Application of titanium in the chemical industry-chlor-alkali

Application of titanium in the chemical industry-chlor-alkali

The chlor-alkali industry is a chemical industry that prepares chlorine gas and caustic soda by electrolysis of an aqueous salt solution. It has a history of more than a hundred years. The chlor-alkali industry is also the earliest industry in the chemical industry to use titanium.The main titanium equipment used in chlor-alkali production are: metal anode electrolytic cell, ion membrane electrolytic cell, tubular wet chlorine cooler, refined brine preheater, dechlorination tower, chlor-alkali cooling scrubber, vacuum dechlorination pump and valve titanium equipment.

(1) Metal anode

The chlor-alkali production process has mercury electrolysis, diaphragm electrolysis and ion membrane electrolysis.In the past, graphite anodes have been used for chlor-alkali anodes.In 1956, the Dutchman Henry.Bill (H. Beer) First proposed that the electrolytic cell uses a metal anode, also known as dimensionally stable anode DSA (Dimensionally Stable Anode), and obtained a patent in 1965. The dimensionally stable anode is an electrode coated with platinum group precious metal oxides on a titanium substrate.In 1968, DeNore, an Italian company, first industrialized titanium anodes in the chlor-alkali industry. Around 1970, the United States, Italy, Japan, Germany, France and other countries quickly switched to metal anodes instead of graphite anodes.In Japan, thousands of tons of titanium have been used as the base material for metal anodes. To produce 10,000 tons of caustic soda, about 5 titanium materials are required.

With the development of China's chlor-alkali industry, the main equipment (electrolytic cell) for caustic soda production has undergone three major changes.The first change was that vertical tanks replaced horizontal tanks. In the early 1960s, (vertical adsorption diaphragm electrolytic cells) were used instead of traditional horizontal tanks, which greatly increased China's caustic soda production, from 193,000 tons in 1957 to 693,000 tons in 1966, an increase of 3.6 times.

The second change is that metal anode electrolytic cells replace graphite anode electrolytic cells. In the 1970s, metal anode (DSA) was used instead of graphite anode. Since 1972, our country has conducted titanium anode tests at Shanghai Tianyuan Chemical Plant and Tianjin Chemical Plant. In 1973, a 20m metal anode diaphragm electrolytic cell test was started, and a 30m metal anode electrolytic cell was gradually used since 1974.In 1978, the state carried out the task of upgrading the metal anode technology of 400,000 tons of diaphragm caustic soda.As of 1981, there were 17 chlor-alkali plants across the country that used a total of 1,217 metal anode electrolysis cells, forming a diaphragm metal anode with an annual output capacity of 670,000 tons of caustic soda, accounting for 30% of the country's caustic soda production capacity, and 95,000 tons of mercury electrolysis capacity using DSA.As of 1996, there were 99 chlor-alkali plants in the country with a total of 8,409 metal anode diaphragm electrolytic cells, with an annual output of 4.2 million tons of caustic soda, accounting for 70% of the country's caustic soda production capacity.Except for a few large chemical plants such as Tianyuan, Tianhua, and Daguhua, most of the metal anode electrolytic cells are manufactured and supplied by professional factories such as Beijing Chemical Machinery Factory and Shanghai 4805 Factory.

The third change is the use of ion membrane electrolytic cells. In the mid-1980s, the use of energy-saving and high-efficiency ion membrane method was promoted to produce caustic soda. Our country introduced ion membrane caustic soda technology and equipment from Japan and other countries, forming a series of 10,000 to 50,000 tons of equipment. The main equipment is ion membrane electrolytic cell, titanium anode liquid circulation tank, fresh brine tank, vacuum dechlorination tower, heat exchanger, tube and pump valve, etc., Titanium equipment and titanium tube are mainly used in anode liquid circulation system, fresh brine system, dechlorination system, wet chlorine gas delivery system and chlorine water circulation system.Titanium pumps are mainly used to transport refined brine, anode circulating liquid, freshwater brine and chlorine water.The amount of titanium used in a 10,000-ton device is about 8t.In June 1986, Yannanxia Chemical Plant introduced Japanese Asahi Glass technology for the first time, with an annual output of 10,000 tons of caustic soda plant. In addition to the three-dimensional electrolytic cell and anode liquid titanium pump supplied by Japan, the remaining six titanium-made equipment are all domestic supporting and supplied by Jinxi Chemical Machinery Factory.By 1990, 11 chlor-alkali plants had adopted ion membrane caustic soda plants with a production capacity of 295,000 tons. In 1995, a total of 27 chlor-alkali plants across the country adopted ion membrane caustic soda plants with a production capacity of 827,000 tons.In 2000, the annual output capacity of China's chlor-alkali industry was 7.5 million tons of caustic soda, 14.71 million tons in 2005, and 23.99 million tons in 2010.

