Electrodes 3 feet in diameter, furnaces holding 150 tons of products reaching temperatures of 2000° C and operating at 5000 amperes, the noise, smoke, gases and high temperatures created rival an inferno and anyone standing near one of these monsters would be in awe, this is what I witnessed during my development.

Crushed ore, coke, steel, scrap, and various fluxing materials are added around the electrodes at the top of the furnace.  The heat of the electric arc and the resistance of this charge generates the intense heat needed to melt.  At these extremely high temperatures, the ore breaks down into its components and combines with the steel to form ferro-alloys.  Periodically, the molten alloy is tapped off from the bottom of the furnace.  The operation generally goes on 24 hours a day.  After being tapped from the furnace, the molten alloy is cooled and solidified into large flat slabs.  It is then crushed into various sizes in lumps weighing approximately 75 lbs. each.  Some may be crushed finer depending on the application.

 

The following alloys were made at Union Carbide Electromet Division: Ferro Chromium, Ferro Tungsten, Ferro Nickel, Ferro Silicon, Ferro Manganese, Ferro Vanadium, Ferro Titanium, and Ferro Molybdenum.

Ferro alloys are a combination of iron and the elements mentioned.  They are used as a ready source of introducing these elements into steel for the purpose of improving steel with respect to its physical properties.  It may sound unbelievable, but 90% of all domestic steel production is treated with some form of Ferro alloy. When operating at these temperatures a series of complex chemical reactions occur with carbon, in the form of coke, acting as a reducing agent and reacting with the metal oxides to form carbon dioxide or monoxide mixtures, and the molten metal which alloys (not a chemical combination) with molten iron. By carefully controlling the stochiometry  ferroalloys are produced.

Ferrosilicon is used in steel making to remove the oxygen which is dissolved in molten steel and which would cause imperfections in steel rails and ingots were it allowed to remain there.  When alloyed to the extent of 10 to 15 per cent, it confers upon iron the property of resisting acids.  Introduced into the steel in lower percentages, it confers upon it unusual electrical characteristics, and the silicon steels are found as a constituent part of our electrical transformers.  The raw materials are silica sand (silicon oxide), iron, and carbon, which at the temperature of the furnace react so that the carbon reacts  with oxygen from the silica, producing silicon metal alloys with the molten iron present.  When the furnace is tapped, the melted ferrosilicon flows out.

Silicon is a brittle solid of silver gray luster, melting around 1400° C.  Large quantities are used as a substitute for carbon when a ferro-alloy of little carbon content is desired.

In a similar manner, ferromanganese is produced by reaction in the electric furnace between manganese and iron ores and carbon; and manganese metal from high manganese ores and carbon, or by the interaction of high manganese ores and silicon.  Ferromanganese enters almost all our steel, the low carbon variety being an essential constituent of the high chromium stainless steels.  The railroads depend upon manganese rail steel.

Ferrochromium, extensively employed in the manufacture of heat,chemical and water resistant chromium alloys, (Stainless Steel) chromium-nickel, chromium-molybdenum, chromium-vanadium steels so necessary for our machines and automobiles, and the corrosion resistant alloys so widely used in our architecture and industries, is made from chrome-iron ore and carbon, or from chrome-iron ore and silicon.  Chromium metal, needed for the manufacture of non-scaling alloys subjected to high temperatures, resistance wires, and alloys for electrical heating appliances, is made by the reduction of chromium ores with silicon.

Tungsten, in the form of ferrotungsten, is an important constituent of our high speed tool steels.  It is made from iron and tungsten ores reacting with carbon in the electric furnace.  The carbon removes the oxygen from the tungsten and iron compounds, and the liberated metal alloy is formed.

 Similarly ferromolybdenum,  a necessary constituent of our aircraft and automotive steels.

Ferrovanadium, which imparts to steel the property of resisting shock and vibration, are made from molybdenum ores and carbon or vanadium ores and carbon.

