CYANAMIDES

the metallic cyanamides, of which calcium cyanamide, cacno, is the only one of technical impor- tance, are derivatives of dyanamide, h2zcne. in practice, calcium cyanamide is always formed by the action of nitrogen on calclum carbide. in 1892 moissan showed that calcium carbide could be prepared easily and in quantity in the electric furnace, and in 1895 a. frank and n. caro (jour. soc. chem. ind., vol. 27, 19083 trans. faraday soc., vol. a9, 1908; zetlsch. angew. chem., vol. 22, 1909) began investigations on the absorption of nitrogen by metallic carbides which have led to the establishment of the modern cyanamide industry. although pure calcium carbide does not absorb nitrogen even at 1,200°c. (moissan), frank and caro found that technical carbides did so readily at much lower temperatures, probably owing to their impurities. formation of cyanamides by nitrogen.—by the action of nitrogen on metallic carbides, mixtures of cyanamides and cya- nides are obtained, the proportions depending on the metal and on the temperature. the general type of the reaction may be illustrated by the case of calcium carbide. caco+no=cacno+c (r) cucn2+c =ca(cn)e “ with alkali carbides, cyanide preponderates largely. with barium carbide about 30° cyanide and 70% cyanamide is ob- tained at 700°-800°c., whilst with calcium carbide cyanamide is practically the only product up to 1100°c. in all cases the pro- portion of cyanide increases with the temperature. these facts explain the formation of cyanide by fusing the mixture of calclum cyanamide and carbon with sodium chloride, cacn2o+c+2nacl=cacl.+ 2nacn (2) calcium cyanamide—the mechanism of the production of calcium cyanamide is by no means clear, in spite of numerous investigations. the chief difficulty in interpreting the results is the effect of impurities, always present in commercial carbide. if the reaction is according to cquation (1), with three solid phases and a gaseous phase, a definite equilibrium pressure of nitrogen should correspond to a given temperature. attempts io determine this equilibrium (caro, matignon, thompson and lombard) led to improbable or conflicting results; m. le blanc and m. eschmann (zeitsch. flektrochem., 17, 20, jan. 1911) could not obtain reproducible values for the equilibrium pres- sures, and attribute this to (2) sublimation of cyanamide prevent- ing its taking part in the reverse reaction; (6) alteration of the carbon rendering it inactive; (c) occurrence of side reactions, the effects of which could not be taken into account. they con- cluded that with impure carbide the system is divariant and that one of the phases is a melt of variable composition. their results were confirmed by v. ehrlich (zeitsch. elektrochem., 28, 529, dec. 1922), who, however, with nearly pure calcium cyanamide, free from carbon, obtained definite equilibrium pressures, which were much lower than the indefinite ones with impure cyanamide —carbon mixture for the same temperature. he concluded that 7380 pure calcium cyanamide decomposes reversibly into calcium sub- carbide and nitrogen, cacno=cac+n, and that his equilibrium pressures vrere for this reaction. he sug- gested the formation of commercial cyanamide in stages, cacs=cac+c cac+nz=cacny, the dissociation of the carbide into subcarbide and carbon had been previously assumed by g. erlwein, c. warth and r. beut- ner (zeitsch. flektrochem., «7, 177, march 1911) to account for their experimental results on heating calcium carbide, but briner and kuhne (jour. chim. phvs., 12, 432) could not confirm this. to sum up, the mechanism of cyanamide formation has not yet been fully explained; the importance of this for the cyanamide industry is obvious. absorption of niirogen.—the rate of absorption of nitrogen by carbide has been much studied. pure calcium carbide does not absorb nitrogen at 1,200°c. with commercial carbide (contain- ing about 10% lime) absorption is rapid and complete at 1,050°— 1,100°c. below 1,000°c. the absorption is less rapid and is in- complete, the nitrogen absorbed tending to a limiting value, which is lower the lower the temperature. these limiting values do not represent true equilibria (foerster and jacoby). pollacci found that the velocity of absorption is proportional to the nitro- gel pressure up to two atmospheres, but that further increase of pressure makes little difference. | many substances when added to calcium carbide lower the temperature at which nitrogen absorption begins and increase the velocity of absorption for a given temperature. the most important of these substances are calcium chloride (tpolzenius) and calcium fluoride (carlson). many other substances, includ- ing