Chlorine and Caustic in ItalyJ
AMALGAM CELL PRODUCTION
RICHARD L. KENYON Associate Editor
in Fallnboration w i t h
PATRIZIO GALWNE Oronrio de Nora,
Impianti Eletuoehimici, M i h n , Italy
A Staffd
- e t r g CoUdorative Repor
c
HU)RINE is electrolytically manufactured commercially by four methods: the electrolyai~of sodium or poteaaiUm chloride brine in the amalgam cell or the diaphragm cell, the eleohlyeis of molten sodium chloride, and the electrolyeis of hyrochlooric acid The electrolyeis of by-product hydrochloric acid has been developed to some extent in Germany ( 8 ) but never has been used d v e l y in the United States. In no country in the extent of its application at all comparable with that of the electrolysis of sodium chloride. The manuIacture of chlorine by the electrolysis of molten sodium chloride in also ~ m ~inl lquantity 88 compared witb the electrolysie of brine. Electrolyeis of brine baa been used most extensively in the umted states for many years, cbieay through the diaphagm oell (1). An example of a major plant using t b i n kind of cell recently has been described (16). The other widely used method for the electrolysis of sodium chloride involve^ the mercury amalgam cell. This type of cell is used exteneively in Europe and Japan and " e n t l y has found increaaioe: 1188 in the United States. Between 1949 and the end of 1962,American capacity employing the amalgam cell hss more than doubled and s o 1 in increasing. An example of a modern amalgam cell developed in the United States in that of the Mathieeon Chemical Cow. (11). Since World War I1 much information brought out of Germany (6, 16, 17) hae greatly influenced cell design and has Stimulated American interest in the amalgam cell. The amalgam cell has long been used widely in Europe and has been ntudied extensively to improve ita design and operation. The de Nora cell is the result of such work by the 6rm Oronzw de Nom, Impinnti Elettrochiraici, Milan, Italy. This cell has been installed extensively in Europe,South Afrioa, Australia, and %uth America since World War I1 and is now enjoying a great increase in popularity in the United Statea.
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Five plan& using the de Nora amalgam cell with a total capmity of approximately 400 tous per day of chlorine were put into operatiou or were under conetmction in North America in 1952. Monsauta-de Nora desiep and wmta'uctiou by Leonard Construdion Co. were used in nearly all thesz plankwbich were adapted to American conditions. The cella in all i n ~ t d a t b ~ ~ am capable of operation at 36,000amp., and in one D B B ~the design included production of 70% rayon grade caustic. Fifty per oent cawtic met the requirements in the other plants, while in one plant the design included facilities for producing both caustic soda and sodium sulfide in varying amounts BS denired up to 65% of the output BS sodium sulfide. ItaliDn Eleotrolytic Chlorine-Caustic Industry IB More than 50 Yearn Old Caustic soda WBE made in Italy by an electrolytic process for the Grst time in 190.2,at Buaai, by %cietA Itallisna di Elettmchimica. At t b i s time there wan a strong demand for cauetic, appreciable q U t i t i e S of which were imported. The problem of finding a profitable use for the chlorine was a deterrent to expansiou of production. %dium hypochlorite and bleaching powder were tbe first use of chlorine in Italy, followed by calcium hypochlorite for the N e and paper induStriee. In 1905, another electrolytic cauntie-chlorine plant WBE put into production by CaEaro, in Breach. Availability of chlorine led to the production of carbon tetrachloride, at Bu~ai,and of c o p p r oxychloride for antifungicide ueea by C a E m . No further pIanta were built until after World War I, when the two existing were enlarged and new plants were built by the following companien: Rumianca at RUmianca, F'iedmout (1915), W A t Materie Coloranti Bouelli (now a part of Monteastini) at Ceaano Maderno, Lombardy (19l8),and Elettrochimica Pomilio, Naples (1918)(4).
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After World War I there was overproduction of chlorine which delayed further expansion. At the start of World War 11, Italian chlorine production was about 40,000 short tons. Following the war there was an excess again, but by 1950 all capacity was being used, and there was an active demand for chlorine and chlorine products. At this time Societh Edison, leading producer of electrical power in Italy, in cooperation with Monsanto Chemical Co., of the U. s. A., formed a new company, Societh Industrie Chimiche Edison, to participate in the growing chemical industry (6). In 1951 the company put into operation a t Porto Marghera, near Venice, its first plant, one of Italy's largest chlorine-caustic installations. That plant uses amalgam cells entirely.
