Calcium carbide Hydrolysis

The residual lime can be upgraded as a fertilizer or for the manufacture of cement, or recyded to the process. Recyde lime is raised to 1100 by the burned gases from the electric furnace, cooled to around and iron particles eliminated in a m netic separator. Recyclii cannot be complete, because it leads to the accumulation of impurities in the electric furnace. It is normally limited to a value between 40 and 60 per cent weight of the lime produced. 

The manufacture of acetylene from caldum carbide is marked by the need to handle large amounts of solids. In fact, although the yield of the hydrolysis reaction is practically quantitative, the manufacture of 11 of acetylene requires 3.1 t of carbide with a purity of 80 per cent weight. 

3 ACETYU3VE lVU aJFACTURE FROM HYDROCARBONS. THERMAL PROCESSES WITH DIRECT HEAT TRANSFER 

The originality of the process resides in the use of aTiigh-power (3200 kW) eieciric. arc furnace supplied v,ith direct current electricity. The Huls arc furnace consists of a vertical tube surmounted by an injection chamber into which the gas is introduced 

The reactor is cooled by water circulation. Quench at the himace exit is provided by water spray or by hydrocarbonSt which are cracked into olefinic compounds. 

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Acetylene as well as its compounds have an acetylenic hydrogen atom with an acid character that can be substituted by a chlorine atom. Thus,with acetylene, calcium hypochlorite and sodium hypochlorite in an acid medium form (this can be formed by calcium carbide hydrolysis) chloroacetylene and dichloro-acetylene, which combust spontaneously in air. 

The manufacture of 1,4-BD is based on availability of acetylene and formaldehyde. Acetylene can be obtained from calcium carbide hydrolysis or from the cracking of crude petroleum, see Figure 7.20. 

Calcium carbide has been used in steel production to lower sulfur emissions when coke with high sulfur content is used. The principal use of carbide remains hydrolysis for acetylene (C2H2) production. Acetylene is widely used as a welding gas, and is also a versatile intermediate for the synthesis of many organic chemicals. Approximately 450,000 t of acetylene were used aimuaHy in the early 1960s for the production of such chemicals as acrylonitrile, acrylates, chlorinated solvents, chloroprene, vinyl acetate, and vinyl chloride. Since then, petroleum-derived olefins have replaced acetylene in these uses. 

The hydrolysis product of calcium carbide is acetylene CaC2 + 2H20 - Ca(OH)2 + C2H2... 

In the early part of this century, coal and coal tar products were the main source of bulk chemicals. Acetylene was the major feedstock, obtained by converting coal to calcium carbide followed by hydrolysis. As the petroleum and natural gas industries developed, ethylene and other products obtained by cracking hydrocar-... 

Cyanamide, H2N =N , is a crystalline solid, mp 45 °C, prepared by hydrolysis of its calcium salt, CaNCN, under mild conditions (e.g. aqueous CO2) in order to avoid further hydrolysis of cyanamide. Calcium cyanamide is made on a large industrial scale by a high-temperature reaction of calcium carbide with molecular nitrogen (equation 18). The cyanamide... 

In 1898 Frank and Caro in Germany developed the cyanamide route, in which very high temperatures were used to fix elemental nitrogen to obtain calcium cyanamide from calcium carbide (Eq. 11.4). Hydrolysis of calcium cyanamide then produced ammonia and calcium carbonate (Eq. 11.5). For many years the cyanamide produced predominantly went into ammonia synthesis. 

As expected, the hydrolysis of calcium carbide in an aqueous solution of NH4CI in the complete absence of copper ions does not produce any detectable amount of polyynes. Thus, the copper ions are involved in the coupling reaction and polyynes synthesis as it happens in a typical Glaser reaction [4],... 

The action of water on MgO slowly converts it to Mg(OH)2 which is sparingly soluble. Oxides of Ca, Sr and Ba react rapidly and exothermically with water, and absorb CO2 from the atmosphere (equation 11.5). The conversion of CaO to calcium carbide and its subsequent hydrolysis (reaction 11.20) is industrially important (see Box 11.3), although, as an organic precursor, eth5me is being superseded by ethene. 

A cold solution of potassium hydroxide is prepared in a tall 250-ml beaker from 20 g of KOH and 50 g of well crushed ice to this are added 15 g of bleaching powder. The mixture is stirred to give a thin slurry, the beaker is placed in an ice-bath, and 3-5 pea-sized pieces of calcium carbide are added during 20-30 min. At the end of this time the beaker is removed from the bath and it is observed that the glass gradually becomes warm owing to hydrolysis of the carbide, that the supernatant foam is self-flammatory when stirred, and that there is then a crackling noise due to partial explosions that can be nicely observed in the dark (fume cupboard ). 

In hydrolysis, the H-OH bond in the water molecule splits and the reactions take place in the presence of the H and OH ions. For example, calcium carbide in water reacts to produce acetylene and calcium hydroxide in solution ... 

The classical method of producing acetylene is from calcium carbide by hydrolysis ... 

The yield of this procedure seems to depend on the quality of the calcium carbide used. TTie purity of the calcium carbide may be estimated ftom the amount of ethyne produced upon hydrolysis of a sample with dilute HCl. For example, pure CaC2 (64 mg, 1 mmol) should afford 22.4 mT, of C2H2. Tlie checker manipulated the CaC2 inside a glove box. 

In Summary Ethyne was once, and may again be in the future, a valuable industrial feed stock because of its ability to react with a large number of substrates to yield useful monomers and other compounds having functional groups. It can be made from coal and H2 at high temperatures, or it can be prepared from calcium carbide by hydrolysis. Some of the industrial reactions that it undergoes are carbonylation, addition of formaldehyde, and addition reactions with HX. 

In the presence of biodegradable wastes, methane and carbon dioxide gases are produced in landfills by anaerobic degradation (see Reaction 22.8.1). Gases can also be produced by chemical processes with improperly pretreated wastes, as would occur in the hydrolysis of calcium carbide to produce acetylene ... 

At the time of the discovery of polychloroprene, acetylene appeared to be the only practical feed stock since no economic route through butadiene existed. Acetylene generated by the hydrolysis of calcium carbide was commercially available, hence for the next three decades all production was based on acetylene technology represented as follows ... 

Salt-like carbides, such as Bc2C, are sometimes called methanides because they produce methane on hydrolysis. Acetylides (methynides) are also salt-like carbides, and contain the (C=C) ions. The use of calcium carbide (calcium ethynide), CaC2, to produce acetylene (ethyne), C2H2, on treatment with water, has been superseded by production from petrochemicals. 

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