Lenntech Water treatment & purification



Growing plants are nourished by certain constituents which they absorb from the soil and air. The chief elements drawn from tho soil are potassium, calcium, sulphur, phosphorus, and nitrogen; other elements such as silicon, iron, sodium, magnesium, and chlorine are taken up to a less degree. The natural weathering of the minerals in the ground usually provide the elements necessary to plant life; but the supply of potassium, phosphorus, and nitrogen may be insufficient and become exhausted by frequent repetitions of the same crops on the same land. The soil becomes less productive, and finally the crops are failures.
To supply this continued drain on the soil, fertilizers are employed. The natural fertilizers, barn-yard manure, urine, and decomposing vegetable mould or muck, will not be considered here, as they need little or no treatment before use.
By artificial fertilizers we understand those manurial substances, prepared from materials which need some special treatment to render them fit for plant food. The chief requisites of a good artificial fertilizer are: - /
1. It must contain at least one substance fit for plant food, and this substance must be easily convertible, by the action of rain and moisture, to such a form that plants can assimilate it.
2. It must be dry and finely powdered, so that it may be evenly distributed over the surface of the ground.
4. It must be cheap.
A complete fertilizer should supply the three essentials, potassium, nitrogen and phosphorus. But the majority of artificial fertilizers afford only one or two of these elements, and are usually sold for certain crops, or for use on particular kinds of soil.
Potassium is generally returned to the soil in the form of sulphate or carbonate (wood ashes), and occasionally as chloride. The preparation and use of these salts have already been considered and also the preparation of ground kainite (p. 1:~5)for this purpose.
Nitrogen is frequently supplied as ammonium salts, or nitrates, particularly sodium nitrate. But many substances used for fertilizers contain nitrogen in organic compounds, which decompose readily in the soil, setting free the nitrogen. Phosphorus is nearly always applied to the soil in some form of calcium phosphate derived from mineral sources 01' from organic matter. The most important branch of the fertilizer industry is the preparation of phosphates.
:Fertilizers are largely made from the waste products of slaughter houses, such as blood, bits of waste meat and other refuse, bones, hoofs, horns, and hair. Tainted meat amI animals which have died of disease are also sent to the rendering tanks. Blood is dried at a moderate heat and crushed. to powder between rolls. It contains about 10 per cent N, and is very uniform in composition.
Bones are very good fertilizing material, supplying both nitrogen and phosphol'lls when used as raw bone; i.e. without treatment other than grinding. But as a rule the bones are extracted with benzine and then boiled, or extracted with steam under pressure to remove the fats and gelatine, after which the residue is ground and used directly for fertilizer as bone meal, the fineness of this" meal" having much influence 011 the rapidity of its decay in the soil. Being more spongy and soft, it yields. its phosphoric acid in a, much shorter time than the hard" raw boue." The latter contains about 22 per cent of phosphoric acid and 4 per cent of nitrogen. But steaming reduces the nitrogen to about 1per cent. while the proportion of phosphoric acil1 is raised to 27 or 28 per cent. Bones are often subjected to destrnctive distilbtion in retorts, by which nearly all the nitrogen is driven out as ammonia" ammonium carbonate, pyridine, and other nitrogenolls organic compounds, boneblack," contains calcium phosphates and other salts, mixed with carbon. This bone-char is extensively used as a decolorizing agent in the purification of sugar, glucose, oils, and other liquids; when it ean no longer be employed for this purpose it is burned with free access of air to form "white-ash," which contains a high percentage of phosphorus. This bone-ash may be used elil'ectly as a fertilizer, but is usually treated with sulphuric acid to for111"superphosphate", which is more soluble than the tricalcium phosphate of the bone. A process for extracting the mineral phosphate from bones by digesting with hydrochloric acid has been practised to some extent. The solution of phosphoric acid thus obtained is neutralized with milk of lime, by which the calcium phosphate is precipitated, chiefly as dicalcium phosphate (ClL2UY20S)' This is sometimes sold as "precipitated phosphate," but the method is more commonly applied to low grades of mineral phosphates (p. 145) than to bones. Garbage containing fatty matter is now collected in many cities and subjected to a rendering process. It is put into steel digesters and subjected to the action of steam at 50 pounds pressure for eight 01' ten hours, when the mass is reduced to a soft pulp which is put into presses and the oily matter pressed out. The press-cake is broken up and dried in revolving steam-heated drums, after which it is powdered, sifted, and used for" filler" in fertilizers, under the name "tankage." It contains nitrogen, phosphoric acid, and a little potash. On cooling the oily matter forms a soft grease, which is used for soap and candle stock. The water which is pressed out of the tankage with the grease contains a large amount of ammonium salts and some potash; it is evaporated to dryness and the residue mixed with the tankage, thus increasing the nitrogen and potash in the latter.
