrane section. Based on a price of 6,000 yuan per ton of crude titanium dioxide, the annual revenue is 750,000 yuan. In the nanofiltration membrane section, about 20,000 tons of ferrous sulfate heptahydrate are recovered annually. At a price of 300 yuan per ton, the annual revenue is approximately 6 million yuan. In the neutralization section, due to the removal of metal ions, the consumption of neutralization agents is reduced, saving about 10,000 tons of neutralization agents annually. Based on a price of 150 yuan per ton of calcium carbide slag, the annual saving is approximately 1.5 million yuan. Additionally, because the high-purity, large-particle gypsum has low moisture content, the produced gypsum can be used as a cement additive or construction gypsum, eliminating 29.75 million yuan in annual landfill fees. Combining the benefits from these three process sections, the total annual economic benefit of the project reaches 38 million yuan.

In terms of economic benefits, affected by the current surge in industrial sulfuric acid prices — with 98% sulfuric acid reaching as high as 2,000 yuan per ton in some regions — Jiuwu Hi-Tech has transformed the fully integrated membrane process of "ultrafiltration – nanofiltration – neutralization – black mud washing" into an "ultrafiltration – nanofiltration – acid recovery" process. The ultrafiltration and nanofiltration stages remain unchanged, while the neutralization and black mud washing stages are replaced by acid recovery from the nanofiltration permeate.

If the operating energy consumption required to concentrate 5% dilute sulfuric acid to 50% is converted to a cost of 639 yuan per ton of sulfuric acid (on a 100% basis), based on a daily output of 4,000 cubic meters of 5% dilute acid and a local sulfuric acid price of 1,000 yuan per ton, the annual economic value would be 238.26 million yuan.

Considering that landfill fees and industrial sulfuric acid prices vary by region, enterprises can select the appropriate process based on their specific conditions to maximize economic benefits.

In terms of economic benefits, multiple resource recovery measures provide strong support for cost reduction and efficiency improvement: the ultrafiltration membrane recovers metatitanic acid from acidic wastewater, while the nanofiltration membrane recovers ferrous sulfate and titanyl sulfate. This enables an annual recovery of approximately 200 tons of titanium dioxide and 15,000 tons of ferrous sulfate heptahydrate, effectively increasing the yield of titanium dioxide and ferrous sulfate, and generating nearly 10 million yuan in added annual output value. At the same time, the use of calcium carbide slag is significantly reduced, saving nearly 5 million yuan per year, delivering remarkable economic benefits.

In terms of technical demonstration, Jiuwu Hi-Tech adopted the BOT model, providing a new cooperation paradigm for acidic wastewater treatment in the titanium dioxide industry and lowering the threshold for corporate environmental protection investment. At the same time, the successful application of the integrated membrane process has fully validated the feasibility and superiority of ultrafiltration-nanofiltration technology in the treatment of titanium dioxide acidic wastewater. The closed-loop model established — "acidic wastewater → resource extraction → system reuse" — offers a clear pathway for the industry's transition toward a circular economy.

The titanium dioxide waste acid resource utilization project in Shanghai marks a milestone application of Jiuwu Hi-Tech's membrane technology in the field of waste acid treatment within the titanium dioxide industry, representing another breakthrough for China's specialty membrane technology in the resource utilization of high-difficulty industrial wastewater. In the future, Jiuwu Hi-Tech will continue to focus on the R&D of acid-resistant, alkali-resistant, and anti-fouling specialty membrane materials, continuously optimize integrated membrane processes, and promote the titanium dioxide industry toward "waste minimization, maximum resource utilization, and minimal environmental impact." At the same time, driven by technological innovation, the company will continue to deepen its presence in chemical, new energy, and other fields, continuously expand the application scenarios of membrane technology, and inject strong "membrane power" into China's green and low-carbon industrial transformation and high-quality development.