What are the safety requirements for aluminum electrolysis operations?

Due to the particularity of aluminum production, there are harmful factors and dangerous factors in the production process. Harmful factors mainly include dust hazard, poison hazard, high temperature hazard and noise hazard, and dangerous factors mainly include mechanical injury, falling from a height, electrical injury and fire and explosion hazard.

I. Analysis of Main Harmful Factors

1. Dust hazard

The dust produced in the production process of aluminum plant mainly includes alumina dust, petroleum coke dust and asphalt dust. Alumina dust mainly exists in electrolysis workshop and alumina storage and transportation system; The feeding system, discharging system, kneader, preheating screw machine and grinding system of calcination section produce dust and asphalt smoke; The molding section also produces asphalt smoke; Dust is produced in the process of coarse crushing, batching and screening in the residual electrode treatment section. Crane drivers, electrolytic workshop workers, workers in positions such as powder crushing and screening are greatly endangered by dust. According to TJ36-79 Hygienic Standard for the Design of Industrial Enterprises, the maximum allowable concentration of harmful substances in the air of the workshop is that the alumina dust in the productive dust shall not exceed 6 mg/m3; Other dust (when the content of free silica is lower than 10%) shall not exceed 10mg/m3.

2. Poison hazards

The main toxicants exposed to workers are fluoride, sulfide, asphalt smoke and carbon monoxide. Poisons mainly exist near electrolytic cells and flue gas purification systems.

Aluminum electrolysis takes cryolite-alumina-aluminum fluoride melt as electrolyte and carbon material as electrode. In the process of electrolysis, liquid aluminum precipitates at the cathode and gas is generated at the anode. At the same time, it also emits electrolytic flue gas mainly containing fluoride, dust and other pollutants. At the temperature of 400℃ ~ 600℃, alumina can still contain 0.2% ~ 0.5% moisture. Water in raw materials can react with solid fluoride salt at high temperature, and water entering molten electrolyte can also react with liquid fluoride salt to generate harmful hydrogen fluoride.

Inhalation of excessive fluoride often leads to skeletal fluorosis such as osteoporosis, hyperosteogeny, tooth mottle, etc., and severe cases lose the ability to work. Fluoride also has a strong stimulating and corrosive effect on respiratory mucosa and skin. According to China's health standards, the maximum allowable concentration of fluoride in the air of workplace is 0.5mg/m3. According to the principle of toxicity classification in the current national standard "Classification of Occupational Exposure Poisons", fluoride is Grade II, which is highly harmful.

Asphalt smoke mainly comes from the kneader, grinding system and molding section of the raw anode section of this process. The softening point of coal tar pitch is 100℃ ~ 1 10℃, which belongs to high temperature pitch. There are two main hazards of asphalt to human body: first, because asphalt contains anthracene and other photosensitive substances, long-term exposure to the sun will cause dermatitis; The second is the stimulating effect of asphalt smoke on skin and mucosa. According to the principle of toxicity classification in the current national standard "Classification of Hazard Degree of Occupational Exposure Poisons", asphalt smoke is Grade III, which belongs to moderate hazard.

Carbon monoxide is produced in the anode of electrolytic cell, which is a colorless and odorless gas. It combines with hemoglobin in the blood, resulting in tissue hypoxia. Patients with mild poisoning have headache, dizziness, tinnitus, palpitation, nausea, vomiting, fatigue, rapid pulse, irritability and shallow to moderate coma; In severe cases, there are deep coma, pupil contraction, increased muscle tension, frequent convulsions, incontinence, pulmonary edema and serious myocardial damage. The maximum allowable concentration of carbon monoxide in workplace air in China is 30mg/m3. According to the principle of poison classification in the current national standard "Classification of Occupational Exposure Poisons", carbon monoxide is Grade II, which is highly dangerous.

Sulfides are also produced during electrolysis. Sulfur dioxide is a colorless gas, which has a strong stimulating effect on eyes and respiratory mucosa. In mild poisoning, skin or eye contact can cause inflammation or burns; Severe poisoning can cause pulmonary edema, laryngeal edema, vocal cord spasm and suffocation within a few hours. The maximum allowable concentration of sulfur dioxide in workplace air in China is 15mg/m3. According to the principle of poison classification in the current national standard "Classification of Hazard Degree of Toxic Occupational Exposure", sulfur dioxide can be classified as grade VI, which is a mild hazard.

