I/EC A
COSTS
W O R K B O O K
F E A T U R E
by Walter L Hardy, Foster D. Snell, Inc.
Hooker Type S cells in batteries like this are still preferred for small plants
Chlorine Manufacture Where power is cheap and fuel for caustic concentration expensive, mercury cells are preferable
• ROBABLY the most important single industrial gas is chlorine. It is used in its elementary form, as a disinfectant and bleach, as a raw material, in the synthesis of a wide range of chlorinated organic compounds, many of which, in turn are converted to nonchlorinated end products. About 15% of all chlorine produced is used in the pulp and paper industry; about 60% is consumed in chemical processes. Other end uses include water and sewage treatment, synthetic rubber, metal-
lurgy, bleaching, a n d military applications. T h e greater part of all chlorine produced is obtained as a coproduct of the electrolytic manufacture of caustic soda from sodium chloride. Caustic soda m a y be m a d e by reaction of Solvay process soda ash with slaked lime, when there a r e no immediate markets for chlorine. Chlorine m a y be m a d e from salt by the so-called nitrosyl process or from hydrochloric acid by oxidation. These processes a r e not more comm o n because of the nonavailability
of process know-how, high production cost, and, in t h e case of nitrosyl process, low d e m a n d for by-product sodium nitrate. In most cases, a prospective chlorine manufacturer must also face t h e problem of caustic soda sales a n d the disposal of by-product hydrogen. Frequently, caustic soda is the d e sired primary product and markets must be found or created for t h e by-product chlorine. I n either case, the availability or absence of cheap power, salt or brine, nearby m e a n s of transportation, and markets for VOL. 49, NO. 9
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SEPTEMBER 1957
55 A
I/EC
COSTS
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Λ Workbook
Feature
the products and by-products will determine whether a proposed plant can be operated economically. Most electrolytic chlorine is pro duced in either diaphragm or mer cury cells. A discussion of the several types of cells offered in each category is far beyond the scope of this column. T h e published litera ture includes excellent descriptive matter on electrolytic processes ( 7-4). Diaphragm cells utilize a porous diaphragm to separate a carbon anode and an iron cathode immersed
in brine. Chlorine is discharged at the anodes, rises through the brine, and is collected at the top of the cell. Sodium, liberated at the cathode, reacts with water instantly to form dilute caustic soda and free gaseous hydrogen. In the mercury cells, the liquid metal acts as the cathode. Liberated sodium forms an amalgam with the mercury during electrolysis. T h e amalgam is continuously re moved and reacts with water to form caustic soda; the freed mercury is returned to the cell.
Unit Requirements and Costs of a 100-Ton-Per-Day Chlorine Plant Unit Cost, J/Unit
Diaphragm Cells Lb. /ton Cl2 $/ton CI2
Mercury Cells $/ton Cls Lb./ton Cl2
MATERIALS
Salt Graphite Soda Hydrochloric acid (30%) Caustic soda Mercury Total
0.0052 0.4200 0.0300 0.0180 0.0290 3.3552
3,500 7 13 3 1
18.20 2.94 0.39 0.05 0.03
3 ,400 6 45 17 0 . 36»
$21.61
17.68 2.52 0.81 0.49 1.21 $22.71
UTILITIES
Power, kw.-hr. Steam, lb. Water, gal. Total
0.01 0.001 0.05/1000
3,040" 10,100" 31,000»
30.40 10.10 1.55 $42.05
3 500» 1 ,060» 3 ,260»
35.00 1.06 0.16 $36.22
LABOR
Man-hours Total
2.00
3.7°
7.40 $ 7.40
4. 8»
9.60 $ 9.60
MAINTENANCE
At 3% of total plant cost/year Total
$ 1.10 $ 1.10
$ 1.20 $ 1.20
0.25
1.S0
0.73
0.80
OVERHEAD''
Royalty Administrative expense at 2% of plant cost/year Estimated plant cost Diaphragm, $11,000,000 Mercury, $12,000,000 Depreciation, at 10% of plant cost/year Taxes and insurance at 2% of plant cost/year Total Subtotal
Literature Cited
3.66
4.00
0.73 $ 5.37 $77.53
0.80 $ 7.10 $76.83
$45.60 $31.93
$45.60 $31.23
CREDIT
1.14 tons caustic/ton Ch at 2/3 sales price of $60/ton Total Subtotal Estimated cost of purifying diaphragm caustic Estimated production cost per ton of chlorine
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Kobe, Κ. Α., "Inorganic Process In dustries," pp. 127-34, Macmillan, New York, 1948. (2) MacMullin, R. B., C/iem. Inds. 61, 4 1 50 (1947). (3) Shearon, W. H., Jr., Chrencik, Frank, Dickinson, C
L., I N D . E N G . CHEM.
40, 2002 (1948). (4) Shreve, R. N., "Selected Process In dustries," 2nd éd., p p . 303-8, McGraw-Hill, New York, 1950.
$ 9.50 $41.43
$31.23
• Units/ton. b Overhead and sales costs not itemized above are not considered in above estimate. No credit is included for by-product hydrogen and hydrogen chloride.
56 A
Each type of cell has advantages and disadvantages compared to the other which will influence its selec tion. This column is restricted to a discussion of the cost factors common to both types. T h e table presents material, utility, and labor requirements and costs for both diaphragm a n d mercury cells per short ton of chlorine for a 100-tonper-day chlorine plant. Actual re quirements vary slightly for different makes of cell in each category—the data given represent an average. After allowing credit for the esti mated sales value of by-product caustic soda, the production cost of 1 ton of chlorine is estimated to be $41, using diaphragm cells, and $31, using mercury cells. T h e cur rent sales price is $63 per ton of liquefied chlorine in tank cars. T h e estimated production cost figures are based on $10.40-per-ton rock salt prices and 1 cent per kw.hr. power rates. T h e cost of power represents over half of the estimated production cost. Cheap hydroelec tric or self-generated power will sig nificantly increase the profitability of a chlorine plant. T h e use of natural brines or captive salt deposits will further reduce production costs. Elimination of the charges for concentrating and purifying dia phragm caustic soda will make the operating costs for the two types of cell roughly comparable, where a dilute, regular grade of caustic soda is required. T h e mercury cell can produce high purity, concentrated caustic soda directly and is preferable where power is cheap and fuel for caustic concentration expensive.
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