Sulfur hexafluoride

English name: sulfur hexafluoride

Chemical formula: SF6

Molecular weight: 146.05

CAS registration number: 2551-62-4

EINECS registration number: 219-854-2

Melting point: -62 °C

Boiling point: -51 °C

Water solubility: slightly

Dissolved density: 6.0886kg/m3

Appearance: colorless gas

Molar mass: 146.05

1. Chemical properties

Chemically stable.Slightly soluble in water, alcohol and ether, soluble in potassium hydroxide.No chemical reaction with sodium hydroxide, liquid ammonia, hydrochloric acid and water.It does not react with copper, silver, iron and aluminum in a dry environment below 300 °C.Quartz does not work below 500°C.It reacts with metallic sodium at 250°C and in liquid ammonia at -64°C.It is decomposed when mixed with hydrogen sulfide and heated.At 200 ℃, in the presence of specific metals such as steel and silicon steel, it can promote its slow decomposition.

2. Use

Sulfur hexafluoride has good electrical insulation properties and excellent arc extinguishing properties.Its electric strength is 2.5 times that of nitrogen under the same pressure, its breakdown voltage is 2.5 times that of air, and its arc extinguishing ability is 100 times that of air. It is a new generation of ultra-high voltage insulating material superior to that between air and oil.Sulfur hexafluoride has good insulating properties and arc extinguishing properties, such as: circuit breakers, high-voltage transformers, gas-enclosed combined capacitors, high-voltage transmission lines, transformers, etc.Electronic grade high-purity sulfur hexafluoride is an ideal electronic etchant and is widely used in the field of microelectronics technology.As a refrigerant in the refrigeration industry, the refrigeration range can be between -45°C and 0°C.The electrical industry uses its high dielectric strength and good arc extinguishing performance as an insulating material for high-voltage switches, large-capacity transformers, high-voltage cables and gases.

The mining industry is used as a desorbent to replace oxygen in coal dust from mines.High-purity SF6 is also widely used in metal smelting (such as magnesium alloy melting furnace protective gas), aerospace, medical (X-ray machine, laser machine), meteorology ( Tracer analysis), chemical industry (automobile tires, new fire extinguishers), etc.With the development of today's science and technology, the fields involved in SF6 continue to expand, and it is widely used in more and more basic fields and scientific and technological fields.

3. Overview of risks

Health Hazards: Pure and non-toxic.However, the products produced may be mixed with toxic low sulfur fluoride, hydrogen fluoride, and disulfide decafluoride.

Explosion Hazard: This product is not flammable.

First aid measures

Inhalation: Quickly leave the scene to fresh air.Keep the airway open.If breathing is difficult, give oxygen.If breathing stops, give artificial respiration immediately.seek medical attention.

Fire-fighting measures

Hazardous characteristics: In case of high heat, the internal pressure of the container will increase, and there is a risk of cracking and explosion.

Hazardous combustion products: sulfur oxides, hydrogen fluoride.

Extinguishing method: This product is not flammable.Cut off the air supply.Cool containers with water spray and move containers from fire to an open area if possible.

Emergency treatment of leakage Emergency

treatment: quickly evacuate the personnel from the contaminated area to the upper wind, isolate them, and strictly restrict access.It is recommended that emergency responders wear self-contained positive pressure breathing apparatus and general work clothes.Cut off sources of leaks as much as possible.Reasonable ventilation, accelerate the diffusion.Use immediately if possible.The leaking container should be properly handled, repaired and inspected before use.

Handling and storage

Operation precautions: closed operation, local exhaust.Operators must undergo special training and strictly abide by operating procedures.It is recommended that operators wear filter respirators (half masks).Keep away from flammable and combustible materials.Prevent gas leakage into workplace air.Avoid contact with oxidants.When handling, lightly load and unload to prevent damage to cylinders and accessories.Equipped with leakage emergency treatment equipment.

Storage Precautions: Store in a cool, ventilated warehouse.Keep away from fire and heat sources.The storage temperature should not exceed 30℃.It should be stored separately from flammable (combustible) materials and oxidants, and should not be stored together.The storage area should be equipped with leakage emergency treatment equipment.

Packing method: steel gas cylinder

Transportation precautions: When transporting by rail, it needs to be certified by biological tests, and consignment is carried out according to the certificate of conformity.The safety cap on the cylinder must be worn when transporting in a fresh bottle.The cylinders are generally laid flat, and the bottle mouths should be in the same direction, and should not cross; the height should not exceed the guardrail of the vehicle, and be fastened with triangular wooden pads to prevent rolling.It is strictly forbidden to mix and transport with flammable or combustible materials, oxidants, etc.In summer, it should be transported in the morning and evening to prevent sunlight exposure.It is forbidden to slip away during railway transportation.