The electric furnace produces many other alloys, all of which have special characteristics and uses.  The list is long and includes zirconium, zirconium-ferrosilicon, silicon-zirconium, calcium-silicon, ferrotitanium, ferrophosphorous, ferro-uranium, silico-manganese, each of which in turn contribute as some essential part of our present day complex and fast-moving civilization which is so dependent upon performance utilizing alloy steels.

The industrial history of the ferro-alloys and calcium carbide in the early stages was closely connected.  A number of organizations made carbide and ferro-alloys.  In the United States Electromet is the largest carbide maker and also the largest ferro-alloy manufacturer in the world.

An electric arc steel furnace.  Notice there are three electrodes, 20-inch diameter on the cover and the furnace is tilted for pouring.

 

Synopsis

Calcium carbide is manufactured industrially by reacting highly purified calcium oxide (lime) with coke, in an electric arc reduction furnace at 2000-2200 OC.  The calcium carbide so produced as a molten liquid, cooled in molds and removed in blocks.  The energy requirements are massive 3,100 kW hours per ton, three phase power up to 70 MW.  The furnaces described herein are enormous, nine stories high, containing  three massive electrodes each 20-22inches in diameter operating at  three phase power at 70 MW.  When calcium carbide is reacted with water, in a controlled manner, acetylene gas is generated,  purified, and piped to various plants in the Niagara Falls area.  By-product from this operation is carbon monoxide which is piped to other plants.

Furnace charge:

 Weigh hoppers beneath the coke and the limestone storage bins discharge to converging belt conveyors which mix the lime-coke charge in transit to the carbide furnaces.  Proportions are controlled by adjusting the speed of the belts to maintain 10 to 15% excess of lime over stoichiometric proportions of 56 parts of lime to 36 parts of coke.  The exact proportions are not critical, and experienced operators can control the mix by visual observation.  From the mixing belt the charge is picked up by bucket elevators which carry it to feed hoppers located above the carbide furnaces.

Furnaces.  Furnaces of various sizes have been used at Niagara Falls and some of the smaller ones are still occasionally pressed maintained by a battery of two 20,000-kw. furnaces and two 10,000-kw. furnaces.  German plants have installed furnaces up to 30,000-kw. capacity.  However, EM engineers believe that over-all efficiency drops off somewhere above 20,000-kw. Although the furnaces in this plant are essentially square, circular furnaces with the electrodes introduced in a triangular pattern and elliptical furnaces with in-line electrodes are used.  Changes in design are made largely for construction considerations.  These do not affect the performance of the furnace since most of the reaction takes place within 1 foot of the electrodes.

The Niagara Falls furnaces are essentially firebrick-lined steel boxes with a taphole about 18 inches from the bottom on one side.  The firebrick floor of the furnace is covered with 3 feet of electrode carbon blocks cemented with pitch.  This is the only refractory known which will stand the high temperatures of the operation combined with the high alkalinity of the molten lime.  The sides of the furnaces are not subjected to such vigorous conditions since they are effectively insulated by a non-reacting mass of charge and product. Nine inches of common firebrick serve for these surfaces.

The external dimensions of the two larger furnaces are 41 × 14.5 feet at the top and 29 × 11 feet at the bottom; they are 18 feet high.  The smaller ones are 21 × 27 feet at the top, tapering to 17 × 25 feet at the bottom, and 11 feet high.  In a newly bricked furnace the taphole is about 6 inches in diameter.  However, in use the carbon blocks surrounding the taphole gradually burn away.  The lip of the taphole is a water-cooled casting.

Electrodes.  Each furnace is equipped with three composite electrodes.  Each electrode in the large furnaces consists of five 20 × 22 inch rectangular electrolytic-carbon rods which are held in a line to give a composite cross section of 20 × 110 inches.  The electrodes in the smaller furnaces are constructed of four of these components, similarly arranged.  The rods are attached to the water-cooled, cast-bronze header by means of two horizontal water-cooled bolts.  The components are constructed with a threaded cavity 8 inches in diameter and 10 inches deep at the top and bottom.  In making up a new electrode a graphite threaded plug is inserted for half its length into the tapped hole in the bottom of the new rod.  The remainder of the plug is then screwed into the hole in the top of a stub removed from the burned out electrode.  In this manner the electrodes can be used completely.

