Chromium, chemical symbol Cr, atomic number 24, inperiodic table of ele mentsIt belongs to family Ⅵ B.The element name comes from Greek, originally meaning "color", because chromium compounds have color.The single substance is steel gray metal, which is the hardest metal in nature.The content of chromium in the crust is 0.01%, ranking 17th.Free natural chromium is extremely rare and mainly exists in chromite.Relative density 7.15g/cm³，Melting point 1907 ℃, boiling point 2679 ℃.The resistivity is 12.7x10 ⁻ ⁸ Ω · m (20 ℃).[1]
Chromium is a bluish gray, very shiny hard metal.Chromium compounds are toxic. Chromium on the earth exists in the form of chromite.[3]Chromium is mainly used forstainless steel、electroplateAnd cermets.[1]
Trace chromium is an essential element for life. The toxic dose of chromium is 200mg, and the lethal dose is more than 3g.High valent chromium has carcinogenic effect.[1]
In 1766, Germany's Lehmann, who was a chemistry professor in St. Petersburg, Russia, once analyzed chromium and determined that it contained lead.
In 1797, Auckland discovered chromium oxide.[3]
In 1798, Volkland named the gray needle like metal he found chrome, which came from Greek chrome (color).The Latin name chromium and the element symbol Cr are thus obtained.At about the same time,ClaptottAlso fromChromiteChromium was found independently in.
On October 27, 2017, the list of carcinogens published by the International Agency for Research on Cancer of the World Health Organization (WHO) was preliminarily sorted out for reference, and chromium metal was included in the list of three types of carcinogens.[2]
Physical and chemical properties
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physical property
Metal chrome block, glass sealed chrome display specimen
Chromium is a silvery white lustrous metal. Pure chromium has ductility. Chromium containing impurities is hard and brittle.Relative density 7.15g/cm³，Melting point 1907 ℃, boiling point 2679 ℃.The resistivity is 12.7x10 ⁻ ⁸ Ω · m (20 ℃).[1]
Chromium has a high corrosion resistance. In the air, it oxidizes slowly even in the state of red heat.Insoluble in water.Plating on metal can play a protective role.
chemical property
Reaction of Chromium with Halogen
At 400 ℃ and 200-300 atm, chromium reacts directly with fluorine to form chromium fluoride (Ⅵ) CrF ₆.[3]
Under relatively mild conditions, chromium fluoride (Ⅴ) CrF ≮ can be generated.[3]
Under milder conditions, chromium reacts with fluorine (F ₂), chlorine (Cl ₂), bromine (Br ₂), and iodine (I ₂) to produce the corresponding trihalide chromium fluoride (Ⅲ) (CrF ∨), chromium chloride (Ⅲ) (CrCl ∨), chromium bromide (Ⅲ) (CrBr ∨), and chromium iodide (Ⅲ) (CrI ∨).[3]
Reaction of chromium with acid
Chromium metal can be rapidly dissolved in dilute hydrochloric acid to form hydrogen and an aqueous solution containing chromium (Ⅱ) ions.In fact, Cr (Ⅱ) is represented by Cr (H ₂ O) ₆²+ions.There is a similar situation in sulfuric acid, but pure chromium will not react with acid, for example, chromium will not react with nitric acid (HNO ≮).In fact, chromium is purified in nitric acid.[3]
Preparation method
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Industrial preparation
chromium
The primary mineral used to produce chromium is chromite [Fe (CrO ₂) ₂], which can be oxidized with air in molten alkali to produce sodium chromate (Na ₂ CrO ₄) with a price of+6.After sodium chromate is extracted with water, precipitated, and reduced with graphite, chromium oxide (Ⅲ) (Cr ₂ O ∨) can be obtained.The metal chromium can be obtained by further reducing the oxide with aluminum or silicon.[3]
Laboratory preparation
Another method of producing chromium is electrolysis. First, dissolve Cr ₂ O ∨ in sulfuric acid, and then use the obtained electrolyte for electroplating.[3]
application area
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High carbon chromium stainless steel rod
Chromium is used to make stainless steel, auto parts, tools, tapes and videotapes.Chromium plating on metal can prevent rust, also known as Kedomi, solid and beautiful.