TABLE I. ITALIAN CHLORINE PRODUCTION Total Chlorine, Year Short Tons 44 000 1948 43 000 1949 52,000 1950 65 000 1951 70: 000 (est.) 1952 No. produoers: About 20 Leading producers: Montecatini and subsidiaries led by Societh Chimica de1)'Aniene (Solvay contralied) Rumianca Caffaro Societh Industrie Chimiohe Edison Elettrchimioa Solfuri e Cloroderivati
Analgam Cell Operates Satisfactorily w i t h Partial Purification of Brine
Actual comparison of amalgam and diaphragm cells ( 1 4 ) has indicated that for the U. S., capital investment and production costs are about the same. The amalgam cell is favored where electric power is cheaper and steam is more expensive. An important difference lies in the quality of caustic produced: while the diaphragm cell is equal or slightly favored, economically, for the production of regular grade caustic, the amalgam cell is
agreed to be superior for rayon grade caustic. The increased demand for chlorine has contributed to the fact that most of the increased caustic capacity uses the electrolytic process. While caustic quality requirements favor the amalgam cell, it is of interest that the amalgam has other uses ('7, 8, 11-16). No evaporation or purification steps are needed to make pure caustic of 50 to 70% concentration with the amalgam cell. Economic operation of amalgam cells usually demands resaturation of depleted brine by addition of salt. An external source of make-up salt is required, as the amalgam cell caustic does not require concentration, a source of make-up salt. With diaphragm cells the dilute, impure caustic must be evaporated. Thus salt is recovered for use as the process make-up. This permits the use of the cheap solution from brine wells as the sole source of salt for the diaphragm cell. When recovered salt is available from a diaphragm cell operation, however, addition of an amalgam cell installation may readily be justified. This combination results in a cheaper brine for use in the amalgam cells and gives the advantage of higher grade caustic. For the amalgam cell complete purification of brine is not essential, as for the diaphragm cell, and some impurities such as magnesium and calcium salts and sulfates can be tolerated to a considerable extent. I n comparing diaphragm and amalgam cells for Italy, it should be kept in mind that while the country must import most of its coal, it is a major European producer of cheap hydroelectric power (see Economics section). Thus production of caustic by electrolysis is strongly favored, as may be seen by the decrease in lime-soda caustic production and the rise of electrolytic production (6). For this and other reasons, in Italy, the amalgam cell has economic advantages over the diaphragam cell. Mercury is one of the most expensive raw materials used in amalgam cell construction. Italy produces about 40% of the world supply of mercury, followed closely by Spain. Thus in Italy the amalgam cell maintains the points of advantage which i t has in the American industrial economy and a t the same time finds its less favorable points minimized under Italian conditions. 1163
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INDUSTRIAL A N D ENGINEERING CHEMISTRY WATER INLET TO DECOMPOSERS
Vol. 45, No. 6
NON-CONDUCTING LINING
Speci6c data on the economics of operation are given in a Inter section. With the amalgam cell, other sOhtiOn8, such SE potassium chloride or sodium sulfate, may be electrolyzed, and the amalgam may also be used for the production of sodium sulfide (8) or for the reduction of organic compounds such as nitrobenzene. This d i e c d o n will be limited to the manufactwe of chlorine, sodium hydroxide, and hydrogen, from sodium chloride and water.