However, it is essential that there be no air space between the plug and the rod or the electrode will overheat and break off.  The possibility is avoided by very careful machining of both the plug and the tap.

Paste cannot be depended on to complete the contact in the threaded section although it is sometimes used between the abutted ends of the carbon rods.  The upper half of a new electrode assembly is protected by a thin layer of ordinary cement held in place by 1-inch chicken wire.  This prevents oxidation or burning of the portion of the electrode which is not initially submerged in the furnace charge.

Both the water and the electrical connections to the electrodes are flexible to permit the vertical adjustment of the electrodes.  An automatic mechanism operating a cable drum over the furnace adjusts the height of the electrodes to maintain a constant resistance through the reaction bed.  Under normal conditions 1 to 3 feet of the electrodes are buried in the charge.  If the resistance in the bed increases the electrodes are lowered so as to present more surface to the reactants.  If the resistance of the bed is reduced, usually because of an excess of carbon in the charge, the electrodes will rise and if there is too great an excess of carbon they will “ride out” of the bed completely.  When this condition appears imminent additional lime is shoveled directly onto the top of the bed around the electrodes.  As the bed settles, the lime increases the resistance around the electrodes and allows the electrodes to drop back into normal position.  This condition is somewhat chronic, and it is usually necessary to add lime to the electrode area regularly throughout the melt.

The electrodes in the furnace are gradually burned off at the bottom and must be replaced after about 170 hours of operation.  When such replacement is necessary the electrode assembly is lifted out of the furnace, disconnected from the cooling water and power lines, and replaced by a new assembly.

Although the entire electrode assembly weighs about 16 tons permanent cranes and quick-acting connections enable two workmen to change an electrode in less than 20 minutes.  When new electrodes are introduced into the furnace they are slowly lowered under manual control, with the power on, until they strike an arc through the reactant bed; this avoids the power surge which would occur if the electrodes were allowed to come into the bed by means of automatic control.

Another electrode system is used at the TVA plant in which sliding contacts replace the header used at Niagara Falls.  This arrangement permits the addition of electrode components without removing the assembly from the furnace.

Componental electrodes are gradually being replaced throughout the carbide industry with continuous electrodes of the Söderberg type, and Carbide plans to eventually make the change in their works.  In the Söderberg electrode a paste of buckwheat-sized carbonized anthracite, coke fines, and tar is packed into the top of a 40-foot, thin steel tube.  The entire tube serves as an electrode and is fed into the furnace as it burns away from the bottom.  As the paste moves closer to the surface of the charge it is slowly baked so that when it has reached the 6-foot section below the sliding electrical contacts it has great mechanical strength.  As the electrode is consumed additional tubing is welded to the top and filled with the carbon paste.  Suspension is by means of steel straps progressively welded to the sides of the tube and reeled out as required to feed new electrode into the furnace.  The entire assembly is raised and lowered by an automatic mechanism similar to that used with componental electrodes to maintain constant amperage in the furnace.

Gas Collection.  At the plant four water-cooled refractory hoods extend over the bed of the furnace between and alongside of the electrodes.  These hoods are connected to a common collector outlet which leads to a scrubber.  Water sprays playing on a high speed propeller fan in the scrubber ensure that solid particles entrained in the gas stream are wetted and carried off with the water.  The fan is designed so that it provides almost no suction at the hoods since suction at this point would act as a forced draft on the furnace and burn up some of the coke before it could react.  The gas discharged from the scrubber is mostly carbon monoxide and hydrogen and is fed into the lime kilns to augment the coal flame.  Only a trace of oxygen and carbon dioxide is present.

The gas collection system handles about 1000 cubic feet of gas per minute from each of the large furnaces; this represents almost all the off-gases.  The small amount that leaks around the hoods escapes through roof vents over the furnaces.

