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Vol. 16, No. 10
The Canadian Salt Company’s Processes for the Manufacture of Alkali-Chlorine Products’ By D. A. Pritchard and G . E. Gollop THEC A N A D I A N
FIG.1 - s A N D W I C H
SALT
W O R K S OF
C O . , LTD., W I N D S O R ,
ONT.
T H G C A N A D I A N S A L T C O M P A N Y , LTD
H E R E are two factors of great importance that govern the selection of a site for an electrolytic alkali-chlorine plant. These are cheap salt and cheap power. Underlying the southwestern section of Ontario and portions of Michigan are salt deposits of vast extent in the salina formation of the Silurian Age. The approximate area of this salt basin in Ontario is about 3000 square miles. The salt beds vary in thickness, the upper and thinner layers being interstratified with dolomite and shale. In the salt beds at Windsor a log of one of our wells shows the so-called “deep salt” to be 230 feet thick. To give an appreciation of what this deposit means, a block or square mile of it 230 feet deep would contain over 400,000,000 tons. Passing south into Michigan the salt formation continues under Lake Huron, the St. Claire River, Lake St. Claire, and the Detroit River, but 10 to 20 miles from the border it dips sharply and does not appear again except a t the edges of this vast basin. The depth of the salina a t the center of the basin must be about 4000 feet. At the mine of the Detroit Rock Salt Company, which has a present capacity of 1000 tons of salt per day, and which will shortly be doubled, the workings are 1150 feet below the surface. From Detroit to Wyandotte, a distance of 12 miles along the United States side of the Detroit River, are the alkali plants of the Solvay Company, Michigan Alkali Company, and the Pennsylvania Salt Manufacturing Company, having a total daily capacity of approximately 500 tons of equivalent caustic soda.
T
POWER The Sandwich works of the Canadian Salt Company are a t the extreme western end of the Ontario Hydroelectric Power Commission’s distribution of Niagara power, there being 276 miles of transmission lines between Windsor and the Niagara stations. The energy is brought to Windsor a t 110,000 volts and delivered to the plant at 26,400 volts, 1 Presented by Mr Gollop before Section I3 (Chemistry) at t h e meeting of t h e British Association for t h e Advancement of Science, Toronto, Canada, August 6 t o 13, 1924.
25-cycle alternating current, where it is converted t o direct current a t 250 volts. The company buys from the commission 3000 horsepower and has a steam power plant capable of generating 3600 horsepower. BRINEPURIFICATION Before the raw brine from the wells can be used in the process, it must be treated to remove salts of lime and magnesium, which impurities would otherwise precipitate in the pores of the diaphragm and thereby increase the resistance of the electrolytic cell. This purification is done by heating the raw saturated brine to 85” C. and adding to it a calculated amount of sodium carbonate solution containing a trace of caustic soda. The resulting precipitate of calcium carbonate and magnesia is allowed to settle out and the clear, pure, and warm brine run through a settler heated by coils supplied with hot, condensed water from the salt grainers. Settling by mechanical clarifiers or in tanks is quite satisfactory and is a considerable saving in labor and material over the filter-press methods. The reagent sodium carbonate solution is made by carbonating with boiler flue-gas caustic liquor from the cells. This, in addition to being cheaper than using soda ash, gives a more rapid settling precipitate. After the impurities have settled out the brine is again heated to 85” C., and fed automatically and by gravity to the electrolytic cells, where it is decomposed by the electric current into its component parts. THEELECTROLYTIC CELLS The cells or “electrolytic decomposers” used a t the Sandwich plant are of the diaphragm type and are the invention of an Englishman, Arthur E. Gibbs. The process is undoubtedly one of the most efficient of its kind in operation today. A large plant consuming about 10,000 horsepower is located a t Wyandotte, Mich., and possibly the largest electrolytic alkali installation in the world today is that of the United Alkali Company, of Great Britain, both using the Gibbs cell. I t was a t the former works, the Pennsylvania Salt Manu-
I N D C S T R I A L AND ENGINEERING CHEMISTRY
October, 1924