Fourth, the application of electrical equipment matters needing attention

Sulfur hexafluoride gas has excellent insulation and arc extinguishing properties, and is widely used in electronic and electrical equipment. Its typical application is used as gas insulation in high-voltage switch cabinets of power transmission and substations, power plants, etc. in the power supply department.With the rapid development of sulfur hexafluoride electrical equipment, more and more people are exposed to sulfur hexafluoride gas and the decomposition products of sulfur hexafluoride after the action of electric arc.

5. Source of gas toxicity

The toxicity of sulfur hexafluoride gas mainly comes from five aspects.

1. Some toxic products produced by sulfur hexafluoride gas in electrical equipment when the high temperature arc acts.

2. The sulfur hexafluoride product is impure, and contains highly toxic low sulfur fluoride, hydrogen fluoride and other toxic gases when leaving the factory.

3. The sulfur hexafluoride gas and its decomposition products in electrical equipment react with electrodes (Cu-W alloy) and metal materials (AL, Cu) to generate some toxic products.

4. The decomposition product of sulfur hexafluoride gas in electrical equipment reacts chemically with the moisture in it to generate some toxic products.

5. The sulfur hexafluoride gas and its decomposition products in electrical equipment react with insulating materials to generate some toxic products.Such as insulating parts such as epoxy phenolic glass fiber cloth boards (rods, tubes) containing silicon components; or epoxy resin casting parts, molded parts and porcelain bottles, silicone rubber, silicone grease, etc. with quartz sand and glass as fillers. , generate SiF4, Si(CH3)2F2 and other products.

6. Toxic gases decomposed

1. Thionyl fluoride (SOF2)

A colorless and highly toxic gas, which can invade the lungs, cause acute edema of lung tissue, affect gas exchange, and cause pulmonary hypoxia and congestion, resulting in suffocating death. It has a strong disgusting odor , which can be used as a warning signal.The lethal concentrations for rats and rabbits were 10 × 10 -6 and 50 × 10 -6 (V/V).

2. Sulfuryl fluoride (SO2F2)

Compound, colorless and odorless, has a stimulating effect on lung tissue at higher concentrations, causing alveolar hemorrhage.The lethal concentrations for rats and rabbits were 200 × 10 -6 and 400 × 10 -6 (V/V).

3. Sulfur tetrafluoride (SF4)

A colorless gas with a pungent odor similar to SO2. Its toxicity is comparable to that of phosgene, and it is harmful to the lungs.

4. Sulfur difluoride (SF2)

A boiling point of 35 °C, which is extremely unstable. It is more active after being heated, and is easily hydrolyzed to form S, O2, HF, etc. Its toxicity is equivalent to that of HF.

5. Sulfur fluoride (S2F2)

Colorless at room temperature Gas, with strong toxicity, generates HF when it encounters water, which has a destructive effect similar to phosgene on the respiratory system.

6. Hydrogen fluoride (HF)

A colorless gas or liquid with a strong pungent odor. It is easily dissolved in water to form hydrofluoric acid, which is highly corrosive to general materials.HF has a strong stimulating effect on the skin and mucous membranes, and can cause pulmonary edema, pneumonia, etc.

7. Disulfide decafluoride (S2F10)

A colorless and volatile liquid under normal temperature and pressure, and is a highly toxic substance, with a toxicity of about 300 times that of SOF2.S2F10 mainly invades lung sites, causing pulmonary hemorrhage and pulmonary edema.The lethal concentration for rats was 1 × 10 (V/V).

8. Aluminum trifluoride (AIF3)

White powder, usually adsorbs a large amount of toxic gaseous decomposition products, so it should be regarded as a highly corrosive and toxic substance.ALF3 dust can irritate the skin, cause rashes, and have invasive effects on the respiratory system and lungs.

9. S2F10O

A highly toxic substance, which has a strong invasive effect on lung tissue.The lethal concentration for rats was 20 × 10 (V/V).

7. Personal protective measures

(1) The work site should be strongly ventilated, and the maintenance personnel should be in the upwind position;

(2) Wear gas masks and protective gloves;

(3) Recover the exhausted gas, or use a conduit to discharge the gas into the sewers. Directly discharge to the atmosphere;

(4) Wash the face, neck, arms and hands thoroughly with soap and plenty of water before taking a break or after work;

(5) If accidentally exposed to a large dose of gas, Wash immediately, change clothes, and go to the hospital for observation and treatment in time.