In recent years many closed carbide furnaces have been built, mostly of the circular design mentioned.  This design eliminates the need for gas-collecting devices but does not appear to enhance the heat efficiency of the installation appreciably, since a negligible percentage of the energy applied to the system is lost as radial heat.

Furnace Operation.  The lime and coke charged to the furnace is introduced from the feed hoppers by long tubes, 8 inches in internal diameter, which hang between the electrodes and alongside the outside electrode.  The center feeders have roof-shaped deflectors and the outer ones single-pitch deflectors, which throw the charge against the side of the electrodes.  Curved slide valves between the feeders and the feed hoppers are closed when the feed pipe is vertical and open when it is moved forward or back.  When the operator wants to charge to the furnace he hooks a long rod onto the end of the feeder tube and pushes it back and forth, thus opening the valve and disturbing the charge along the electrodes at the same time.  The furnace charge must be augmented in this fashion every 5 to 10 minutes.

As the furnace continues in operation fused solid impurities in the charge gradually build up a layer on the furnace floor.  Once this layer rises above the level of the taphole it is usually necessary to shut down the furnace and clean it out.  A hump of solid calcium carbide also often forms at the back of the furnace opposite the taphole.  This hump tends to grow slowly until it obstructs the flow of the molten product from the taphole.  When either of these conditions necessitate a shutdown the electrodes are lifted from the furnace and charge is allowed to cool from 4 to 7 days.  The cooled charge is almost a solid mass which must be broken up with air hammers and removed by crane bucket.  Since it is impractical to wait until the entire mass has cooled the hammer operators wear heavy wooden clogs and work down into the furnace as fast as the temperature permits.  Fortunately, this operation is only necessary about every 3 years.

Flow through the taphole is maintained by ramming the charge with 1-inch steel rods about 15 feet long.  The rods can be inserted in the bed for about 30 seconds before they become so flexible that they are ineffective.  However, the rods can be removed, cooled in the air, and then re-used.  If the flow becomes excessively sluggish the taphole is “needled”; the needle is a graphite electrode 4 inches in diameter and 4 feet long connected to one phase of the power source and suspended from an overhead running hanger.  When it is inserted in the taphole it strikes an arc through the charge to the main electrodes and raises the temperature in the vicinity of the taphole.  When it is desired to shut off the flow from the taphole, the operator merely shovels about a bushel of cold carbide into the opening.  This treatment produces sufficient local cooling to stop the flow of molten carbide.  To reinstitute the flow the taphole is opened by use of the “needle.”

A potential of about 200 volts and as much as 70,000 amperes across the electrodes produces a temperature of 2000° to 2200° C. in the molten charge of the furnace, although in the actual heating arc much higher temperatures are attained.  Under these conditions the furnace product contains about 90% calcium carbide although 95% pure product can be made under ideal conditions.  The two large furnaces discharge continuously at a rate of about 6 tons per hour into a bucket conveyor.  This conveyor doubles back on itself to provide air cooling for the carbide.  Preliminary cooling reduces the temperature from 4000° to 300° F.

The smaller furnaces follow an older procedure whereby the charge is tapped intermittently into cooling cars which hold approximately 2000 pounds.  These cars are about 2 ½ feet wide by 4 feet long by 3 feet high, constructed of 2-inch plates of cast steel.  When the product is handled in this manner cooling must be accomplished by storing the cars in an open-sided shed for about 24 hours.

Coke

Carbon for the carbide reaction at Niagara Falls is obtained from various sources.  Most of it is obtained as metallurgical grade coke although a high degree of mechanical strength is not essential.  At present the coke supply comes from vertical retorts or beehive ovens.  However, there is apparently no reason why coke made in other types of ovens, which can process expanding coals not suitable for metallurgical coke, could not be used if it were available.  Some work has also been done which indicates that so-called low temperature char could be used.  Charcoal has been used, but its low density makes it unsuitable for furnace charging.  Actually any sort of carbonized material having a low ash content and low electrical conductivity is suitable.  As coking grade coals become increasingly scarce, it is probable that some substitutes will be used.

