Slag-forming mixtures

The use of slag-forming mixtures is an essential factor in ensuring technical reliability of casting process and formation of high quality surface of cast billets.

Slag-forming mixtures are necessary for the following items:

  • reducing friction between solidifying steel and the mould due to the formation of a molten slag film;
  • insuring uniform heat transfer from steel to the mould;
  • inclusion assimilation by liquid slag;
  • preventing secondary oxidation of steel;
  • thermal insulation of the bath surface to prevent a partial crystallization in this zone.

Slag mixture comprises the following components, in % wt.: fluorine-containing material 16-24, silicate lumps 8-12, silicon oxides-based material 8-12, boron oxides-containing material 12-18, cement — balance. The technical effect is an increase in capacity for assimilation of aluminium oxides and improvement of technological properties and billet surface quality.

Known slag-forming mixtures:

- for continuous casting of steel; these types contain amorphous carbon, fluoride material, silicate lumps, cement and other components. Due to a high graphite content in mixtures (5-20 %), a 0.01-0.02 % increase in carbon content is observed in steel.

- with a lower graphite content (2-15%); when these mixtures are used, carburizing low-carbon and ultra low-carbon (electrical) steel grades also takes place.

- for continuous casting of mainly low-carbon stainless steel grades; mixture of this type contains 0.5 - 4.0 % graphite, 21-32 % calcium fluoride, 27.5 - 36.5 % Portland cement, 2-6 % mica (perlite), 25-42 % primer silicate enamel wastes (B2O3 = 5.5–8.7 %) and 0.5-3.0 % ferrosilicon or magnesium-containing material.

However, the presence of graphite in the mixture does not prevent surface and bulk carburization of steels, including low-carbon and ultra low-carbon grades (non-grain-oriented, transformer or relay grades containing less than 0.045 % carbon). Due to primer enamels (wastes), ferrosilicon and (combustible) magnesium, this mixture has a complex composition and contains not common materials.

The slag-forming mixture of optimal composition contains 20 % fluorspar concentrate, 10 % silicate lumps, 10 % SiO2-based material, 15% datolite concentrate and 45 % slag Portland-cement.

Graphitized electrodes

In the industry, graphitized electrodes are used to produce high-grade and low-carbon alloyed steels, special steels and ferroalloys in submerged-arc and steelmaking furnaces. Graphitized electrodes are characterized by a higher electrical conductivity, increased press-formability, sufficiently low resistivity and high resistance to thermal shocks. Small amounts of ash in the graphitized electrodes allow improving quality of the produced metal.

Technology for manufacturing graphitized electrodes includes raw material treatment — crushing and calcination, mixing fillers with carbon loam, forming, baking, impregnation, graphitization and machining. Extrusion and vibro forming are used to produce compacted semi-products.

Graphite electrodes are consumed in steelmaking and submerged-arc furnaces for producing high-quality alloyed and low-carbon steels, ferrous alloys and special alloys. They are also used in various electrothermic furnaces and installations to produce steel, pig iron, non-ferrous metals and special alloys. They are intended for feeding electricity to the furnaces. The electrodes are usually completed with nipples and have corresponding sockets provided at both ends of the electrode. Individual electrode sections are joined with one another by the nipples and sockets and continuously fed into the furnace. They are also used in iron foundry for arc metal cutting. Their use significantly reduces the reject rate.

Soot, graphite or anthracite may be used as raw materials for the production of electrocarbonic products. To produce stick electrodes, crushed mass with a binder, for instance, coal tar and sometimes water glass, is extruded through a mouthpiece. Products of more complex shapes are manufactured in proper press-tool dies. Carbonic semi-products are then baked. The form of carbon that will be present in the final product is determined by the baking conditions. At high temperatures, induced transformation of carbon into graphite is achieved, this process being called the graphitization. Graphite electrodes are made on the basis of petroleum coke and carbon loam and have high thermal shock resistance.