3. High temperature hazards

According to the current national standard "Classification of High Temperature Work", the work with an average WBGT index equal to or greater than 25℃ is high temperature work. The electrolysis temperature of aluminum reduction cell is as high as 940℃ ~ 960℃, which is the main production heat source. Production equipment such as calcination, roasting, continuous kneading, preheating screw and asphalt melting in carbon section are all productive heat sources. The data show that when the ambient temperature reaches 28℃, people's reaction speed, calculation ability, sensory sensitivity and sensory motor coordination function decrease obviously. High temperature reduces labor efficiency and increases operation error rate. Mainly reflected in the body's temperature regulation, water and salt metabolism and circulatory system. High temperature can also inhibit the central nervous system, so that workers' attention is distracted during operation and their muscle working ability is reduced, which leads to injury accidents.

4. Noise hazards

The equipment that produces noise mainly includes purification system fan, carbon system crusher, ball mill, molding machine, feeder, vibrating screen, conveyor, roasting flue gas purification system fan and dust removal fan. In the ball mill workshop, the noise level caused by coke grinding is as high as 100dB(A). The noise level generated by using pneumatic slag shell crusher near the electrolyzer in the electrolysis workshop reaches 100dB(A). Noise can lead to hearing loss or even deafness, or neurasthenia, cardiovascular diseases and digestive system diseases. Noise affects information exchange and promotes misoperation.

Second, the main risk factors analysis

1. Lifting machinery injury

The multifunctional crown block used for high-level electrolysis in aluminum plant is a bridge crane, and its functions include: crusting electrolyte, adding alumina to electrolytic cell, replacing anode, lifting the anode bus frame hoist, installing and overhauling the lifting work of electrolytic cell, discharging aluminum and lifting bags, in addition, it can also lift other heavy objects.

Common accidents of bridge crane are as follows: ① Heavy objects fall: damaged spreader or hoisting container, loose binding of objects, improper hook, sudden power failure of electromagnetic sucker, failure of hoisting mechanism parts (especially brake failure and broken wire rope) will all cause heavy objects to fall. ② Squeezing: There is no good safe passage on both sides of the crane track or there is not enough safe distance between the crane track and the building structure, which makes the running or rotating metal structure pinch people; The wrong operation of the running mechanism or the failure of the brakes cause the car to slip and cause injuries. (3) Falling from a height: personnel fall from a height of more than 2m above the ground to cause injuries during crane installation, disassembly, inspection, maintenance or operation. ④ Electric shock: When the crane is working near the electric wire, any part or suspension of the crane is too close to the high-voltage charged object, which will cause electric shock injury. ⑤ Other injuries: Other injuries refer to injuries such as wringing, grinding and poking caused by contact between human body and moving parts; Hydraulic components of hydraulic crane are damaged, resulting in high-pressure liquid injection injury; Strike injury caused by flying objects; Loading and unloading high-temperature liquid metal, inflammable, explosive, toxic and corrosive dangerous goods, injuries caused by falling or loose packaging, etc.

2. Risk of mechanical injury and falling from a height

There are several kinds of mechanical injuries: ① squeezing; (2) collision and impact; ③ Contact: including pinch-off, shearing, cutting and scratching, jamming or winding, etc.

The main equipment of electrolysis process includes: advanced multifunctional crown block, mop cleaner, vibrating screen, crusher, hoist, residual extreme pressure off-line, ferrophosphorus ring pressing off-line, aluminum guide rod straightener, etc. The main equipment of carbon process includes: ball mill, crusher, screening machine, preheating screw machine, continuous kneader, vibration forming machine, anode baking furnace multi-functional unit, etc. All kinds of moving parts that are easy to be approached by operators are dangerous parts of machinery, and the processing area of mechanical processing equipment is also dangerous parts. If the rotating parts of these mechanical equipment are exposed or the protective measures and necessary safety devices are not perfect, it is easy to cause personal injury accidents.

Fall injury: the damage treatment process includes cleaning, screening, crushing, quantification and other processes. Because the equipment is installed on different planes, there are different forms of operating platforms, trenches, lifting openings, potholes and protective fences. If there are no protective measures or defective protective measures, workers are in danger of falling and falling at any time.

3. Electric injury

Electrical accidents can be divided into electric shock accidents, electrostatic accidents, lightning accidents and electrical system failures.

(1) Electric shock accident

Electric shock accidents can be divided into electric shock and electric injury.

Electric shock: the electrolytic reduction tank is operated in series with low voltage and high current, so the electric shock event is usually not serious. However, a serious electric shock accident may occur at the connection point between the high-voltage power supply in the power workshop and the networking circuit in the electrolysis workshop.

