Titanium equipment chemical cleaning, by Xu Yazhou, Henan Industrial Vocational and Technical College, Nanyang, Henan 473000. This document includes course notes and a successful case of titanium equipment cleaning. Titanium is an excellent engineering material known for its strong corrosion resistance. In recent years, it has been widely applied in the chemical and light industries due to its outstanding thermal conductivity, high mechanical strength, lightweight, and corrosion resistance. It is commonly used in manufacturing reactors, heat exchangers, and other industrial equipment. In the salt industry, the common process involves evaporating and crystallizing sodium chloride solution into salt. The best heating equipment for this process is a titanium-made heat exchanger. Pingdingshan Zhongyan Xiaolong Co., Ltd. installed a tube-type heat exchanger with a heat exchange area of 1020 m². Each device has a tube height of 6300 mm and contains 1299 titanium tubes with a wall thickness of 1.5 mm, which are expanded between the upper and lower tube sheets. During operation, concentrated brine is evaporated under very high negative pressure through the tubes, while superheated steam flows through the shell to heat the material. These units operate in series, referred to as the first effect and first effect heater based on the flow direction of the material. However, during operation, a problem arises where scales form on the outer layer of the titanium tubes in the shell-heating chamber, mainly composed of magnetite (iron oxide). These scales are hard and brittle, with a length of about 6 mm. Their thermal conductivity is much lower than that of titanium, significantly reducing heat transfer efficiency and affecting production. Since the first effect heater is almost scale-free, the carbonate scale is easier to remove, but the effective scale in the heater is more challenging, making it the main focus of the cleaning process. The iron scale may originate from impurities in the water or iron salts in the salt mine. During the mining process, water is injected into the ground to dissolve the solid salt mine, producing brine that is sent to the salt-making equipment for evaporation and crystallization. Testing revealed a small amount of organic impurities with a density of about 1.43. Physically removing the iron scale is nearly impossible due to the severe vibration of the long 6300 mm titanium tubes under stress. When slightly vibrated, the tubes collide, and the dense arrangement prevents high-pressure water from entering the equipment. Therefore, chemical cleaning is the ideal method, but it requires a suitable cleaning agent. Laboratory testing was conducted by sending scale samples to a professional chemical cleaning unit. After receiving the samples, tests were carried out according to production requirements and equipment characteristics. Although titanium is resistant to corrosion, it is a highly reactive metal, and the case is expensive. The factory has strict requirements for chemical cleaning, especially for key equipment used in salt production. During the test, it was found that strong acid heating can dissolve iron scale, and iron oxide can also be dissolved via redox reactions under acidic conditions. However, strong alkali solutions were later rejected by the factory due to manufacturer restrictions—cleaning must be done under weak acid or base conditions, with pH controlled between 3.5 and 10.5. A chelating agent was used instead. In the lab, citric acid (234 g) was used with ammonia to adjust the pH to 3.5–4.5. To improve the cleaning effect, a small amount of 901 or similar additive was added, and the titanium specimen was heated for 8 hours. The scale was completely dissolved without any change in the titanium’s quality or hardness. After confirming the plan's safety, a final solution with pH 4 and appropriate corrosion inhibitors was prepared. Once the cleaning plan was approved by the chief engineer office and faxed to Baoji Titanium Factory, the technical department confirmed that the plan would not damage the titanium equipment and was safe for implementation. Due to the large shell-side volume of the heater, controlling steam flow was difficult. Direct steam introduction caused tube vibrations, potentially damaging them. To prevent this, a large amount of steam was introduced into the cleaning system, while a small amount was added to the shell side to control the flow rate and prevent tube collisions. Heated solution was forced to circulate, and the entire cleaning process was conducted in a circulating state, with solids removed promptly. Throughout the project, the factory’s quality inspection department monitored the process. The central laboratory tested acid concentration, pH, iron ion levels, corrosion inhibitor concentration, temperature, cleaning time, temperature rise, heating duration, circulation flow, rinsing flow, and rinse temperature. All data were recorded and provided to the factory. A 1.5 m³ corrosion-resistant tank was used, approved by the factory’s security department, with tap water sourced from a nearby fire hydrant. The cleaning agent was prepared and injected into the container, with heating starting at 150°C and reaching 90°C in one and a half hours. After seven hours of cleaning, two ammonia supplements were added. Finally, the system was rinsed with hot water, and the inspection hole was opened. The descaling rate exceeded 98%, with no iron scale remaining on the titanium tubes, revealing their original condition. The results matched the lab findings, and both parties were satisfied. Construction precautions included ensuring all pipe valves and connections were in good condition. Ammonium citrate has strong rust-dissolving properties, so pipes with rust must be carefully inspected, especially welds, to avoid issues during the process. In conclusion, chemical cleaning of titanium equipment is a technically demanding task with high-value equipment. Careful planning, strict adherence to the cleaning process, proper organization, and quality control are essential. Problems arising during production should be taken seriously. The cleaning process is determined based on test results, and some indicators may need adjustment depending on the chemical reaction.

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