and the remaining hydroxyl ion reacts with the sodium ion to yield sodium hydroxide, producing the over-all reaction
+
+
+
(Ag)Na HX, -C NaOH 'MIS (Hg) The moat important factom inftuencing the electrolysis are
anodic and cathodic current density (a), anode to cathode distance, final concentration of the amalgam, the rate of flow of mercury, and the concentration, purity, and temperature of the electrolyte. High current density decreasea the &e and cost of the elecThe Cell and D-mpwer Are Primary Parta of an Amalgam Cell trolytic cell but increases voltage drop and energy consumption. While these factors must be balanced, the current density must An amalgam cell consiats principally of two compartments: be high enough to give total hydrogen polarieation on the (1) the cell, in which the brine is electrolyzed between graphite cathode to prevent unde6rahle hydrogen liberation there. anodes and a mercury cathode to produce chlorine gas and sodium Another factor which must he o b m e d in the prevention of amalgam and (2) the decomposer, in which sodium amalgam is hydrogen formation is the level of the sodium concentration in decompoeed in the presence of graphite to produce d u m hythe amalgam: hydrogen formation increases aa the concentradroxide and hydrogen labile regmerating mercury. tion rises. The graphite anodes dip into the brine solution from above The cathodic current density 80 far adopted has varied from and the mercury flows along the flmr of the cell coucurrently 1 to 3 amp. per Square inch with a sodium concentration of with the brine. The result of direct current flow is the forma0.06 to 0.2% sodium or more. The anodic current density tion of chlorine and sodium amalgam. calculated for the area facing the cathode is slightly highN a C l + (Hg) 4 (Ag)Na '/L% because of a somewhat smaller area, hut the actual density is lower for Vwved anodes. In the decomposer an electrochemical reaction oecum in which COnWSdY, Cathodic area is determioed hy the current density sodium amalgam is anode, the =tho&, and s~ to be u d . For a 10,ooO-amP. cell with a current density of 200 dium hydroxide, formed by decomposition of the dgm, is the electrolyte. sodium is ionized and into s ~ l ~att i ~ ~ m P . per Square foot (1.39 Smp. pes square inch), s cell with an active bottom area of W square feet would be required. .nna. The distance between cathode and anode should be 88 small N a e-Ns+ aa possible, to reduce loss of energy, but if it is to0 mall secondary I W C ~ ~ Otake ~ S place, particularly the direct attack of sodium Hydrogen ions from the aqueous electrolytesolution reduced amslgamby the chlorine anodes allow at the cathode escaue of chlorine. thus pivine smaller bubbles. and can be pl&d nearer to t& cathode. H+ +e-'/&
+
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June 1953
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Mercury. Taking a 10,000-amp. cell as an example, the amount of sodium (atomic weight 23) produced per minute is
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which means that 143 kilograms of mercury must flow each minute to obtain a sodium concentration of 0.1% a t the cell outlet. Assuming a current efficiency of 9574, the flow of mercury must be 10 liters per minute. The speed and the thickness of the mercur stream are important for good cell operation. They may g e varied within certain limits, depending on the material used for the cell floor and its wettability with mercury. It has been found that a speed of 36 feet per minute and a thickness of 1 / ~inch are generally satisfactory. The slo e of the floor is usually 0.2 to 0.4%. Electrolyte. The electrofyte, a substantially saturated solution of sodium chloride, should be pure in the ideal case. Impurities can be tolerated to various degrees. Chromium, nickel, vanadium, and heavy metals in general are very harmful to amalgam stability and even less than 1 p.p.m. may be dangerous; a combination of such impurities can further lower this limit, and for the same reason iron is especially dangerous. Magnesium and calcium can be tolerated up to 10 p.p.m. and 100 p.p.m., respectively, but to higher levels when iron is not present. Sulfates increase anode graphite consumption and decrease current efficiency. The desirable pH value for the feed brine, once controversial, is accepted today as being 4 to 5.5. While 50' to 70" C. has been considered the optimum temperature range for the brine, it has been found that temperature has no appreciable direct influence on the process. Amalgam Decomposition. The speed of amalgam decomposition is a direct function of the effective current density in the decomposer and of the internal resistance and temperature of the caustic. The total quantity of current involved, which is, of course, the same as that in the cell, as it is equal to that required to reduce the sodium ions, is constant; thus the cathodic and anodic surfaces can be substituted for the current densities. As the current will pass through the shortest circuit in the electrolyte, which is approximately the line of contact of the three phases, amalgam-graphite-caustic, it can be concluded that the factor determining the speed of decomposition is, apart from temperature, the sum of the lengths of those lines of contact.
An increase in the sum of the contact lines or in the temperature will increase directly the speed of decomposition of the amalgam. Time also must be considered in regulating the decomposition. The amalgam must remain in contact with the graphite in the decomposer long enough to be converted back to mercury. I n vertical type decomposers the amalgam trickles by gravity through the graphite packing, and the retention time is short in comparison with that re uired for a horizontal trough-type decomposer. The mercury %oldup is correspondingly smaller. The active surface area of the graphite in the vertical decomposer is so great that retention time of 10 to 15 seconds is sufficient to complete decomposition under practical conditions. I n this type of decomposer highly concentrated caustic can be produced without difficulty by providing thermal insulation and applying heat. Typical de Nora Plant Uses Cell with Capacity of 12,000 Amp.