facturing Company, that the cell was first installed in 1907.2 There have, of course, been improvements in seventeen years of operation, but the original invention has been faithfully adhered to. Fig. 3 shows the present cell. The cell itself is inclosed in an upright iron container, cylindrical in shape, and is assembled by placing inside this container a cylindrical and perforated cathode sheet on the inner side of which is secured a diaphragm of a special grade of long-fiber asbestos paper. Inside the cylindrical cathode and as near to it as practicable are placed 24 graphite sticks, 2 x 2 x 36 inches long, suspended in a circle from a domeshaped head. The head, the ring it fits, and the bottom around which the cathode with its diaphragm is formed are made by the company from a mixture of cement and asbestos. The joints between the various divisions of the cell are SO designed that they can be made gastight by a chlorineresistant putty. These cells are arranged in a series of seventy individual cells placed in two parallel rows of thirty-five each, and are connected so that the current must travel in series through the whole seventy under an applied voltage of 250 volts. Each cell will take approximately 3.57 volts and a current of 1000 amperes. Practically the only departure from the original design has been in the manner of feeding the brine. By an ingenious arrangement the cell is kept full of hot brine. Some of the cells are operating with the original underfeed, although the type of feed just described is being installed. The chlorine gas generated is removed through a pipe connection into a gas main kept under slight suction. The caustic soda, together with some undecomposed brine, trickles down the outside of the cathode sheet to the bottom of the cell container and thence into a receiving line into storage tanks which supply the evaporator department. The cell operates a t a current efficiency of 93 to 95 per cent over the anode life of better than a year, a power efficiency of 56 to 57 per cent, and a voltage efficiency of 60 to 61 per cent. It will produce in 24 hours, per square foot of floor space taken, 5 poundb of caustic soda and 4.43 pounds of chlorine. The caustic will run 120 grams per liter of sodium hydroxide, and
FIG 2-CELL
ROOM
the chlorine 95 per cent or better. The cell itself is remarkably simple, while the ease of its assembling and replacement does not demand expensive labor in building or supervising while in operation.
EVAPORATOR DEPARTMENT Tke caustic cell liquor is pumped into storage tanks, which in turn feed the evaporators. Two double-effect Scott 2
U S P a t e n t 874,064: British Patents 27,830 and 28,147 (1907).
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evaporators with a salt separator for each effect are used. The problem of evaporating the cell liquor containing 180 grams per liter of salt is no mean one, and requires an equipment with a very rapid liquor circulation to keep a clean heating surface and provide for a rapid heat exchange between the steam and liquor. Then again, inasmuch as the liquors corrode the tubes from which the heating surface is constructed, it is necessary to have an apparatus in which t h e tubes can be easily replaced. These features are well provided for in the evaporator used.
SECTION THRUG/t3sk C€LL Fro 3
I n this apparatus the uptake area is distributed over the whole area of the evaporator, so that each nest of heating tubes has its own downtake. The tubes are expanded metal to metal into the tube sheets, so that gaskets are not necessary for fastening them into place. The tubes being carried vertically in the evaporator, the vapor generated by evaporation assists in propelling before it the liquor in the tube. The vapor and liquid separate in the vapor chamber which is placed a t the upper portion of the evaporator, and the vapor passes off through a catchall to be used in the next evaporator, or to the condenser. Although some installations use triple-effect evaporation, double-effect is used here and the vacuum is controlled on each of the two effects in such a way as to obtain the greatest temperature difference between the steam on the outside of the tubes as compared with the liquor inside the tubes. Operating in this manner is very economical, particularly as exhaust steam from the engines is used for doing the work in KO. 1 evaporator. It seems to be advantageous to have the steam around the tubes as the latter are really immersed in a bath of steam whatever the condition of the liquor in them. This may not obtain in the reverse case. Should the liquor be weak, the steam would under certain conditions condense in the tubes.