In the ion membrane electrolytic cell, the temperature of the cathode and anode rooms is about 90℃, the anode chamber has chlorine gas and salt solution, and the cathode chamber has 30%~35% concentration of caustic soda solution. The general operating current density of the ion membrane electrolytic cell is 30~40A/dm.Under such harsh conditions, the material use and anticorrosive structure of the electrolytic cell must be fully considered when designing the electrolytic cell.The anode part of the ion film electrolytic cell (refers to the anode and the part in contact with the anode liquid), countries around the world without exception have chosen titanium metal (or corrosion-resistant titanium alloy) with good corrosion resistance in the anode liquid.

The schematic diagram of the ion membrane caustic soda ion exchange membrane, as shown in the figure, the two electrodes are isolated with an ion exchange membrane. Salt water is added from one side and pure water is added from the other.After passing through the current, chlorine gas is generated from the anode side and hydrogen gas is generated from the cathode side.The ionic membrane only allows sodium ions to pass through, so sodium hydroxide is produced from the cathode side.

In addition to the electrolytic cell of the main equipment of the ion membrane caustic soda device, the main parts used in the titanium equipment are: brine system-liquid level meter; anode liquid system-anode liquid tank and chlorine scrubber; fresh brine system-dechlorination tower, fresh brine distributor, instrument cooler; sodium hypochlorite system-cooling, absorption tower, distributor; chlorine gas system-wet chlorine gas cooler; hazard removal system-heat exchanger, hazard removal fan.

(2) Wet chlorine cooler

While electrolysis of table salt produces caustic soda, a large amount of hot wet chlorine gas is generated, which can be used after cooling and drying.There are two ways to cool hot and humid chlorine gas: direct water spraying and indirect cooling by tubular coolers.Direct cooling will not only produce a large amount of chlorine-containing chlorine water, which will seriously pollute the environment, but also a large amount of chlorine gas is lost, sulfuric acid is consumed, and the working conditions in the workshop are poor.The material of indirect coolers has been graphite coolers, glass tube coolers, ceramic coolers, plastic coolers, etc., but there are many problems such as poor corrosion resistance, easy to break, and easy to age. Stainless steel indirect coolers can only be used for 8 to 10 days and need to be stopped for repair.The test results show that titanium is extremely corrosion-resistant in an environment of high temperature and wet chlorine gas, with an annual corrosion rate of 0.0025mm.The use of titanium coolers in the production of chlor-alkali industry can shorten the cooling and drying process, reduce chlorine gas loss, reduce environmental pollution, and create conditions for the stable operation of compressed gases and achieve a high degree of drying.

In 1963, Russia began to use titanium chlorine gas coolers, with a heat transfer area of 140m. Titanium pipes for conveying wet chlorine gas were also used, with a diameter of 300~600mm and a length greater than 500m.Almost all wet chlorine gas coolers used in the chlor-alkali industry in Russia are made of titanium.Allied Chemical Company in the United States uses titanium instead of graphite to make coolers in the chlor-alkali industry. The original graphite tube was used for 2 to 3 years, and a 78m titanium cooler completed the cooling capacity, while the graphite cooler required 140m.

China's first titanium cooler was manufactured in 1965 by Jinxi Chemical Machinery Factory. The heat transfer area is small, only 16.8m. Since 1973, chlor-alkali plants in Shanghai, Tianjin, Beijing, Liaoning, Guangdong and other provinces and cities have successively used titanium tubular coolers with good results.There are hundreds of titanium tubular coolers in our country.

(3) Pumps and valves

In the production of chlorine gas by membrane electrolysis and mercury electrolysis, the titanium pumps used in potassium hypochlorite and sodium hypochlorite are the most economical.The Georgia-Peefick company in the United States uses a titanium pump to pump a salt solution at 85℃. The salt solution contains 270~320g/L NaCl, NaCl crystals and more than 0.5g/L of free chlorine. The service life of the titanium pump is up to 10 years.

The second Beijing Chemical Plant uses a cast titanium 6BA-12 pump, Dg100Dg globe valve and HTB-701l water ring ceramic vacuum pump titanium impeller in the new vacuum dechlorination process.These titanium pumps and impellers have a long service life.