The industrially important sources of water may be thus summarized : -
1. The sea.
2. Rain water.
3. Surface waters, consisting of
a. Flowing waters (streams).
b. Still waters (ponds, lakes, etc.).
4. Ground waters, furnished by
b. Shallow wells (penetrating but one geological stratum).
c. Deep wells (passing through more than one such stratum).
The impurities contained in water depend upon the character of the ground with which it has been in contact; they may be classed as soluble and insoluble. The more common soluble impurities are calcium chloride, calcium sulphate, calcium bicarbonate, magnesium chloride, magnesium sulphate, magnesium bicarbonate, sodium chloride, sodium sulphate, sodium carbonate, and organic matter. The usual insoluble impurities are sand, clay, and organic matter. Insoluble suspended matter can be removed by allowing the water to stand and drawing off the clear portions from the sediment, or by filtration. If the water contains much organic matter, a layer of coke or charcoal dust is sometimes put in when building a sand filter. A water may contain large amounts of soluble impurities, and yet answer very well for washing and levigating, while for use in condensers and cooling apparatus this scarcely need be considered at all. Soluble matter may be injurious for some purposes and beneficial for others, but as a rule water carrying much suspended matter must be purified.
The soluble impurities cause the most difficulty in technical work, especially when the water is to be used in steam boilers.
According to the nature of these impurities, water is hard, soft, saline, or alkaline.
Hard water contains one or more of the salts of calcium, magnesium, iron, or aluminum in solution, and is usually defined as one which precipitates soap from solution. Hence it is customary to determine hardness by titration with a standard soap solution.
Temporary hardness is caused by the presence of the bicarbonates of calcium or magnesium, while permanent hardness is principally due to the neutral chlorides and sulphates of these metals. The neutral carbonates of calcium and magnesium are insoluble, but if carbon dioxide is present in the water, they dissolve, probably forming the bicarbonates CaH2(C03)2 and MgH2(C03)2'
Soft waters contain very little mineral matter. Rain water as it falls is very soft, and if it could be collected uncontaminated would he far the best for most purposes. Natural soft waters usually fall upon ground nearly free from lime or magnesia, and collect in streams or ponds by percolating through the soil. Very often the soil contains peat or other decayed vegetable matter, from which the water may derive organic impurities or coloring substances, which affect its use for many purposes. Peaty waters often contain organic acids or other material this causes them to attack iron or lead.
Saline and alkaline waters are those in which large quantities of soluble sulphates, chlorides, or carbonates occur. They frequently contain bromides and other salts. Sea-water and certain mine waters containing sulphates of copper, iron, or other metals, are the most important of the saline group. Alkaline waters, e.g. the" alkali" waters of the western states, contain considerable quantities of the alkaline carbonates or sulphates.
The purification of water for use in the industrial arts often presents considerable difficulty owing to the nature of the impurity of the magnitude of the plant necessary for large works. When possible the quality of the water should be considered in locating the works, so that little or no purification may be necessary. However, in most localities the boiler water needs some treatment.
The following are the usual methods of removing temporal hardness: -
If possible, it is best to use only the clear calcium hydroxide solution obtained by allowing the undissolved lime to settle, but this requires much space for precipitating and settling tanks. If the "milk of lime" is used, the proper quantity of quicklime is carefully weighed. slaked in a small amount of water, and the" milk" then thoroughly mixed with the water to be purified. This is Clark's process. The
sludge of calcium carbonate is best removed by the filter press.
These are very effective, but are too expensive for most purposes.
'1'he permanent hardness is less easily remedied, for in every case treatment of the water leaves some substance more or less (deleterious in solution, as shown in the following reactions:-
Magnesium and iron salts react like the calcium salts, though the iron precipitates as hydroxide when sodium carbonate is used.
When water containing soluble impurities is evaporated in a boiler, a more or less coherent deposit called boiler scale forms on the plates and tubes. This is chiefly composed of carbonate and sulphate of calcium, while in some cases magnesium hydroxide, magnesium sulphate, iron hydroxide or oxide, silica, and organic matter
are present. Calcium carbonate alone forms a porous and non-adherent scale, which is easily. removed by "blowing off" the boiler.
Calcium sulphate forms a hard, compact scale, which adheres very firmly. Scale formation is detrimental in several ways; since it is a poor conductor of heat, the fires must be driven harder; it separates the water from the boiler plates, which are thus overheated and rapidly burned out. :Moreover, the tubes and feed pipes become
clogged, and their efficiency is much reduced.