EVAPORATOR OPERATION The jet water condenser is turned on; No. 1 pan is filled with cell liquor and KO. 2 pan with intermediate liquor of 60" Twaddle. Either exhaust or live steam is admitted to the steam belt of the first effect, which boils under 18 inches vacuum at 81O C. The vapor from N o . 1 pan passing a t the same time into the steam belt of No. 2 pan boils the liquor it con-
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tains a t 54" C. under 28 inches vacuum. The vapor from this pan is condensed in the water eductor. Boiling is continued until the second-effect liquor reaches 80" Twaddle, during which time the liquor in No. 1pan has been concentrated to about 58" Twaddle. As the batch is boiling the liquor is continually circulated from the bottom of the pan through the salt separator and out the top of the latter, being admitted to the pan through a connection just above the cone. No. 2 pan draws its feed from the top of No. 1 separator and No. 1 pan from the cell-liquor storage tanks. This procedure takes about 4 hours. During this time the pans have been operating double effect, but when the gage on the steam belt of the first effect shows a pressure of 8 pounds per square inch the so-called by-pass stage is said to be reached and the steam is by-passed to No. 2 pan. The increase of pressure is due to the steam in the first effect not being condensed, this by reason of the rise in the boiling point of the liquor approaching the temperature of the steam. KO. 1 and No. 2 pans are now separated and No. 2 is run as a single effect, still obtaining its feed from the former. I n about 1 hour the liquor is concentrated to 90" Twaddle. The remainder of the batch from No. 1is run over a bed of socalled "strong" salt left from a previous batch of finished liquor, then drained, and it is from this liquor that No. 2 pan then obtains its feed. Concentration is carried on until the hydrometer shows 104" Twaddle. The time consumed to concentrate the intermediate liquor from 240 grams per liter (60" Twaddle) to 750 grams per liter (104" Twaddle) is about 7 hours. As the liquid enters the bottom of the salt separator conical it is given a centrifugal action by means of the spiral attached to the bottom of the conical. The salt is thereby thrown to the outer wall of the separator, where it gradually piles up until, when the separator is full, it contains mushy salt mixed with liquor around a small, central core of the liquid itself. When a batch is completed, the contents of the second effect and its separator are unloaded by a centrifugal pump into a cylindrical drain tank with a conical bottom, where it is allowedto settle for 48 hours. Separating this cone from the rest of the tank is a finely perforated monel metal screen, and on this screen most of the salt suspended in the strong l i q u o r xs retained. This liquor is then drained slowly into cooling tanks, which reduce its temperature to 30' C. The solubility of salt (NaC1) in liquor of this strength and at 30" C. is approximately 16 grams per liter, and since the soluble portion must go to the finishing pots, it is quite important to settle out practically all suspended salt in order to make h i g h - t e s t finished caustic. The typical finished liquor shows on analysis : NaOH.. . . . . . . . . . . . NaC1.. . . . . . . . . . . . . NazSOa.. . . . . . . . . . .
Fe . . . . . . . . . . . . . . . .
with first-effect liquor, one wash with cell liquor, or liquor of like strength, and then with hot water orkrine until the wash water shows but 1 gram per liter of sodium hydroxide. A final few minutes' wash with hot water removes most of the sodium sulfate that entered the process when the soluble gypsum was removed from the brine. Upon refrigeration of this liquor a salable Glauber's salt is recovered.
FINISHING The clear, settled "strong liquor" is pumped to tanks in the finishing shed and thence into open-fired pots 10 feet in diameter and 7 feet deep at the center. The pots are filled to within 1 foot of the top and fired briskly until foaming commences (which is about 280" C. a t this plant). Care must be taken from this point until the temperature rises to 360" C., when foaming stops. Up to the time of the initial foaming strong liquor is fed to the pot slowly in order to maintain the original level. After the period of foaming the feed can be run in more rapidly so that the pot is full at the end of the run. The remaining water is boiled out over a period of 2.5 days. In this time the pots are brought up to 500' C. and then allowed to cool gradually. At 450' C. coming down, 3 pounds of sulfur are added to decompose compounds of iron and manganese, and then the pot is allowed to settle for 24 hours. It is poured a t 340" C. with a special centrifugal pump into sheet steel drums for solid, or onto iron trays for broken caustic. This supersedes the older method of baling out with a large ladle into a pouring trough. A pot the size given pours 16 tons of finished caustic. Within the past two years the plant has made considerable flake caustic. This convenient form is produced on an internally water-cooled cylinder, which rotates with its face slightly immersed in a shallow pan of molten caustic. The film is picked up on one portion of the cylinder and removed as it rotates past a knife scraper. The operation is continuous. To the best of the writers' knowledge very little published information is available to describe the reactions involved in the sulfuring of pots, but it is assumed3the following reactions occur: 6NaOH 4s = Na2$Oa 42NazS 3&0 (Na25208 heat = NazS -I-
Grams per liter 750 17.5 0.6
0.17
WASHING THE SALT The salt left from the strong liquor batch has two washes
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FIG.~ - - L I Q U I D CHLORINE PLANT
+ + + so, + O ) *NazFe204 + 2Na2S = 2FeS + 3Na20 + 0 (Na2O + HzO = 2NaOH) 2FeS + 7 0 Fez08 + 2S02 =
* Sodium ferrate is also present. LIQUIDCHLORINE
Since the inception of the alkali-chlorine cell, the disposal of chlorine has been the ever-present problem. The end of the war found the producers with equipment to make more chlorine than the market could possibly absorb, and the price of chlorine, bothinliquidand bleaching powder, dropped to very lorn levels. At the same time the market for liquid chlorine has expanded to t h e d e t r i m e n t of bleach sales. a Theory of J. H. Hubel, chief chemist, Canadian Salt Co., Ltd.