lime, act similarly, and this explains the action of commercial carbide. the rdle of these additions has been investigated by g. bredig, w. fraenkel and fe. wilke, f. foerster and h. jacoby, rudolphi, g. pollacci and others (zertsch. elekirechem., vol. 13, march 1907; vol. 14, sept. 1908; vol. 15, nov. 1909; zettsch. anorg. chem., §4, 170), and has been connected with the lowering of the melting-point of the mass which they cause; but how this increases the velocity of absorption, whether by removing pro- tecting layers of reaction products from the carbide, by allowing the nitrogen to dissolve in the liquid mass, or by other means, has not been settled. reaction (1) is strongly exothermic. the heat of reaction is about 77,800 calories at 25°c. technical production.—on the commercial scale a mixture of calcium cyanamide and carbon (cacn2+c) is produced by heat- ing ground calcium carbide in an atmosphere of nitrogen to about 1,000°-1,100°c. the nitrogen is generally prepared by the fractional distillation of liquid air by the linde or claude process, and should be as free as possible from moisture, co, cos and especially oxygen, as these act on either the carbide or cyanamide and diminish the nitrogen content of the final product. discontinuous process.—in the discontinuous process a vertical steel drum lined with firebrick is fitted loosely with a thick paper or thin, perforated, sheet-iron cylinder. down the centre of the retort passes a carbon rod concained in a cardboard tube to separate it from the carbide. this rod is connected to an alter- nating current supply and is used as a heating resistance to start the reaction. the ground carbide is packed between the inner tube and the outer cylinder. the lid is luted on airtight. nitrogen is admitted by a pipe through the outer casing. the charge con- sists of 300-800 kilos of carbide. a battery of such units is connected to the current supply. the heated carbon resistor starts the reaction at the centre. after a time the current is cut off as the reaction produces sufficient heat to maintain the charge at about 1,000°c. and so complete the reaction. the drums are allowed to cool, the contents ground and treated with sufficient water to slake the lime and decompose any uncombined carbide. the ground product is sometimes mixed with a little oil to render it dustless before packing in bags. the crude cyanamide, known as ‘nitrolim,” ‘lime nitrogen,” ‘ kalkstickstoff”’ or cyanides “ stickstoffkalk,” contains 20-22°% of nitrogen, corresponding ty 57-63% of cacn:z and about 20% lime, 7-8% silica, iron oxide and alumina and 14% of graphitic carbon. continuous process ——in one type of continuous furnace the carbide is packed in perforated metal boxes which are sent through a tunnel filled with nitrogen and heated, electrically or by external gas firing, to the necessary reaction temperature. another common type is a vertical iron shaft lined with firebrick and divided tnternally into sections by shelves. the carbide is fel in at the top and heated by arcs between carbon electrodes. it meets an ascending stream of nitrogen and is made to fall slowly from shelf to shelf by scrapers until it is discharged at the bottom. in the carlson process calcium fluoride is added to the carbide to increase the velocity of absorption and to lower the reaction temperature. uses of nitrolim.—nitrolim may be used directly on most soils as a nitrogenous fertiliser. when treated with water or steam under pressure it gives up practically the whole of its nitrogen as ammonia (q.v.): cacno+3h2o0 = cacos+ 2nh3 the addition of alkali increases the yield of ammonia by dimin- ishing the formation of polymers of cyanamide. by fusing a mixture of nitrolim, common salt and calcium carbide in an are furnace sodium cyanide is produced by reaction (2) above. by rapidly cooling the melt the reverse reaction is prevented and the cyanide preserved (chem. met. eng., 22, 265; can. chem. and met., 6, 129). by suitable treatment numcrous organic compounds (urea, dicyandiamide, guanidine, etc.) may be pre- pared from cyanamide (jour. ind. eng. chem., 5, 159). it also forms the basis of ‘‘ ferrodur,’’ used for case-hardening iron. the world’s production (1918) was 668,800 metric tons. bibliography.—final report of nitrogen products committee; joseph knox, fixation of atmospheric nitrogen (1921); a. j. allmand, the principles of applied electrochemistry, p. 643 (1924); j. r. partington and l. h. parker, the nitrogen industry (1922). (j. kn.) cyanides ! (sce gold, 12.190). —the world’s consumption of cyanides has increased by about 50% since 1910, and, owing to improvements in manufacture, the price is now 10 to 20% lower than that ruling in 1910-3. metallurgy.