Application of graphite electrodes:

  • Graphitized electrodes of EG (ЭГ) grade are used in arc steelmaking, refining, ferroalloy and submerged-arc furnaces.
  • Graphitized electrodes of EGP (ЭГП) grade are used in high power arc steelmaking furnaces and ladle furnaces.
  • Graphitized electrodes of EGSP (ЭГСП) grade are used in ultra high power electric arc furnaces and ladle furnaces.

Imported graphitized electrodes are marked according to marking systems of the foreign manufacturers.

Graphite electrodes for cutting (also called plates, rods or slabs) are made of GE (ГЭ) grade graphite similar to the material of graphitized electrodes for electric arc steelmaking furnaces by its production method and physico-mechanical properties. In iron and steelmaking, graphite electrodes are used for arc cutting thick metal including armour and ingot risers as well as for cast fettling (removal of burning-ins, joint fins, blisters, etc.).

In electroerosion broaching machines, graphite or copper electrodes similar in shape to workpieces serve as tools. The desired shape is formed by these electrodes made of isostatic graphite, e. g. of the Japanese grade ISEM-3 (И-3).

Fluorspar

Fluorite, also called fluorspar, (derived from the Latin word fluo — to flow) is a mineral composed of calcium fluoride CaF2. It is fragile and has different shades — yellow, green, blue, cyan, coral, violet and sometimes purplish-black; colourless crystals are rare. Colour zoning is a distinctive feature. The colouration is caused by defects of the crystal structure which is highly responsive to radiation exposure and heating. The mineral sometimes contains impurities of rare-earth elements and, in some deposits, uranium and thorium can be detected.

Properties

Cubic syngony, crystals of the cubic, octahedral and cuboctahedral form. Penetration twins are common. The mineral has perfect octahedral cleavage ({111}) caused by weak binding in the octahedral networks. Hardness of fluorite is 4 on the Mohs scale. The fracture is shell-like and brittle. Specific gravity of the mineral is 3.18 and increases up to 3.3–3.6 for yttrium and cerium varieties. The melting point is 1360 °C. Fluorite is diamagnetic. In the blowpipe flame, the mineral is friable, it burns and melts slightly at the edges. It dissolves in concentrated hydrochloric acid releasing HF that corrodes glass. Pure crystals of fluorite are highly transparent in a wide spectrum range from the vacuum ultraviolet band to the far-infrared band, fluoresce brightly in cathode and ultraviolet rays and glow when heated (exhibit thermoluminescence). Fluorite is a typical fluorescent mineral, it fluoresces when heated or exposed to ultraviolet light. Actually, the term fluorescence proposed by G. Stokes comes from the name of this mineral (and not vice versa, as some believe). The Latin name of fluorine, fluorum, is also derived from this mineral’s name.

Varieties:

  • antozonite — dark violet fluorite,
  • chlorophane — green fluorite,
  • ratovkite — earthy or fine-grained variety of fluorite,
  • yttrofluorite — up to 15-18 % calcium is replaced by yttrium.

Colour zoning is a distinctive feature. Colouration is determined by impurities of rare-earth elements as well as chlorine, iron, uranium and thorium. It can also be caused by defects of the crystal structure, which is highly responsive to radiation exposure and heating. The mineral may show colour zones. Pure fluorite is colorless and crystal clear and has a glassy luster, but usually it is green, violet, yellow and other colours, due to impurities or radioactive effects (yellow shade). Dark violet fluorites contain more strontium, and samarium is present in green varieties. Rare-earth elements, some heavy metals and excessive calcium ions give it different colours.

Application

In metal sector, the mineral is used as a fusing (fluxing) agent to form low-melting slags. It is this application that the mineral’s name (fluid) is related to. In the chemical industry, fluorite is used to produce fluorine, artificial cryolite for manufacture of aluminum by electrochemical methods, as well as a number of fluorine compounds. In the ceramic industry, the mineral is used to produce enamels and glazes. Transparent colourless variety of fluorite crystals is used in optics for the manufacture of lenses. Fluorite crystals doped with rare-earth elements or iron can be used in quantum light generators. Treatment of the mineral with sulfuric acid gives hydrofluoric acid that can be used to create etched patterns on glass.