Alkali waste

Alkali waste. This is a secondary source and is often obtained from the waste of chemical plants. Such material will contain calcium oxide and/or calcium carbonate. 

Alkali waste Obtained from chemical plants a lot of chemical processes require bases Ca(OH)2 is a preferred base. After the reaction, CaC03 is sometimes left behind. 

The acid and alkali wastes are pumped from the acid-alkali wastewater sump [T-30] into the acid-alkali treatment module [T-31], Metering pumps controlled by pH instruments feed either acid or caustic to the module as required to maintain an acceptable alkalinity for the formation of metal hydroxides prior to discharge to the precipitator consisting of a mixing tank [T-98], a surge tank [T-99], and a sedimentation clarifier [T-101], The pH is adjusted to a value of 8.5 for optimum metal hydroxide formation and removal. 

The oxide of manganese having been used a second time, may be again recovered a third and a fourth time and if no lime or alkali-waste has been UBed in neutralizing the excess of acid, it may be recovered on indefinite number of times. 

Mr. Gossage surmounted the pentethlonic acid difficulty, by causmg the reaction to take place at a high temperature. H1b process is peculiarly applicable to the reproduction of the sulphur contained in the lime compound or alkali waste thrown away by the soda manufacturers. As is web known the whole of the sulphur used in the manufacture of the alkali finds... 

Sulphur is also obtained from alkali waste (p. 56). 

The sulphides of the alkali metals and of the alkaline earth metals are readily decomposed by weak acids, even by carbonic acid.3 Ily the use of these sulphides and dilute sulphuric acid it is possible to obtain hydrogen sulphide in a high degree of purity. The hydrosulphides of the alkaline earth metals arc convenient sources of hydrogen sulphide, yielding the gas on treatment with carbon dioxide or even by merely heating.4 The carbon dioxide method is applied on a technical scale in the extraction of sulphur from alkali waste (see p. 50). 

CAUTION Chloroform/acetone is explosive if it comes in contact with strong alkali. Waste solvent should be stored in a labeled container strictly used for disposal of these solvents, and not mixed with other materials. Disposal is conducted according to implemented safety guidelines of the institution. If there is a possibility of the waste turning alkaline, some hydrochloric acid should be added to ensure it remains acidic. 

Sodium monosulphide is manufactured by reducing sodium sulphate with charcoal, and can also be obtained in solution from the alkali-waste of the soda manufacturer.11... 

Sodium thiosulphate, Na2S203.—The thiosulphate is manufactured from the alkali-waste of the Le Blanc process by atmospheric oxidation in presence of sodium sulphate, which reacts with the calcium thiosulphate first formed to yield sodium thiosulphate 1... 

The sodium carbonate is lixiviated with water to separate it from the calcium sulphide or alkali-waste, and from other impurities such as sodium chloride, sulphate, silicate, and aluminate calcium oxide, sulphite, and thiosulphate iron oxide and alumina. 

Figure 3.7 Flow sheet of alkali waste liquid treatment.

Alkali waste hquid NaCl, Na2C03, or Na2S04 can be recovered by oxidized roasting. 

Excess of ammonia must be present to prevent formation of sulphocarbonate. At the end ol the process this is distilled off, CO., gas is driven through the liquid to expel the sulphuretted hydrogen (as in the Claus-Chance treaiment.of alkali.waste), and any precipitated calcium carbonate is filtered off. The soluble calcium sulphocyanide is treated with sodium carbonate and converted into sodium sulphocyanide, thus —... 

The gypsum produced in reaction (15.1) is disposed of at a secured landfill site whilst the water, often required to be slightly alkaline, is sent to a sewer. For processes using sodium hydroxide, depending upon the environmental requirements, the sodium sulfate is sent to a sewer or recovered as a soKd, via an evaporator/crystallization process, and sold for uses such as household detergent. Other plants have found outlets for the dilute acid in areas such as alkali waste neutralization. 

In spite of the success of the Leblanc process, it was evident from the outset that its operation involved serious drawbacks. Although the hydrogen chloride pollution problem was largely solved by the Gossage towers, there remained the problem of the residue left behind after the black ash had been extracted with water. The principal constituent of this alkali waste was calcium sulphide, but it also contained calcium hydroxide and unchanged coal. There was no alternative but to dump it near the alkali works, where it released hydrogen sulphide when acted upon by acidic rain water. Not only did this cause a serious pollution problem, but also all the sulphur of the original sulphuric acid was wasted. 

This is aimed at preventing the discharge of noxious and offensive gas in alkali and other scheduled works. It also deals with the condensation of acid gases and the separation of acids and other substances from alkali waste. Parts have been repealed by The Control of Pollution Act 1974 and The Health and Safety (Emissions into the Atmosphere) Regulations 1983. 

The entire electroplating process generates complex wastewater streams. For the purpose of treatment these can be considered to occur in four segments — acid, alkali, chromium and cyanide streams [29]. The acid and alkali wastes are combined and the pH adjusted. The chromium is reduced from Cr to Cr " with sulphur dioxide at pH 2-3. The cyanides are destroyed with chlorine or hypochlorite. Destruction of cyanides is essential for effective precipitation of the heavy metals, as they form strong complexes with the metals and thereby increase... 

Corrosion of metals or reactions between metals and alkali wastes, e.g. hydrogen is emitted when zinc or aluminium is attacked by alkalis... 

The Claus process to recover and recycle sulfur in the Le Blanc process, based on the procedure suggested by C. F. Claus in 1883, was introduced in 1887 by A. M. Chance. Alkali waste containing calcium sulfide was suspended in water, and hydrogen sulfide was generated by pumping carbon dioxide through the sluny ... 

Figure 4.19 Deionization plant. 1, Multi-media prefilter 2, cation exchanger 3, anion exchanger 4, process water 5, raw water sump 6, regeneration chemicals 7, acidic waste 8, alkali waste.

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