—the extraction of gold and silver from their ores is still the principal use of cyanides. two grades of sodium cvanide (90% and 97°53 nacn respectively) and, since 1916, an impure calcium cyanide containing cyanogen equivalent to 48 to 50°, nacn are in common use. while the principle of the methods employed is the same as before, the process is no longer regarded merely as a way of recovering gold which cannot readily be won by amalgamation. when the ores are very finely ground, so as to pass through gauze with 150 to 200 wires per in., the time needed to dissolve the precious metals is very much reduced, a more complete extraction is obtained (95% or more is not un- usual), very dilute cyanide solutions, containing considerably less than 0o-1°5 nacn, can be employed and, finally, many ores formerly regarded as refractory can be treated successfully. the economic handling of finely divided ores presented diffi- culties which have only gradually been overcome. each mining field has developed methods to suit the peculiarities of its ores and its own local conditions. ‘ in general the ore is first crushed roughly in presence of cyanide solution; the mixture of ore and solution then passes to a tube mill for fine grinding. the pulp passes on to some form of hy- draulic classifier which separates the coarser particles of ore and returns them to the tube mill for regrinding. the separation of the cyanide solution (which now contains the precious metals) is carried out either by settling and decantation or by filtration; in the most recent plants these operations are continuous. in the recovery of gold and silver from the cyanide solutions zinc dust is now often used in place of zinc shavings. the dust is stirred into the solution in suitable quantity and the precipitate of finely divided gold or silver which forms very rapidly is filtered off at once. the consumption of zinc is smaller and the bullion is recovered at shorter intervals than in the older process. cyanides control of insects and other pests —the value of hydrocyanic acid as an insecticide was first recognised in 1886 by d. w. coquillett, who used it in the control of scale insects on citrus trees in california. the methods of work were standardised by the investigations of r. s. woglum in 1907-10 (u.s. dept. of agriculture, bureau of entomology, bull. 90). in 1916, more than 1,300 tons of sodium cyanide were consumed in the fumigation of citrus trees in california alone (g. p. gray, j. ind. and eng. chem., 10, 301, 1918) and the method is now practised in florida, porto rico, australia, japan, south africa, spain and italy. it was first tried on deciduous trees in 1894 in virginia by coquillett and in 1909 a. l. quaintance (u.s. dept. agriculture, burcau of entomology, bull. 84) showed that it was effective against the san jose scale on apples; subsequently the fumigation of nursery stock, before planting, was made compulsory in several states of the united states. uses as fumigant.—hydrocyanic acid was first used for the de- struction of verminin the sleeping carriages of the cape govern- ment railway in 1898 and its use for similar purposes spread so rapidly that the towncouncil of johannesburg found it necessary, in 1916, todraw up by-laws for the licensing of fumigators and for regulating the practice. hydrocyanic acid was authorised as a fumigant in the united states quarantine regulations of 1910, but it was first used on a large scale for killing rats in ships, as a measure of plague control, at new orleans in 1914; since then it has been used as'a routine measure so that, for example, of 1.425 ships fumigated at the port of new york in the year 1921-2, all but ro were treated with hydrocyanic acid. the method was first used in germany in 1917 against weevils in a flour-mill, in italy in 1919 against rats in granaries and in ships, and in great britain in 1922 for the fumigation of ships and greenhouses. (p. g. stock and g. w. monier-williams, afinisiry of health, rep. on public health, no. 19.) in the fumigation of plants it is important to secure a concen- tration of hydrocyanic acid which will kill the insect pest with- out injury to the plant. plants are more severely injured when they are exposed to sunlight than they are in the dark; fumiga- tion is therefore done at night and at temperatures between 38° and 65° fahrenheit. there is considerable variation in the sus- ceptibility to damage of different plants; roses and sweet peas for example are easily burned, lemons are much less affected than oranges, and olives may even be fumigated in daylight. in order to obtain the necessary