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Although liquid chlorine has been made by the compression system for many years, it has been by the so-called tower system that the recent expansion of the industry in Canada and the United States has been brought about. Chlorine derived from the cells is drawn through a cooler and thence through a 40-foot drying tower countercurrent to a flow of strong sulfuric acid to remove the moisture from the gas. The cooler and tower remove a t a maximum 50 pounds of water per ton of gas passing through them. This renders possible the liquefaction of the gas in iron equipment, for dry chlorine does not attack iron. Induced from the drying equipment by a Xash pump operating in sulfuric acid, the dry gas is delivered to the towers of the liquefying plant at a pressure above 3 pounds. It is there entrained through a suitable suction head with a falling column of strong sulfuric acid contained in a 4-inch line, or “leg,” as it is called. Falling through a distance of 90 feet into a compression chamber, the gas pressure is increased thereby t o 35 pounds per square inch gage, or 50 pounds absolute, the acid continuing its cycle. Under this pressure the gas rises through double cooling coils kept at -25” C., where it liquefies. The liquid chlorine runs from the cooling coils by gravity through a trap into a steel vessel properly insulated, the capacity of which is about 5000 pounds. From this it is loaded either into storage tanks, 15-ton tank cars, or 150-pound cylinders, by air under 100 pounds pressure which is previously dried by passing through a series of towers containing strong sulfuric acid. Chlorine gas as it leaves the cell room shows on analysis 95 per cent chlorine, 1.2 per cent carbon dioxide, 1 per cent hydrogen, a trace of carbon monoxide and the remainder air. The conditions necessary for the liquefaction of chlorine leave the carbon dioxide, hydrogen, and air as a residual gas, and since a process cannot operate a t 100 per cent efficiency, this residual, or “by-gas,” contains a considerable percentage of chlorine. It is also enriched by liquid which vaporizes when the latter is transferred from blow bottles to storage tanks or other containers. A typical “bygas” analysis shows 50 per cent chlorine, 5 per cent carbon dioxide, 4 per cent hydrogen. This gas, a t present, is used up in b l e a c h - m a k i n g . Those who have had bleach-making experience can sympathize with the bleach-maker having to deal with a gas containing 5 per cent carbon dioxide. However, by adding sufficient fresh gas from the cell room to dilute the carbon dioxide below the point where it is harmful, this gas has been disposed of. For a production of 600 tons of liquid chlorine per month it is necessary to make in the bleach department 350 tons of bleach in order that high-test bleach may be produced.
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the lime and gas together. The one most successful up to the past few years is that known as the Hasenclever mechanical bleach chamber. A more recent development is at present operating a t the Sandwich works. There is also an installation of four of these machines, known as Rudge cylinders, a t the Wyandotte plant of the Pennsylvania Salt Manufacturing Company. This machine is the development of A. Rudge, Gateshead-onTyne, England.4 I n England a t the plants of the United Alkali Company, where they were developed, there are six in operation. The machine is a horizontal cylinder 65 feet long by 3 feet in diameter, closed a t one end by a cast-iron head into which operates a lime-feeding device, and a t the other by a discharge chute fitting with a gastight cover into the drum being filled. The lime is propelled in the horizontal cylinder by its own slope and chiefly by six complete spiral helixes extending the length of the machine and riveted to its shell. These so-called "lifters," although themselves stationary relative to the cylinder, literally propel the lime as it falls from the slope of one to that of the next below it. The gas properly refrigerated and diluted is driven by a fan countercurrent to the flow of lime. The spent gas is drawn from the lime end of the machine through a dust chamber by another fan and 4 Canadian Patent 194,216 (November 25, 1919); U.S Patent 1,330,495 (February 10, 1920).
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discharged into a scrubber. The cylinder is cooled by water sprayed over its surface from a pipe running lengthwise with and above the machine. The heat of reaction is removed by this cooling water, refrigerated gas) evaporation of excess moisture in the lime, and the great volume of gases flowing out of the machine. The best operating conditions may be summed up by the followingrequirements: lime, 27 to 30 per cent water; gas entering the machine, about 12 per cent chlorine a t a temperature of 5' to 8' C. The exit temperatures will run 55" to 60" C. The speed of rotation may vary from one revolution in 1 minute 15 seconds to one revolution in 3 minutes depending upon the production desired. The present installation will produce 1 ton of high-test bleach per 18 square feet Aoor space per week, which is approximately four times greater than our best chamber production. Under proper running conditions the bleach produced is of a uniformly higher strength and has better keeping qualities. A sample packed in a steel drum with a loose-fitting head and subject to all the changes in temperature and humidity of Windsor climate lost but 2 per cent of its available chlorine in 9 months. With the mechanical chamber the disagreeable loading and packing out of chambers is eliminated. Further development of bleach machines is dependent on the demands of the market for a high-test, stable bleach, for which a fair price will be paid.