Chromium can be used to make stainless steel.Red and emerald colors also come from chrome.As the most important metal in modern science and technology, the chrome nickel steel fused with different percentages is ever-changing and has an incredible variety.
The toxicity of chromium is related to its existing valence state. Hexavalent chromium is 100 times more toxic than trivalent chromium, and is easily absorbed by the human body and accumulated in the body. Trivalent chromium and hexavalent chromium can be transformed into each other.Natural water does not contain chromium;The average concentration of chromium in seawater is 0.05μg/L；Drinking water is lower.The pollution sources of chromium include sewage discharged from chrome ore processing, metal surface treatment, leather tanning, printing and dyeing, etc.
Due to the brittleness of chromium alloyMetallic materialsThe use is still under study, and chromium is mainlyferroalloy(such as ferrochrome) is used to produce stainless steel and various alloy steels.Metallic chromium used asaluminium alloy、Cobalt alloy、titanium alloyandSuperalloy, resistance heating alloy, etc.Chromium oxideIt can be used as a light resistant and heat resistant coating, as well as an abrasive, a colorant for glass and ceramics, and a catalyst for chemical synthesis.Basic chromium sulfate(trivalent chromium salt) used as a tanning agent for leather.Chrome alum、DichromateUsed as mordant, impregnant and various pigments for fabric dyeing.Chromium plating and chromizing can make steel and copper, aluminum and other metals form a corrosion resistant surface, which is bright and beautiful, and is widely used in furniture, automobile, construction and other industries.In addition, a large amount of chrome ore is also used to make refractory materials.
It can be used for anodic aluminum oxide in aviation and other industries.[3]
In the production of refractory materials, chromite is used for making bricks and forming.[3]
By the end of 2021, the global chromite reserves are 2.042 billion tons.[5]
China is short of chromium resources, with 13.095 million tons of proved reserves and 10.951 million tons of retained reserves, mainly distributed in Tibet, Xinjiang and other places.There are 56 places of origin of chromite, mainly including Xinjiang Sartuohai, Tibet Lobsa, Inner Mongolia Hegenshan, Gansu Dadaoerji, etc.[6]
Safety measures
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Environmental hazards
Trivalent chromium has almost no harmful effect on human body, and there is no report of industrial poisoning.Chromium entering the human body is accumulated in human tissues, and the metabolism and removal speed is slow.When chromium enters the blood, it mainly binds to globulin, albumin and r-globulin in plasma.Hexavalent chromium can also pass through the erythrocyte membrane. Within 15 minutes, 50% of hexavalent chromium can enter the cell and bind with hemoglobin after entering the erythrocyte.The metabolite of chromium is mainly excreted from the kidney, and a small amount is excreted through feces.Hexavalent chromium is mainly a chronic toxicity to human beings. It can invade the human body through the digestive tract, respiratory tract, skin and mucosa, and accumulate in the liver, kidney and endocrine glands in the body.Those that enter through the respiratory tract tend to accumulate in the lungs.Hexavalent chromium has a strong oxidizing effect, so chronic poisoning often begins with local damage and gradually develops into irremediable.When invading the human body through the respiratory tract, it starts to invade the upper respiratory tract, causing rhinitis, pharyngitis, laryngitis and bronchitis.
Health hazards
To human skin
Chromium compounds do not damage the intact skin, but when the skin is abraded and exposed to chromium compounds, it can cause injury.The incidence of chromic skin ulcer is relatively high, which is mainly related to the length of contact time, skin allergy and personal hygiene habits.Chromic ulcer mainly occurs in hands, arms and feet, but it can occur wherever the skin is damaged.The root of the nail is the exposed part, which is easy to accumulate dirt, and the skin is also the most vulnerable to damage, so these parts are also easy to form chrome sores.Before the formation of chromic ulcer, the skin initially appears red and swollen, with a sense of itching. Without proper treatment, it can invade deep.The ulcer is covered with a hard scab of secretion. The surrounding part is raised, the center is deep and full of carrion, the edge is obvious, grayish red, local pain, the ulcer is inverted cone, the ulcer surface is small, generally not more than 3mm, sometimes it can be as large as 12-30mm, or as small as the size of a needle tip. If the treatment is ignored, it can be placed deep into the bone for further development, severe pain, and slow healing.