8. Electrical properties of sulfur hexafluoride gas

Regarding the electrical properties of sulfur hexafluoride gas, we will discuss its electrical insulation properties at room temperature and arc extinguishing properties at high temperatures respectively.

In a uniform electric field and under the action of power frequency voltage, compared with sulfur hexafluoride gas, oil and nitrogen, the dielectric strength of sulfur hexafluoride gas is about five times that of nitrogen, and when the pressure is greater than 8 atmospheres, its dielectric strength The strength can exceed that of transformer oil.

Next, we discuss the factors affecting the breakdown voltage of sulfur hexafluoride gas:

The breakdown voltage of sulfur hexafluoride gas is not only related to pressure, but also related to the smoothness and cleanliness of the electrode surface; the cleaner and smoother the electrode surface, the more The higher the breakdown voltage.

Under the same pressure, the breakdown voltage of sulfur hexafluoride gas increases with the increase of the contact distance, but it is not a linear relationship, but has a meaning of saturation, and the greater the pressure, the more serious the saturation; Therefore, in practice, we cannot strengthen the insulation by increasing the contact distance alone.

In addition, the breakdown voltage of sulfur hexafluoride gas is also related to the geometry of the electrode and the contact area; if the electrode shape makes the electric field more uniform, the breakdown voltage will be higher, which should be noted here, unlike the vacuum switch, The electrode material has no obvious effect on the breakdown voltage of sulfur hexafluoride gas.

The reason why sulfur hexafluoride gas has better insulating properties is because sulfur hexafluoride gas is negatively charged, which means that sulfur hexafluoride molecules can absorb free electrons in the gas and become negative ions. The quality of this negative ion It is much larger than the mass of free electrons, so the movement speed is greatly reduced. In addition, the number of free electrons in the gap is reduced, and it is difficult to form a breakdown channel.So its insulation performance is better.In addition, this electronegative property is also very advantageous at high temperatures, that is, during arc extinguishing.

The arc extinguishing performance of sulfur hexafluoride gas is also very good, which is due to the following reasons:

Sulfur hexafluoride gas will decompose and dissociate under the action of the arc. The polyatomic structure molecules are decomposed into single atoms or charged particles. The gas starts to decompose into low fluoride at about 2000 °C, starts to dissociate at about 4000 °C, and starts to dissociate at 6000 °C. When the temperature is higher than 10000 ℃, the sulfur hexafluoride gas is completely freed; this internal change will affect the thermal conductivity and electrical conductivity of the gas, which greatly enhances its thermal conductivity and electrical conductivity.

The thermal conductivity of the gas is enhanced, and the heat dissipation of the arc is accelerated, which is conducive to the rapid cooling of the insulating medium in the gap after the arc is extinguished, which is conducive to the compounding of low-fluoride compounds into sulfur hexafluoride, and at the same time, it is beneficial to restore the insulation, which greatly reduces the arc. Re-ignition is conducive to quenching.

It may be a little difficult to understand the reasons why the enhanced electrical conductivity of gas is beneficial to arc quenching.The reason is this: the enhanced electrical conductivity is indeed beneficial to arcing. From the volt-ampere characteristic curves of several gas arcs, it can be found that the volt-ampere characteristic curve of sulfur hexafluoride gas, that is to say, under the same current condition, The arc voltage of sulfur hexafluoride gas, and the arc energy is Uh*I, also, the arc can be maintained even when the current is very small, and will not break, so that the phenomenon of interception will not occur, which is also sulfur hexafluoride Where the gas is superior; we know that the current sulfur hexafluoride circuit breakers are all extinguished when the arc current crosses zero, the arc energy is small when the arc is ignited, and the temperature of the arc and the decomposed gas are relatively small. It is very beneficial to the recovery of the dielectric strength of the gap after the current crosses zero, making it difficult to re-ignition or re-ignition after the arc is extinguished, so the sulfur hexafluoride gas is not only conducive to arc ignition, but also conducive to extinguishing.

In addition, sulfur hexafluoride The electronegative nature of the gas (the characteristic of adsorbing free electrons) and the secondary recombination characteristic (the low-valent fluoride decomposed in the arc is rapidly reduced to sulfur hexafluoride molecules after the arc is extinguished), these characteristics also make the sulfur hexafluoride gas no matter Whether the initial medium strength, medium recovery speed or final medium strength is relatively high.