Carbonaceous materials other than coke are presently added to the charge of the carbide furnaces.  The addition of petroleum coke reduces the conductivity and ash content of the reaction bed.  The use of petroleum coke is limited almost entirely by its availability.  Percentages up to 40% have been used; however, Cyanamid currently adds something less than 10% depending upon the supply situation.  Raw anthracite coal may also be used in the charge since it has a high carbon content and relatively low conductivity.  The Niagara Falls plant sometimes uses as much as 10% of this material in the carbon charge, again depending on its availability.

Coke, as it is dropped from the hopper ears, passes between toothed rolls with a clearance of 2 ½ inches between the rolls.  Rubber belt conveyors then carry it through a series of three similar mills which reduce the particle size to an ultimate maximum of 1 inch.  After leaving the fourth mill the coal is screened, and the material over 1 inch is recycled to the feed of the No. 1 mill.  The properly sized material goes to two parallel rotary dryers, heated by a countercurrent stream of flue gas from a pulverized coal fired combustion chamber.  The coke is discharged with a moisture content of less than 1%.  The dry material is screened again and conveyed to one of nine 40-ton storage hoppers.  These are the first holding hoppers in the coke preparation mill.  All coke must either be processed directly from the railroad ears to this point or stored on outside piles as it is received.

Lime

Lime for the carbide furnaces is produced at Niagara Falls by burning the limestone from quarries at Beachville, Ontario, 100 miles from Niagara Falls.  The material is quality-controlled at the quarries and crushed to size there.  Both 2-inch and 1-inch limestone are received at the plant.  These two size grades are blended, after burning, using 60% of the larger size.

As the limestone is received in the pits below the unloading tracks it is conveyed to seven 240-ton hoppers.  From these hoppers a belt conveyor feeds seven rotary kilns, each with a capacity of 100 tons of lime per 24-hour day.  The kilns are heated directly by the combustion of about 50 pounds of pulverized coal per minute.  The Niagara Falls kilns are 125 feet long.  Practice in recent years has tended toward longer lime kilns – up to 300 feet.  These longer kilns improve the heat efficiency of the operation as much as 50%, requiring perhaps 6,000,000 B.t.u. to burn a ton of lime as compared with 9,000,000 B.t.u. required by the Niagara Falls kilns.  However, exhaust gas from the kilns at Niagara Falls is passed through waste heat boilers which reduce its temperature from 1100° to 450° F.  The discharge from the boilers along with some uncooled gas is then used in the tunnel dryers in the briquetting department.  Another portion of the off-gases is used in spray-drying operations in another department.  These procedures result in heat efficiencies comparable to those which would be obtained from longer kilns.

The discharge from all of the kilns falls into a single bucked conveyor which carries the burnt lime into a rotary cooler.  This cooler was constructed by Cyanamid engineers of two concentric cylinders with an annulus of 12 inches.  Six longitudinal plates support the central cylinder and divide the annulus into six separate compartments to reduce breakage of the lime.  Water running over the outside cylinder and air passing through the center reduces the temperature of the lime from 1000° to 300° F.  A steel apron conveyor receives the cooled material and carries it to a screen with 0.090-inch openings.  The material that passes through the screen is conveyed to the briquetting department, the oversized material going to storage hoppers with a combined capacity of 800 tons.  About 2 to 5% of unburned calcium carbonate is left in the kiln product to produce optimum efficiency in the carbide furnaces.