Chromic dermatitis and eczema can also occur when exposed to hexavalent chromium. The skin of the affected area itches and forms blisters. Dermatitis can occur after a few days of contact with chromium pollutants in people with skin allergies. The chromium allergy period can be as long as 3-6 months. Eczema often occurs in exposed parts such as hands and forearms, occasionally in feet and ankles, even in the face and back.
To respiratory tract
The common occupational disease of respiratory tract exposed to chromium salt is chromium rhinitis. The early symptoms of this disease are nasal mucosa congestion, swelling, nasal cavity dryness, itching, bleeding, decreased sense of smell, increased secretion of mucus, frequent sneezing, etc., followed by nasal septum ulceration. The ulceration site is generally 1.5 cm in front of the lower end of the nasal septum soft bone, without obvious pain.Chromic rhinitis can be divided into three stages according to the degree of ulcer and perforation:
Erosive rhinitis, the mucosa of nasal septum is broken, showing gray white spots.
Ulcerative rhinitis, nasal septum thinning, concave defect of nasal mucosa, thick scab cover on the surface, pale nasal mucosa, and obvious olfactory decline.
The nasal septum is perforated. The cartilage of the nasal septum can be seen to form a round triangular hole with yellow scab at the perforation. The nasal mucosa is atrophic and the nasal cavity is dry.
Eyes and ears
Contact of the eyelids and cornea with chromium compounds may cause irritation and ulcers. Symptoms include conjunctival congestion, foreign body sensation, tingling tears, and impaired vision. In serious cases, it may lead to corneal epithelial detachment.
Chromium compounds erode the tympanic membrane and the outer ear and cause ulcers only occasionally.
For gastrointestinal tract
Ingestion of hexavalent chromium compounds by mistake can cause thickening of oral mucosa, edema and formation of yellow crust, nausea and vomiting, sometimes with blood, severe abdominal pain, hepatomegaly, and even circulatory failure, unconsciousness, and even death.Hexavalent chromium compounds are carcinogenic when inhaled and can cause lung cancer.
Systemic poisoning
The symptoms are headache and emaciation, gastrointestinal disorders, liver failure, kidney damage, increased monocytes, increased blood calcium and phosphorus.
Toxicological data
All chromium compounds are highly toxic.Chromium (Ⅵ) compounds are highly toxic and carcinogenic, while chromium (Ⅲ) compounds are less toxic.Chromium compounds are important pollutants.[3]
Storage and transportation
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Storage method
Store in a cool and ventilated warehouse.Keep away from kindling and heat sources.Should be connected withOxidantAcids shall be stored separately, and mixed storage shall be avoided.Equip with corresponding types and quantities of fire-fighting equipment.The storage area shall be equipped with appropriate materials to contain leakage.[4]
Transport method
The equipment for transporting ferrochrome must be covered to avoid stormy weather during transportation.
Before loading ferrochrome, first check whether the vehicle at the place of transportation can operate safely, then check whether there is any leak in the carriage, check whether the equipment at the place of transportation has rain proof measures, check whether the packaging of goods is complete, and whether all goods are sorted out, and then put them into the transportation equipment in order.
After the car is filled with ferrochrome, the handover procedures must be handled, and the train will depart after confirmation.
In the process of transportation, the driver should regularly check the vehicle, check the ceiling, and check the stacking of goods in the carriage. The hidden danger of general accidents is from leakage. Because the vehicle bumps and shakes on the way, the package is often damaged.
The manufacturer of high carbon ferrochrome reminds that although ferrochrome is not a dangerous article, it is also necessary to pay attention to the route during transportation. It is not allowed to choose some complicated and difficult road sections for the sake of road proximity. At the same time, it is not allowed to park in urban streets and densely populated areas. In addition, during the transportation of ferrochrome, it is not allowed to mix with hard materials, nor strong vibrationRub, trample, smash, and do not throw.
Physiological function
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Chromium is an essential trace element for life, which participates inglucoseMetabolism of.Chromium can affect the expression of insulin.Chromium compounds are highly toxic unless they are traces of chromium.[3]