concentration of gas, plants growing in the open are covered with a tent; the form in general use is an octagonal sheet of light, closely woven cotton cloth measuring usually from 36 to 45 ft. across. by means of two light poles this is drawn over the tree so that it hangs down touching the ground all roundit. the distance round the bottom of the tent is then measured by a tape and the distance over the top of the tree read off from a scale which is painted on the tent itself; the quantity oi cyanide required for an enclosure of the size defined by these measurements is then read off from a table which was constructed by woglum (foc. cit.) as a result of his experiments. the table is based on the use of 1 oz. of potassium cyanide (or 3 oz. of sodium cyanide) for 1oo cu. ft. of enclosed space for trees 11 to 12 {t. high; smaller trees require rather more and larger ones less owing to the relatively greater or smaller leakage through the tent cloth. the exposure to the gas is usually one hour. in calculating the dosage allowance is also made for the kind of insect to be attacked; for example, the full dosage is needed for purple scale in california and for the “‘ serpeta ” in spain, but a } dosage suffices for black scale and about § dosage for phleothrips oleae on olive trees in spain. the hydrocyanic acid is generated by mixing together sodium cya- nide (1 oz.) concentrated sulphuric acid (1 fluid oz.) and water (2 oz.). the water is placed in a 2 to 3 gal. stoneware jug, the sulphuric acid then added, within the tent, the charge of cyanide is then thrown in and the tent closed at once. in more gas-tight enclosures, such as greenhouses, a smaller dosage is sufficient; 4 oz. of sodium cyanide to 1,000 cu. ft. will eradicate aphides, in other cases a repetition of the treatment after a suitable interval may be needed, because the eggs and 731 pupae are more resistent than the larvae and mature insects; most scale insects require about 23 oz. per 1,000 cubic feet. in the fumigation of ships and mills modifications of the pro- cedure are necessary owing to the larger quantity of gas required and the difficulty of protecting the operator from danger. the compartments to be treated are first closed as nearly as possible gas-tight; wooden barrels, placed so as to distribute the gas uni- formly, are charged with water and sulphuric acid and the cya- nide placed in hinged receptacles which can be tipped by means of wires communicating with the exterior. the holds, cabins, etc., are opened up 2 hours after generating the gas and several hours allowed for it to escape before anyone is allowed to enter; in calm weather ventilation is accelerated by a fan. the officer in charge finally makes an inspection, wearing an oxygen-breathing appa- ratus and carrying a cage of rats; if these are unaffected the ship is certified safe. there are several modifications of this method. the necessity of handling strong sulphuric acid and difficulties in disposal of the poisonous acid residues have led to the intro- duction of pure liquid hydrocyanic acid. this was first manu- factured commercially in california in 1917 and in south africa in 1920. in california the liquid, which contains 2 to 4% water, boils at 27°c., and has density 0-7, is supplied in iron drums holding too ib. each. it is applied by means of a portable vessel, containing 17 to 21 lb., with attached hand pump. fach stroke of the pump delivers a measured quantity of the liquid to a spraying nozzle which is pushed into the tent. another method passes the measured volume of liquid into a heated tube where it is vaporised and delivered into the tent as gas. in south africa the liquid is put up in sealed glass tubes which are broken within the space to be fumigated. : liquid hydrocyanic acid is unstable, more especially in presence of alkalis; it polymerises to a black substance resembling charcoal, with evolution of a considerable quantity of heat. as the speed of polymerisation increases with temperature the reaction may become explosive. (walker and eldred, j. ind. and feng. chem., 17, 1,074, 1925.) for this reason the transport and storage of liquid hydro- cyanic acid are attended with risks which have contributed to con- fine its use to districts in which it is possible to distribute it from factory to consumer by motor transport. the usual dosage for ships, mills, etc., (in oz. nacn per 1,000 cu. ft.) is:—for mosquitos 0-5, fleas 2-5, rats and mice §, lice to, bed bugs 5, with exposures of hali an hour for the smaller quantities up to 2 hours for the larger. when liquid hydrocyanic acid is used 22 c.c. is equivalent to rt oz. of sodium cyanide. case hardening.