Briquetting

Very fine particles of lime or coke cannot be charged to the carbide furnaces because they tend to clog the charge and may result in explosive release of product gases.  They may also reduce resistance of the bed and cause the electrodes to ride up out of the charge and result in very inefficient operation.  However, about 10% of fine material (under 0.1 inch) is produced in the coke milling and the screening of the burnt lime produces about 7 tons of dust per kiln per day.  This material is collected from the screens collectors for reclaiming; it is combined in the proportions of 2 parts of carbon to 3 parts of lime and made into a paste by the addition of about 10% of lime slurry.  The paste is extruded as a slab, 2 × inches, onto a belt conveyor.  As it travels along the belt it is separated into 2- to 3-foot lengths and scored with knives into 2 × 3 inch rectangles.  The scored slabs on sheet-metal trays are placed in rolling racks, holding about 50 slabs, and rolled into one of three tunnel dryers.  Lime kiln exhaust gases which have passed through a dust collector provide the heat for the dryers.  The temperature of the dryer is held at 350° to 400° F. by adjusting the proportion of gas taken directly from the kilns to that taken from the waste heat boiler.  The briquets pass through the dryer in about 8 hours, are dumped into transfer cars, and taken to the furnace feed conveyor belt where they will be introduced into the furnace charge.

Calcium Carbide Production by Electrometallurgy

Weigh hoppers beneath the coke and the limestone storage bins discharge to converging belt conveyors which mix the lime-coke charge in transit to the carbide furnaces.  Proportions are controlled by adjusting the speed of the belts to maintain 10 to 15% excess of lime over stoichiometric proportions of 56 parts of lime to 36 parts of coke.  The exact proportions are not critical, and experienced operators can control the mix by visual observation.  From the mixing belt the charge is picked up by bucket elevators which carry it to feed hoppers located above the carbide furnaces.

Furnaces.  Furnaces of various sizes have been used at Niagara Falls and some of the smaller ones are still occasionally pressed into service in emergencies.  However, primary production is maintained by a battery of two 20,000-kw. furnaces and two 10,000-kw. furnaces.  German plants have installed furnace3s up to 30,000-kw. capacity.  However, Cyanamid engineers believe that over-all efficiency drops off somewhere above 20,000-kw.  Although the furnaces in this plant are essentially square, circular furnaces with the electrodes introduced in a triangular pattern and elliptical furnaces with in-line electrodes are used.  Changes in design are made largely for construction considerations.  These do not affect the performance of the furnace since most of the reaction takes place within 1 foot of the electrodes.

The Niagara Falls furnaces are essentially firebrick-lined steel boxes with a taphole about 18 inches from the bottom on one side.  The firebrick floor of the furnace is covered with 3 feet of electrode carbon blocks cemented with pitch.  This is the only refractory known which wills stand the high temperatures of the operation combined with the high alkalinity of the molten lime.  The sides of the furnaces are not subjected to such vigorous conditions since they are effectively insulated by a nonreacting mass of charge and product.  Nine inches of common firebrick serve for these surfaces.

The external dimensions of the two larger furnaces are 41 × 45.5 feet at the top and 29 × 11 feet at the bottom; they are 18 feet high.  The smaller ones are 21 × 27 feet at the top, tapering to 17 × 25 feet at the bottom, and 11 feet high.  In a newly bricked furnace the taphole is about 6 inches in diameter.  However, in use the carbon blocks surrounding the taphole gradually burn away.  The lip of the taphole is a water-cooled casting.

Electrodes.  Each furnace is equipped with three composite electrodes.  Each electrode in the larger furnaces consists of five 20 × 22 inch rectangular electrolytic-carbon rods which are held in a line to give a composite cross section of 20 × 110 inches.  The electrodes in the smaller furnaces are constructed of four of these components, similarly arranged.  The rods are attached to the water-cooled, cast bronze header by means of two horizontal water-cooled bolts.  The components are constructed with a threaded cavity 8 inches in diameter and 10 inches deep at the top and bottom.  In making up a new electrode a graphite threaded plug is inserted for half its length into the tapped hole in the bottom of the new rod.  The remainder of the plug is then screwed into the hole in the top of a stub removed from the burned out electrode.  In this manner the electrodes can be used completely.

However, it is essential that there be no air space between the plug and the rod or the electrode will overheat and break off.  The possibility is avoided by very careful machining of both the plug and the tap.

