I/EC A
COSTS
W O R K B O O K
FEATURE
by Walter L. Hardy, Foster D. Snell, Inc.
Costs of Producing Sodium Cyanide Material balances and comparative costs are discussed for two methods of sodium cyanide production
is well known to the layman as the preferred poison in innumerable mystery novels. However, only a small part of the 100 to 150 ' million pounds of sodium cyanide consumed annually in the United States is used in this manner. The greater portion, by far, finds employment as a fumigant, in the refining of gold, silver, zinc, and lead, as a dye intermediate, and for surface hardening of steels. Sodium cyanide could also serve as a source of hydrogen cyanide which is employed in the manufacture of acrylonitrile, adiponitrile, and the acrylate resins. Most of the hydrogen cyanide required for these OODIUM CYANIDE
products, however, is now produced directly by reaction of hydrocarbons and ammonia. Most new cyanide capacity now being installed or planned is designed to produce hydrogen cyanide gas. As a result, national sodium cyanide requirements exceed domestic capacity. Although it is relatively simple to make sodium cyanide by reaction of hydrogen cyanide gas with caustic soda, the cost of this type of processing is such that it has been found economical to import from 8 to 12 million pounds of sodium cyanide per month from abroad. Sodium cyanide may be produced by several methods other than by the NH,
BAG CYCLONE FILTER
I
SODIUM OR SODA ASH
GAS COOLER
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L_KO>
reaction of hydrogen cyanide and sodium hydroxide. Of these, two are considered here. The best known of these is the reaction of metallic sodium, ammonia, and carbon to form sodium cyanide and hydrogen gas. The other, not believed to be in use in this country, involves the reaction of sodium carbonate, ammonia, and carbon to produce sodium cyanide, hydrogen gas, and carbon monoxide. The reaction is endothermic. In the metallic sodium process prepared charcoal is slowly added to molten sodium in a steel vessel. Ammonia is introduced gradually until the reaction is complete. A
I.
N5
I
l|
COMPRESSOR
PRODUCT TO MOLDS
et
4É
BY-PRODUCT GASES
REACTION FURNACE
FILTER FURNACE
CHARCOAL· TREATING GRINDING Flow diagram for production of sodium cyanide VOL. 48, NO. 12
·
DECEMBER 1956
41 A
I/EC
COSTS
»
A Workbook
terial Balances Metallic Sodium Process
Sodium Carbonate Process
Materials Required per Ton of Sodium Cyanide Produced (Pounds) Sodium 940 Sodium carbonate ... 2400 Charcoal 520 1300 Ammonia 695 1200 By-Products Obtained per Ton of Sodium Cyanide Produced Carbon monoxide 1720 Hydrogen 210 120
temperature of about 1500° F. is maintained in the reactor. The reaction actually takes place in two steps, although these are combined in practice: 2 N a + 2NH 3 2NaNH 2 + 2C
> 2NaNH 2 + H 2
(1)
>• 2NaCN + 2H 2
Table II.
(2)
Feature
The product sodium cyanide is removed as a liquid and cast into desired shapes or flaked. In the sodium carbonate method soda ash and treated charcoal react in the same manner with ammonia at about 2000° F. Heat is added by direct firing of the reaction vessel with fuel or recycled by-product gases. When the reaction is complete, the resulting mass is filtered and the cleaned molten sodium cyanide is cast or flaked to its final form. The process reaction is : Na2COa + 4C + 2NH3 > 2NaCN + 3CO + 3H2 Nitrogen gas may be used to purge the reaction system and to blow the molten cyanide from the reactor to the filter. The gases evolved may be used for ammonia or methanol synthesis, if such facilities are located
Estimated Cost of Erected Plani
(3 tons of sodium cyanide per day) Metallic Sodium Process Raw materials and by-product storage $22,500 Reaction pots and supports 39,200 Charcoal treating unit 9,500 Dust collectors, bins, etc. 6,000 Molding equipment 4,800 Total erected equipment cost $ 82,000 Land, 1 acre at S1000/acre $ 1,000 Building, including foundations, excavation, utility lines, etc. 30,000 Railroad siding 6,000 Total building and land cost $ 37,000 Total erected plant cost $119,000
Table III.
Sodium Carbonate Process $ 32,200 68,000 15,800 9,700 4,800 $130,500 $ 1,000 37,000 6,000 $ 44,000 $174,500
Estimated Production Cost
(3 tons of sodium cyanide per day, 24 hours per day, 260 days per year) Per Day Metallic sodium Sodium carbonate process process Labor, excluding maintenance labor $ 76.80 Unskilled at «1.60 hour $ 38.40 100.80 100.80 Semiskilled at $2.10/hour 24.00 24.00 Supervision at $3.00/hour 40.32 32.64 Reserve at 20% Total labor $195.84 $241.92 Utilities Fuel at $0.80/million B.t.u. $ 72.00 $ 8.00 Power at $0.015/kwh. 36.00 24.00 9.00 8.00 Steam at $1.00/1000 lb. $117.00 Total utilities $ 40.00 Maintenance, including maintenance labor at 5% $ 33.80 $ 23.00 of total erected plant cost Depreciation at 10% of total erected plant cost $ 67.60 $ 46.00 Other, to cover taxes, insurance, etc., at 4% of erected plant cost $ 27.00 $ 19.20 Total daily processing cost $324.04 $487.32 $ 0.0811 Processing cost/pound product $ 0.0540 0.0535 Material cost/pound product 0.1015 Total production cost per pound of sodium cyanide $ 0.1555 $ 0.1346
42 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
nearby. They may also be recycled in part as fuel for the reactor. Charcoal treatment for both processes is identical and consists of heating to redness to eliminate moisture, oxygen, and oxygen compounds. The treated charcoal is cooled and ground before it is added to the reaction vessels. Material balances for the two processes are given in Table I. Raw material cost will be about $122 per ton of sodium cyanide for the sodium carbonate process against about $208 for the metallic sodium process. If the gaseous by-products are credited at fuel rates of 80 cents per million B.t.u., net raw material costs will be $107 per ton of sodium cyanide for the sodium carbonate process and $203 per ton of sodium cyanide for the metallic sodium process. The two processes are compared below for a capacity of 3 tons of sodium cyanide per day. Estimated erected plant costs are given in Table II. The smaller volume of reactants and by-product gases involved in the metallic sodium process bring the fixed investment down to about $119,000 as compared to an estimated $174,500 for the sodium carbonate process. Production costs for the two processes are estimated in Table I I I . These are seen to be 5.4 cents per pound of sodium cyanide for the sodium process and 8.1 cents per pound of sodium cyanide when sodium carbonate is the starting material. However, the higher processing cost in the soda ash process is more than offset by the savings in raw material cost. With sodium cyanide now selling for 17.8 cents a pound in carload quantities, the profitability of a small plant utilizing metallic sodium is low and it is understandable that much of our sodium cyanide needs are imported. The soda-ash process, however, shows greater potential, with a gross profit of about 4 cents per pound for a 3-ton-per-day plant. This is equal to $62,400 per year. Larger plants would have improved profit possibilities, because production costs would be lower. They would, as well, make more feasible the sale of by-product gases at higher than fuel rates for ammonia or methanol synthesis.