—since 1910 case hardening by immersion in fused sodium cyanide has come into use more especially in the united states and canada. mild steel absorbs carbon from fused sodium cyanide and is so converted, superficially, into steel which can be hardened by rapid cooling. the articles to be treated are suspended in a bath of fused sodium cyanide prefer- ably diluted with sodium chloride or carbonate to contain about 50% of nacn. the most suitable temperature is 850°c. the rate of penetration of carbon into mild steel is 1/100th in. (0-25 mm.) per hour, or about 50% more when 3 °% nickel steel is used. the rate of penetration is independent of the percentage of cyanide in the bath, but the carbon content, and therefore, the hardness after quenching, increases with it. the method has great advantages for small, accurately shaped machine parts since surface oxidation and distortion due to uneven heating are avoided and the depth of case can be controlled very exactly. the operation is rapid, an immersion of 10 to 15 min. being sufficient in some instances. electro-plating.—close-grained, adherent deposits of several metals, especially silver and gold, are obtained by the electrolysis of solutions of their double cyanides. such solutions are now largely used for the deposition of copper, zinc and brass. owing to the bettcr adhesion and more even distribution of the coating it is claimed that a smaller quantity of zinc will prevent corrosion of iron when the zinc is deposited by electrolysis than when it is applied by the older dipping method. by the use of pure cyanides the baths are easily maintained at the strength suitable for rapid deposition of the metals. the following baths are in use for “ still " plating, expressed in oz. per imperial gallon of water:—copper plating, sodium cyanide 3-9, 7352 cuprous cyanide 3-75, soda ash 1-5, sodium thiosulphate 0-3; zinc plating, sodium cyanide 3-1, zinc cyanide 3-75, caustic soda 3-75, ammonium chloride 2-5; brass plating, sodium cyanide 5-6, copper cyanide 3-75, zinc cyanide 1:25, soda ash 1-5, ammonium chloride 0-4. flotation.—intimate mixtures of the sulphides of lead, zinc, copper and iron with gangue often occur in nature. ‘the sepa- ration of the minerals is done by agitating a mixture of the pow- dered ore, water and a little oil withair. a froth is formedin which the minerals predominate. g. e. sheridan and g. g. griswold (u.s. patents 1,421,583 and 1,427,235 of 1922) have found that the addition of a small quantity of sodium cyanide and, prefer- ably, zinc sulphate to the liquid, previously made faintly alkaline with soda ash, prevents the flotation of zinc sulphide and of iron pyrites without affecting the flotation of the sulphides of lead or copper. the iron and zinc may be separated by a second treat- ment in more strongly alkaline solution, the zinc alone floating. the process is already in use at several mines in north america. manufacture—in tg10 cyanides were produced mainly (1) from sodium, ammonia and charcoal, (2) from the nitrogenous constituents of the sugar beet remaining after removal of the sugar, (3) from the hydrocyanic acid of crude coal-gas which was recovered as ferro-cyanide, a part of which was converted into sodium cyanide by fusion with sodium (see 22.529). ferro- cyanides are now more economically made from cyanides, but the two first-mentioned processes continue to produce. in addition to these, processes are used in which the nitrogen is derived from the atmosphere. the most important of these is based on the erlwein & frank process (u.s. patent 708,333, 1902), in which calcium cyanamide is fused with common salt. this process had been worked experimentally in germany with little success. in 1916, it was installed at niagara and improved by raising the temperature of fusion from 950° to 1,200-1,.400°c., by adding calcium carbide in order to supply active carbon (the reaction is essentially cacne2+c=ca [cn}2) and by cooling the product rapidly so as to avoid the reversion of cyanide to cyanamide which occurs at 400-700 degrees. (u.s. patents 1,282,395, 1918 and 1,282,405, 1918). a product containing cyanide equivalent to 48-50% nacnn is obtained in this way which, under the name “ aero cyanide ” has found extensive use in the extraction of gold and silver and in the preparation of hydrocyanic acid for fumigation. much of the cyanide in this material appears to be present as the cal- cium salt which is readily hydrolysed, ca(cn).+2h,0 = 2hcn +ca(oh)s. for this reason powdered “ acro cyanide” has found some application as a fumigant where the residual lime is not harmful, since mere exposure to moist air liberates hydro- cyanic acid. (t. e.) cycle of trade: sce trade cycle. ~ cycling (see 7.682).