Electrical injury: In aluminum electrolysis production, its energy is mainly direct current energy, accounting for about 97% of the total energy consumption. In the electrolytic cell series, the series voltage reaches several hundred volts to several thousand volts. Although people set the zero voltage at the midpoint of the series, the voltage between the two ends of the series is still as high as about 500V V V. Once it is short-circuited, personal and equipment accidents will easily occur. In addition, direct current is used for electrolysis and alternating current is used for electrical equipment on the tank. If direct current flows into the AC system, it will cause equipment accidents. Therefore, many parts of the electrolyzer must be insulated. For example, the cable in the electrolysis workshop has not taken effective flame retardant measures to prevent the cable layer from being damaged, the grounding zero connection measures of electrical equipment are not perfect, the power supply of temporary and mobile equipment (including hand-held power tools and sockets) has not used leakage protectors, or the performance of leakage protectors is unreliable. , will lead to electric shock casualties.

(2) Electrostatic hazard accidents

There are electrostatic hazards in calciner, gas pipeline of calciner, dust removal pipeline of carbon production system and oil-fired boiler system.

(3) Lightning injury accident

The height of the sieve crushing part in the electrolysis workshop is over10m, and the height of the exhaust pipe of the dust removal and smoke exhaust system in the calcination section, green anode and residual electrode treatment section is over 20m, which is in danger of being struck by lightning in a thunderstorm day. Therefore, attention should be paid to lightning damage.

(4) Electrical system failure accidents

The main hazards of electrical system failures are: ① lines, switches, fuses, sockets and plugs, lighting appliances, motors, electric heating appliances, etc. May be the source of fire. (2) The uncharged objects are abnormally charged due to electrical system failure, which may lead to electric shock accidents. For example, the metal shell of electrical equipment is charged due to poor internal insulation; When the high-voltage confederacy is grounded, there is a high step voltage near the grounding point, which may cause an electric shock accident.

4. Danger of fire and explosion

(1) Fire hazard of materials

① Asphalt: One of the raw materials used in the preheating screw kneader of the engineering raw anode section is coal asphalt. The softening point of coal tar pitch is 100℃ ~ 1 15℃, and the flash point is more than 200℃. Asphalt is a mixture of polymer and organic matter. According to the fire hazard classification of production and storage projects in the Code for Fire Protection Design of Buildings (GBJ 16-87), coal tar pitch belongs to Class C. Under certain conditions, it will burn violently and have fire hazard.

② Petroleum coke: Petroleum coke is one of the main raw materials of prebaked anode, which needs to be crushed twice in the process of manufacturing anode, and dust will be formed after crushing. According to the fire hazard classification of production and storage projects in the Code for Fire Protection Design of Buildings, petroleum coke belongs to Class C. ..

(3) Heavy oil: Heavy oil is flammable, and its vapor will burn when exposed to open fire and high heat energy. According to the fire hazard classification of production and storage projects in the Code for Fire Protection Design of Buildings, heavy oil belongs to Class C. ..

④ Gas: The gas used in the power frequency furnace is producer gas, with a relative density of 0.4 ~ 0.6, an explosion concentration limit of 20% ~ 70%, an autoignition point of 700℃ and a low calorific value of 5,900 kJ/m3. Gas and air can form explosive mixed gas, which will cause explosion when exposed to open fire and high heat energy. According to the fire hazard classification of production and storage projects in the Code for Fire Protection Design of Buildings, gas belongs to Class A. ..

⑤ Light diesel oil: Light diesel oil is flammable, and its vapor and air can form an explosive mixture, which will cause combustion and explosion when exposed to open fire and high heat energy. According to the fire hazard classification of products produced and stored in the Code for Fire Protection Design of Buildings, light diesel oil with flash point below 60℃ belongs to Class B, and light diesel oil with flash point above 60℃ belongs to Class C. ..

(2) Danger of fire and explosion in oil tank area

Fire and explosion accidents in oil tank farm are generally caused by the following situations: ① Oil vapor escapes and gathers with air to form explosive gas. When the concentration reaches the explosion limit, it will explode when it meets an open flame. ② Out-of-control of oil products: running, overflowing, dripping, leaking and spilling. ③ The fire source is out of control: equipment repair and welding, open flames, electrical appliances, engines, static electricity, lightning, etc. Strengthen the safety management and monitoring of the oil depot, strictly control the fire source, prohibit smoking and the use of naked flames, prevent the occurrence of iron impact and electric spark, and the electrical equipment in the oil depot should meet the requirements of fire prevention and explosion prevention. These are all necessary measures to prevent fire and explosion in oil tank farm.