The decomposition of bicarbonates of calcium and magnesium by heat, which has already been mentioned, may take place in the boiler. Then, too, calcium sulphate is rendered less soluble by the high temperature and pressure within the boiler, and is deposited as scale. If the water carries magnesium sulphate, this deposits as monohydrate salt (MgS04* H20). Magnesium chloride is very troublesome, for it not only forms a scale, but also attacks the iron of the boiler, the probable decomposition being shown in the following equation:-
For the removal of "temporary hardness," and to a certain extent, of calcium and magnesium sulphates as well, "feed water heaters" are advantageous. The water is heated in these, and the scale-forming matter deposited, before it is delivered into the boiler.
A recent suggestion * is the use of sodium bichromate within the boiler, as a precipitant for both "temporary" and "permanent" hardness. The reactions involved are the following:-
It is claimed that the calcium and magnesium chromates precipitate in the boiler as a loose, non-adherent mass, which is removed by "blowing off" daily. It is further claimed that the free chromic acid does not attack the boiler iron.
Much care is necessary to avoid an excess of the chemical added. As a rule, the water should be treated before it goes into the boiler, but if the scale-forming material does not amount to more than ten grains per· gallon, the purification may be done in the boiler itself, followed by a daily" blowing off." A good circulation of water in
the boiler tends to keep the precipitated material loose so that it may be easily blown out.
Attempts are made to prevent the scale from attaching itself by introducing oil with the water. Kerosene is said to be the best for this purpose; but animal or vegetable oils may be decomposed by the heat and pressure of the steam, forming glycerine and free fatty acid. This acid may injure the boiler or the pipes and valves of the pumps. Mineral oils do not decompose in this way, but may enter into the scale themselves.
A great many proprietary "anti-scale" preparations are sold, many of which are of no particular value. Most of them are to be used inside the boilers. Some are supposed to act chemically on the impurities, and others are mechanical, preventing the adherence of the scale. The former usually contain soda-ash, caustic soda, barium hydroxide, or sodium phosphate or sulphite. Tannin, usually in the form of sodium tannate, is sometimes employed, by which the calcium and magnesium are separated as tannates.
Saline and alkaline waters are troublesome in a boiler, causing "priming," i.e. the passage of foam and water through the steam chest with the steam. Sodium carbonate and sulphate are particularly liable to induce priming, hence an excess of these salts must neither be added nor formed in the process of purification. One of
the worst complications in boiler water is the simultaneous presence of large quantities of alkaline chlorides and sulphates, together with magnesium sulphate. The magnesium sulphate forms a bad scale, and yet its removal, by means of sodium carbonate, makes the water liable to prime. This difficulty is of frequent occurrence in some of the western states, and no satisfactory method of treating such waters has yet been proposed. In the case of locomotives, the usual remedy is to transfer them periodically to other parts of the road, where a soft water can be obtained, which will dissolve part of the scale, and thus loosen it so that it may be blown out.
In regard to the character of the water needed for the various manufacturing processes, only a few general remarks can be made here. A soft water is desirable for a soap works or a bleachery, since the insoluble calcium soaps would otherwise be precipitated and cause a loss of soap, or injury to the goods. A hard water will sometimes cause unevenness in the shade of color in dyeing, but for some dyes, e.g. Turkey red on cotton, it is beneficial, water containing calcium sulphate is considered best for use in brewing.
Hardness of the water is of little consequence to the paper maker, but suspended matter or deep color is bad. Suspended matter or soluble coloring matter is also injurious for a bleachery, a dye works, a starch factory, or in fact for almost any industry where the water comes in direct contact with the goods or product.
Organic Chemistry for the industry
Inorganic Chemistry for the industry
Lixiviation Levigation Evaporation Distillation Sublimation Filtration Crystallization Calcination Refrigeration Density Fuels Liquid fuels Gaseous fuels Water Sulphur Sulphur Derivatives Sulphuric Acid Sulphuric acid burners Fuming Sulphuric acid Salt Hydrochloric Acid Soda Industry Caustic Soda Treatment of tank Ammonia Soda Cryolite Soda process Chlorine Industry Electrolytic Chlorine Hypochlorites Chlorates Nitric Acid Nitrates Ammonia Potash Industry Fertilizers Lime, Cement Cement Glass Ceramic Industries Pigments Bromine Iodine Phosphorus Boric Acid Arsenic Compounds Peroxides Oxygen Sulphates Alum