(from Ullmann`s) Seawater Process
The first economically successful recovery of bromine directly from the sea was carried out near Wilmington, North Carolina (USA), using an air-blowing process originally developed for brine operations by H. H. DOW [28]. The use of air rather than steam is necessary because the cost of steam to heat ocean water, with its bromine content of only about 65 mg/L, would be prohibitive. The Wilmington plant has since been abandoned, but the process is still employed, e.g., at Amlwich (Wales) and Hayle (Cornwall).
Water from the ocean is pumped to the top of blowing-out towers, with sulfuric acid and chlorine being added just above the pumps so that mixing takes place during the ascent. About 1.3 kg of 10 % sulfuric acid per ton of water is required to neutralize the natural hydrogen carbonates and bring the pH to 3.5; 15 % excess chlorine over the theoretical requirement is used [29].Air is drawn up through the towers, sweeping out a mixture of bromine and chlorine (or bromine chloride) from the descending ocean water. The air is drawn next through absorber towers in which it is scrubbed counter-currently by sodium carbonate solution. The several reactions which take place may be summarized approximately by the equation:
3 Na2CO3 + 2 Br2 + BrCl -> NaBrO3 + 4 NaBr + NaCl + 3 CO2
To remove spray from the air, small packed chambers are interposed between the absorber towers and the fans. When the alkalinity of the scrubber solution is nearly depleted, the solution is brought first to a storage tank and then to a reactor in which it is treated with sulfuric acid and steamed to release bromine:
NaBrO3 + 5 NaBr + 3 H2SO4 --> 3 Br2 + 3 Na2SO4+ 3 H2O
The bromine is condensed and used in the manufacture of ethylene dibromide.
In a modification of the process [30] , [31] , which was used for a number of years at Freeport, Texas, halogens blown from the ocean water are reduced with sulfur dioxide and absorbed in a water spray:
Br2 + SO2 + 2 H2O --> 2 HBr + H2SO4
BrCl + SO2 + 2 H2O --> HBr + HCl + H2SO4
The resulting aqueous solution of mixed acids is treated with chlorine in a conventional steam-out tower and the bromine is recovered. The mixture of hydrochloric and sulfuric acid remaining is used for acidifying the raw ocean water.
The SO2 process has been described [32]; further details of its operation at Freeport and the automatic controls employed there are available [33]. Normal flow of incoming gulf water is 568 m3/min; the rubber-lined riser pipe is about 12 m tall and 2.1 m in diameter. A glass electrode at the top of the riser measures the pH to within 0.02 unit and controls "butterfly" valves in a return line on the acid supply pumps. Oxidation potential is controlled at about 970 mV with a platinum electrode and a saturated calomel reference electrode, regulating the addition of chlorine. Oxidation potential measurements are also used to control the mixing of sulfur dioxide with the free halogen vapor stream and the chlorination of the reduced acid mixture.
Analytical methods used for bromide in brines and ocean water have been described [34]. Usually the van der Meulen method [35] or one of its various modifications is employed. Economic factors important in the location of seawater bromine plants, especially as they were related to the siting of the Amlwich operation, have been discussed [36]; temperature of the water has a significant effect on the efficiency of the blowing-out process [32].
Because of the large volume of ocean water processed, the bromine-depleted stream must be discharged at some distance from the intake and in a favorable location from the standpoint of prevailing currents, to avoid dilution. Aside from this, waste disposal is not a problem since both the acidity and the free halogen content of the outgoing water are very low. In contrast, the waste stream from a strong brine process requires neutralization and, in some instances, reduction. In most cases, spent well brine is returned to the ground via disposal wells to maintain hydrostatic pressure within the formation. Acidity or free halogens should be absent to minimize corrosion of the system. Lime or caustic soda are customarily used for neutralizing acidity. The use of semiburnt dolomite with sodium thiosulfate has been recommended [37].
[28] Dow Chemical, US 460370 (1891).
[29] L. C. Stewart, Ind. Eng. Chem. 26 (1934) 361.
[30] Dow Chemical, US 2143223, 1939.
[31] Dow Chemical, US 2143224, 1939.
[32] C. M. Shigley, J. Metals 3 (January, 1951) 25.
[33] P. Hart, Bull. Agric. Mech. Coll. Tex. 5 (3) (1947) no. 2, 59;Instruments 20 (1947) 956.
[34] J. Haslam, G. Moses et al., Analyst. (London) 75 (1950) 343, 352, 357, 371, 383.
[35] J. H.van der Meulen, Chem. Weekbl. 28 (1931) 238.
[36] H. Fossett, Chem. Ind. (London) 1971 no. 41, 1161.
[37] H. Kloth, B. Peter, Bergakademie 22 (1970) no. 10, 628.