Several chlorine plants have been built by de Nora in Italy. Their chief details are described below with reference to a plant having a daily capacity of 50 tons of chlorine and using the de Nora type 20-T cell with rated capacity of 12,000 amperes. The design of this cell allows an overload of 50% or more when the electrical equipment installed has sufficient capacity. This feature gives flexibility to operation and to the capacity of the plant. The brine system can be overloaded by taking advantage of standby units and of the liberal sizing of pipes and equipment. Rock Salt Makes Brine that Requires Only Partial Purification
Practically all plants use rock salt from the mines of Sicily. The crude salt is shipped by 600-ton barges to the plant where it is crushed by a jaw-crusher and fed into the saturators. An average analysis of the salt used is:
HzO SO8 CaO MgO Fe-A1 Insolubles NaCl
From saturators, part of the brine goes to purification system and part directly to settling tanks
Per Cent 0.15 0.13 0.14 0.032 0.01 0.07
99.46
Power is derived from the high tension network of the area and is stepped down. for feeding the direct current generating equipment. I n Italy the standardization of the frequency is not yet complete. However, according to a recent law, it will soon be 50 cycles. Feed brine is prepared by addition of salt to the depleted brine coming from the cells. The depleted brine leaves the cells with a sodium chloride concentration of about 270 grams per liter and is pumped to the saturation tanks, which are cylindrical reinforced concrete structures 23 feet high by 8 feet internal diameter, each having a capacity of 8000 gallons, sufficient to saturate the brine required for 20 tons per day of chlorine. The depleted brine stream is introduced near the bottom of each saturator through a rubber-lined pipe, 6 inches in internal diameter. Saturated brine overflows a t the top, after passing upward through the salt. Three tanks are in constant use, but any one may be by-passed for repairs. The brine is made up to an approximately saturated solution, containing about 310 grams per liter. Make-up water seldom is required, as the amount introduced with solutions added to precipitate impurities compensates for processing losses. Solid impurities collecting in the bottoms of the saturators are dumped from the bottom discharge about once a month. Rather than purify all the brine, which is not essential to meet the requirements of the de Nora cell, one third to one fifth of the flow is chemically treated so as to reduce the concentration of impurities in the entire feed to the desired level. As the cell is I
Depleted Brine Is Resaturated in Reinforced Concrete Tanks by Addition of Crushed Rock Salt
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F i m 2.
DEPLETED BRINE
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Flow Sheat of Typicdl Installation for Pmduation of Chlorine and Caustic in Italy by de Nora Amdg.m Cell
huwlgti3. - -
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1N.D U S.TR I A L ,AN:D: rE:%GlIN.EZEIN0 C R E M I S T R Y
1161
The e l e o t r o l y ~is normally carried out with the following
Tbe unp& portiqfg the brine h a d by a controlling weir directlyfrom the'mtanatok to the a h b g Deohlorination Of ,&pm$W to bepurihd m scoomplished by air stripping in tno oylinddd.steel toners 2'/, feet i n k 1 dismW&t. The tawem m flooded, rather *.'of h@rC&die lIez+tIlus saving a ermore, experience haa that flooded towers are no%kas efficient than packed towers. Air is bubbled tbmugb the brine solution which fills the towers. Chlorine re claimed in tbls Stripping operation is piped to an e i plant section where it is used for the manufacture of e u m bypochlorite.. An only an a m g e of one 6fth of the b+e is p d e d in this proceeq a datively small amount of dilute chlorine is produced. The dechlorinated brine frecttOn Bows by gravity from the stripping towera to the reactor teka. These are cylindrical r e i n f o r d conmte tanka lined with acidproof brick; internal diameter is approximately 13 by 18 feet and capacitiea about lS,&N gallons each. Two rue %me&& in & and two pairs nre used alternatively. In the 6rat tank, barium chloride is added for the precipitation of d a t e ; in the m n d , caustic soda and sodium carbonate are added for the precipitation ob oagnesium and calcium.
w.
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.a
COPPER A N W E COLLAR
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maximum concentration of the main impuritiea in the brine: Calcip ion, grama/liter 1.2 Bulfate ion, grams/liter 3 Msnnmium ion. erwnfitar 0.1
~ I Y p.p.m. ,
I
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