—during the period following 1910 the evolution of the bicycle was not maintained, mechanical changes being less frequent and not so drastic as had been the case during the early pneumatic era. after 1918 interest in every branch of cycling increased to a marked extent, and this had its origin in the attitude taken up by the big cycle makers, who, desiring the freedom from trouble which mass production provides, discour- aged invention and development and allowed design to stagnate. business conditions —-thus an opportunity arose for the as- sembler or small maker, generally an active cyclist himself, and invariably a working mechanic. numbers of such men started in business, making a specialty of bicycles built to order and com- plying with the customer’s own specification. these assemblers purchased parts from the various houses specialising on the pro- duction of fittings for the trade and built them up into complete machines. under their aegis, supported by a growing demand from the large number of club riders of both sexes, the light roadster bicycle was evolved, a type which the large manufac- turer had persistently ignored for more than a generation. in addition to reducing the weight of his product, the small maker incorporated certain improvements which hard riders were quick to appreciate; among these were the drop-out rear wheel and the caliper brake, both of which, it is worth noting, were old ideas, the advantages of which had not been realised cycle of trade—cyprus when they originally appeared on the market. the light-weight bicycle with small (26 in.) wheels gave cycling a new lease of life; contemporaneously with its advent came a movement to popularise rational dress among lady cyclists. the fashion of wearing short skirts, aided by the adoption of breeches, by the many women who took up agricultural work during the war, enabled the bifurcated garment to overcome the prejudice and to score an almost complete triumph over the skirt. to-day nearly 90°% of club women and keen riders wear rationals for their more serious cycling, though skirts are retained for shopping expeditions and short journeys. many wearers of rational dress use diamond-framed machines. officials of the big factories were somewhat slow to note the tendency of demand, and allowed the small makers to obtain a ~ long lead in the production of light bicycles. ultimately they began to realise the position, and many of them designed light and speedy models to compete with the locally built machines. these may now be procured from most of the large manufac- turers, but the small assembler still enjoys the great advantage of being able to comply with the whims of his client, and the ability to build “ to order” without undue interruption of his works routine. another feature of the post-war period has been the revival of interest in tandem riding. his, again, has been fostered by the little maker with the encouragement of club riders. racing.—road racing has flourished exceedingly. in europe, america and australia competitions are run on ordinary racing lines, the riders starting together, or, in the case of handicaps, in the order of their time allowance, the first man to finish being the winner. in great britain and ireland the time trial system is in vogue, and has proved an extraordinary success. so great has the number of events become that, in 1922, the most promi- nent cycling clubs came together and formed the road racing council, to which 38 clubs are affiliated, and which has compiled regulations for the conduct of road trials. the most popular distances are 25, 50 and 100 m., while several races at 12 and 24 hours are organised each year. one of the best things the r.r.c. has done for the sport has been to limit the number of entrants for any race run under its jurisdiction, and in the more impor- / tant events not more than roo riders may take part. track racing continues to enjoy great popularity in france, germany, belgium, switzerland, holland, norway, denmark, america, italy and australia, but in great britain, spain, nor- way, south africa and certain other countries, the scarcity of up-to-date tracks prevents the development of this branch of the sport. the annual races for the championships of the world have been continued, the union cycliste internationale having taken over the series which were inaugurated by its predecessor, the international cyclists’ association. in 1921 the motor- paced race for the amateur title was dropped, and its place as a distance test was taken by a road race, the distance of which is usually from too to 120 miles. (i ws jag)