PH-Control

In addition to the collector, polyvalent ions may show sufficiently strong adsorption on oxide, sulfide, and other minerals to act as potential-determining ions (see Ref. 98). Judicious addition of various salts, then, as well as pH control, can permit a considerable amount of selectivity. 

Chemical Manufacturing. Chemical manufacturing accounts for over 50% of all U.S. caustic soda demand. It is used primarily for pH control, neutralization, off-gas scmbbing, and as a catalyst. About 50% of the total demand in this category, or approximately 25% of overall U.S. consumption, is used in the manufacture of organic intermediates, polymers, and end products. The majority of caustic soda required here is for the production of propylene oxide, polycarbonate resin, epoxies, synthetic fibers, and surface-active agents (6). 

Miscellaneous. Hydrochloric acid is used for the recovery of semiprecious metals from used catalysts, as a catalyst in synthesis, for catalyst regeneration (see Catalysts, regeneration), and for pH control (see Hydrogen-ION activity), regeneration of ion-exchange (qv) resins used in wastewater treatment, electric utiUties, and for neutralization of alkaline products or waste materials. In addition, hydrochloric acid is also utilized in many production processes for organic and inorganic chemicals. 

Health and Safety. The U.S. FDA has affirmed R%- and S(—)-maHc acid as substances that are generally recognized as safe (GRAS) as flavor enhancers, flavoring agents and adjuvants, and as pH control agents at levels ranging from 6.9% for hard candy to 0.7% for miscellaneous food uses (42). R%- and A(—)-maHc acid may not be used in baby foods. MaHc acid is also cleared to correct natural acid deficiencies in juice or wine (43). 

Static mixers are used ia the chemical iadustries for plastics and synthetic fibers, eg, continuous polymeri2ation, homogeni2ation of melts, and blending of additives ia extmders food manufacture, eg, oils, juices, beverages, milk, sauces, emulsifications, and heat transfer cosmetics, eg, shampoos, hquid soaps, cleaning Hquids, and creams petrochemicals, eg, fuels and greases environmental control, eg, effluent aeration, flue gas/air mixing, and pH control and paints, etc. 

Organics related to use of amines for pH control. Value is plant specific. 

Alkalinity Control. Water-base drilling fluids are generaHy maintained at an alkaline pH. Most mud additives require a basic environment to function properly and corrosion is reduced at elevated pH. The primary additive for pH control is sodium hydroxide [1310-73-2] in concentrations from 3 to 14kg/m (1—51b/bbl). 

Decomposition by Caustic Scrubbing. The waste gas stream is led through packed towers where a sodium hydroxide solution is introduced at the top of the towers. Venturi scmbbers can also be used. Makeup sodium hydroxide is added under pH control (32). 

The new Uquid laundry detergents, with no phosphates, have developed a use for alkan olamines. In nonenzyme formulations, they contribute alkalinity, pH control, and enhanced product stabiUty. In enzyme products, alkan olamines contribute to the stabiUty of the enzyme in water solutions (107). 

A variety of appHcations, including photography, employ alkan olamines for pH control. Reports have described formulations including mon oeth an olamine and triethanolamine in films and processing (221—224). 

Fenoprofen, 2-(3-phenoxyphenyl)propionic acid, is made into its monohydroxyalurninum or dihydroxyalurninum salt by reaction of the sodium salt of the acid with aluminum nitrate or chloride under pH control (90,91). The aluminum salt, which is hydroly2ed in the stomach, is more palatable for arthritis treatment (92,93). 

Fig. 5. Fermentative production of amino acids (140). A, pure culture B, inoculation C, boiler D, air compressor E, air filter F, seed tank G, ammonia water for pH control H, fermenter I, sterilizer , culture media K, preparation tank L, centrifugal separator M, ion-exchange column N, crystallizing...

The success of a reverse osmosis process hinges direcdy on the pretreatment of the feed stream. If typical process streams, without pretreatment to remove partially some of the constituents Hsted, were contacted with membranes, membrane life and performance would be unacceptable. There is no single pretreatment for all types of foulants. Pretreatment methods range from pH control, adsorption (qv), to filtration (qv), depending on the chemistry of the particular foulant. Some of the pretreatment methods for each type of foulant are as foUow (43—45) ... 

Succinic acid is Generally Recogni2ed As Safe (GRAS) by the U.S. PDA (184) and is approved as a flavor enhancer, as a pH control agent in condiments, and for use in meat products. It causes irritation to the eyes (185), skin, mucous membranes, and upper respiratory tract. LD q in rat is 2260 mg/kg. Succinic acid, like most materials in powder form, can cause dust explosion. 

Sugar is destroyed by pH extremes, and inadequate pH control can cause significant sucrose losses in sugar mills. Sucrose is one of the most acid-labile disaccharides known (27), and its hydrolysis to invert is readily catalyzed by heat and low pH prolonged exposure converts the monosaccharides to hydroxymethyl furfural, which has appHcations for synthesis of glycols, ethers, polymers, and pharmaceuticals (16,30). The molecular mechanism that occurs during acid hydrolysis operates, albeit slowly, as high as pH 8.5 (18). 

A fermentation such as that of Pseudomonas dentrificans typicaby requires 3—6 days. A submerged culture is employed with glucose, comsteep Hquor and/or yeast extract, and a cobalt source (nitrate or chloride). Other minerals may be required for optimal growth. pH control at 6—7 is usuaby required and is achieved by ammonium or calcium salts. Under most conditions, adequate 5,6-dimethylben2imida2ole is produced in the fermentation. However, in some circumstances, supplementation maybe required. 

Boiler feed-water systems that use dernineralized or evaporated makeup or pure condensate may be protected from caustic attack through coordinated phosphate and pH control. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development of high caustic or acid concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate. Sufficient disodium phosphate must be available to combine with all of the free caustic in order to form trisodium phosphate. 

Fig. 5. Coordinated phosphate—pH control avoids both acid and caustic corrosion. To convert psi to kPa, divide by 6.895.

Fig. 12. Calcium carbonate scaling of a surface condenser due to poor pH control.

Modifications of the basic process are undersoftening, spHt recarbonation, and spHt treatment. In undersoftening, the pH is raised to 8.5—8.7 to remove only calcium. No recarbonation is required. SpHt recarbonation involves the use of two units in series. In the first or primary unit, the required lime and soda ash are added and the water is allowed to settie and is recarbonated just to pH 10.3, which is the minimum pH at which the carbonic species are present principally as the carbonate ion. The primary effluent then enters the second or secondary unit, where it contacts recycled sludge from the secondary unit resulting in the precipitation of almost pure calcium carbonate. The effluent setties, is recarbonated to the pH of saturation, and is filtered. The advantages over conventional treatment ate reductions in lime, soda ash, and COg requirements very low alkalinities and reduced maintenance costs because of the stabiUty of the effluent. The main disadvantages are the necessity for very careful pH control and the requirement for twice the normal plant capacity. 

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