Paste cannot be depended on to complete the contact in the threaded section although it is sometimes used between the abutted ends of the carbon rods.  The upper half of a new electrode assembly is protected by a thin layer of ordinary cement held in place by 1-inch chicken wire.  This prevents oxidation or burning of the portion of the electrode which is not initially submerged in the furnace charge.

Both the water and the electrical connections to the electrodes are flexible to permit the vertical adjustment of the electrodes.  An automatic mechanism operating a cable drum over the furnace adjusts the height of the electrodes to maintain a constant resistance through the reaction bed.  Under normal conditions1 to 3 feet of the electrodes are buried in the charge.  If the resistance in the bed increases the electrodes are lowered so as to present more surface to the reactants.  If the resistance of the bed is reduced, usually because of an excess of carbon in the charge, the electrodes will rise and if there is too great an excess of carbon they will “ride out” of the bed completely.  When this condition appears imminent additional lime is shoveled directly onto the top of the bed around the electrodes.  As the bed settles, the lime increases the resistance around the electrodes and allows the electrodes to drop back into normal position.  This condition is somewhat chronic, and it is usually necessary to add lime to the electrode area regularly throughout the melt.

The electrodes in the furnace are gradually burned off at the bottom and must be replaced after about 170 hours of operation.  When such replacement is necessary the electrode assembly is lifted out of the furnace, disconnected from the cooling water and power lines, and replaced by a new assembly.

Although the entire electrode assembly weighs about 16 tons permanent cranes and quick-acting connections enable two workmen to change and electrode in less than 20 minutes.  When new electrodes are introduced into the furnace they are slowly lowered under manual control, with the power on, until they strike an arc through the reactant bed; this avoids the power surge which would occur if the electrodes were allowed to come into the bed by means of the automatic control.

INSERT PICTURE OF CARBIDE FURNACE HERE

Another electrode system is used at the TVA plant in which sliding contacts replace the header used at Niagara Falls.  This arrangement permits the addition of electrode components without removing the assembly from the furnace.

Componental electrodes are gradually being replaced throughout the carbide industry with continuous electrodes of the Söderberg type, and Cyanamid plans to eventually make the change in their works.  In the Söderberg electrode a paste of buckwheat-sized carbonized anthracite, coke fines, and tar is packed into the top of a 40-foot, thin steel tube.  The entire tube serves as an electrode and is fed into the furnace as it burns away from the bottom.  As the paste moves closer to the surface of the charge it is slowly baked so that when it has reached the 6-foot section below the sliding electrical contacts it has great mechanical strength.  As the electrode is consumed additional tubing is welded to the top and filled with the carbon paste.  Suspension is by means of steel straps progressively welded to the sides of the tube and reeled out as required to feed new electrode into the furnace.  The entire assembly is raised and lowered by an automatic mechanism similar to that used with componental electrodes to maintain constant amperage in the furnace.

Gas Collection.  At the North American Cyanamid plant four water-cooled refractory hoods extend over the bed of the furnace between and alongside of the electrodes.  These hoods are connected to a common collector outlet which leads to a scrubber.  Water sprays playing on a high speed propeller fan in the scrubber ensure that solid particles entrained in the gas stream are wetted and carried off with the water.  The fan is designed so that it provides almost no suction at the hoods since suction at this point would act as a forced draft on the furnace and burn up some of the coke before it could react.  The gas discharged from the scrubber is mostly carbon monoxide and hydrogen and is fed into the lime kilns to augment the coal flame.  Only a trace of oxygen and carbon dioxide is present.

The gas collection system handles about 1000 cubic feet of gas per minute from each of the large furnaces; this represents almost all the off-gases.  The small amount that leaks around the hoods escapes through roof vents over the furnaces.

In recent years many closed carbide furnaces have been built, mostly of the circular design mentioned.  This design eliminates the need for gas-collecting devices but does not appear to enhance the heat efficiency of the installation appreciably, since a negligible percentage of the energy applied to the system is lost as radiant heat.

Furnace operation.  The lime and coke charged to the furnace is introduced from the feed hoppers by long tubes, 8 inches in internal diameter, which hang between the electrodes and alongside the outside electrode.  The center feeders have roof-shaped deflectors and the outer ones single-pitch deflectors, which throw the charge against the side of the electrodes.  Curved slide valves between the feeders and the feed hoppers are closed when the feed pipe is vertical and open when it is moved forward or back.  When the operator wants to charge to the furnace he hooks a long rod onto the end of the feeder tube and pushes it back and forth, thus opening the valve and distributing the charge along the electrodes at the same time.  The furnace charge must be augmented in this fashion every 5 to 10 minutes.

As the furnace continues in operation fused solid impurities in the charge gradually build up a layer on the furnace floor.  Once this layer rises above the level of the taphole it is usually necessary to shut down the furnace and clean it out.  A hump of solid calcium carbide also often forms at the back of the furnace opposite the taphole.  This hump tends to grow slowly until it obstructs the flow of the molten product from the taphole.  When either of these conditions necessitate a shutdown the electrodes are lifted from the furnace and charge is allowed to cool from 4 to 7 days.  The cooled charge is almost a solid mass which must be broken up with air hammers and removed by crane bucket.  Since it is impractical to wait until the entire mass has cooled the hammer operators wear heavy wooden clogs and work down into the furnace as fast as the temperature permits.  Fortunately, this operation is only necessary about every 3 years.

Flow through the taphole is maintained by ramming the charge with 1-inch steel rods about 15 feet long.  The rods can be inserted in the bed for about 30 seconds before they become so flexible that they are ineffective.  However, the rods can be removed, cooled in the air, and then re-used.  If the flow becomes excessively sluggish the taphole is “needled”; the needle is a graphite electrode 4 inches in diameter and 4 feet long connected to one phase of the power source and suspended from an overhead running hanger.  When it is inserted in the taphole it strikes an arc through the charge to the main electrodes and raises the temperature in the vicinity of the taphole.  When it is desired to shut off the flow from the taphole, the operator merely shovels about a bushel of cold carbide into the opening.  This treatment produces sufficient local cooling to stop the flow of molten carbide.  To reinstitute the flow the taphole is opened by use of the “needle”.

A potential of about 200 volts and as much as 70,000 amperes across the electrodes produces a temperature of 2000° to 2200° C in the molten charge of the furnace, although in the actual heating arc much higher temperatures are attained.  Under these conditions the furnace product contains about 90% calcium carbide although 95% pure product can be made under ideal conditions.  The two large furnaces discharge continuously at a rate of about 6 tons per hour into a bucket conveyor.  This conveyor doubles back on itself to provide air cooling for the carbide.  Preliminary cooling reduces the temperature from 4000° to 300° F.

The smaller furnaces follow an older procedure whereby the charge is tapped intermittently into cooling cars which hold approximately 2000 pounds.  These cars are about 2 ½ feet wide by 4 feet long by 3 feet high, constructed of 2-inch plates of cast steel.  When the product is handled in this manner cooling must be accomplished by storing the cars in an open-sided shed for about 24 hours.

Carbide Milling.  Carbide from the cooling hopper or from a conveyor running from a pit into which the chill cars are dumped is fed into a jaw crusher with a jaw clearance of 6 inches.  A bucket conveyor distributes the jaw crusher discharge between two cone crushers which further reduce the size of the carbide to a maximum of 1 inch.  The 1-inch carbide is collected in a feed hopper.  This hopper, together with similar bins for fluorspar and recycled +6-mesh calcium cyanide, feed though variable speed screw conveyors to a ball mill which ensures an intimate mixture of the components while pulverizing them to –100 mesh.  The mill is divided into three chambers having progressively smaller steel balls.  Because atmospheric moisture will react with the carbide to form explosive acetylene, the carbide mills and the conveyors are blanketed with nitrogen to exclude both moisture and oxygen.  This nitrogen is supplied at ½ inch of water pressure from the liquid air machines that serve the calcium cyanamide ovens.