Other nitrogenous waste from various industries -leather scrap, wool waste and dust from shoddy and felt mills - are used to sOllie extent; but these, though very rich in nitrogen, are very slow in decomposing, and are so light when powdered that they are easily blown away.
The press-cakes from various oil industries (e.g. the manufacture of cotton-seed, rape, and castor oils) are often ground for fertilizer. Sometimes the cake is burned for fuel and the ashes used for fertilizing, but in this case the nitrogen is lost, only the potassium and phosphorus being returned to the soil. In the manufacture of fish oils there is a considerable amount of residue from which the oil has been pressed. This is known as "fish scrap," and consists of the scales, bones, fins, and meat of the fish. It contains about 7 per cent of nitrogen and nearly 16 per cent of phosphorus pentoxide. It is dried (usually by exposure to the sun) and then crushed to a rather coarse powder. It is a valuable fertilizer, decaying rapidly in the soil and feeding the plants continually. Peruvian guano was formerly of great importance as a fertilizer, but now the beds are nearly exhausted. It consists of dried excrement, feathers, and carcasses of sea fowl, and is rich in nitrogen and phosphoric acid. It is found in certain islands near the coast of Peru and Chili, and also on the mainland at the base of the Andes, near the sodium nitrate beds (p. 119). The region is dry and hot, and the guano has been thus presened with a high percentage of nitrogen, largely as uric acid and its salts. It needs no preliminary treatment before spreading on the soil.
Fresh guano, collected yearly from Y~rious islands in the South Pacific, is damp, and contains a large amount of ammonium carbonate; this must be" fixed" by mixing with sulphuric acid, to prevent loss of the nitrogen.
Fossil guanos, consisting of fossil excrement and remains of birds and reptiles, are found in the West Indies, Bolivia, Chili, and the South Pacific islands. Since more or less rain falls in these climates, the soluble ammonium salts and nitrates have been washed out, leaving only the calcium phosphate. Some of theqe guanos have entered into combination with the rocks on which they were deposited, thus altering their original chamcter considerably; e.f1. some of them contain a large amount of calcium sulphate. l"ossil guanos are prepared in the same way as phosphate rock (See below.) The largest source of phosphoric acid is now phosphate rook, especially ((]Jatite and ]Jho.~]Jhol'ite. These are found in large deposits in Belgium, Germany, -France, Spain, Algiers, Canada, South Carolina, Florida, and the West Indies. At present the United States deposits are the most important.
Apatite [:{Ca31'~08+ CaF~ . (CaCI2)J is a crystalline mineral, occurring in large deposits in Canada and Spain. The former are very extensive, and are found in Ontario, between the St. L[twrence and Otta"'a rivers, and iu Quebec Province, along the Gatineau and du Liene rivers. The mineral sometimes OCCUl'S in veins and pockets (bonanzas) of nearly pure, massive apatite; and in other cases as distinct, hexagonal crystals 01' nodules, disseminated in calcite or pyroxene. The material is sold on a guarantee of 75 or 80 per cent of calcium phosphate, and to secure this degree of purity, "cobbing" * and hand-picking must be employed. The ore being exceedingly brittle and the gangue rock very hard, there is much loss in the" fines," from which it is not profitable to separate the phosphate rock.
Apatite varies in character from a moderately hard rock, to a soft and friable mass, called "sugar." The color varies much, but is generally blue-green or red-brown. The tricalcium phosphate being quite insoluble, the mineral must be treated with sulphuric acid to form "superphosphate." Hut since more or less calcium fluoride and chloride is present, considerable acid is uselessly consumed, and a special condensing apparatus is necessary to retain the vapors of hydrofluoric and hydrochloric acids set free, or a nuisance is created. Apatite also requires a rather strong acid (1.78 sp. gr.) for its decomposition, while the calcite and other minerals connected with it being acted upon, cause considerable loss of acid.
These objections do not apply to the phosphorites of the United States and Europe, and the cost of mining is not so great. As a result the Canadian mines are now nearly all closed, and there seems little probability that they will be opened for many years to come. Phosphorites are amorphous rocks of varying composition, but all containing a large percentage of tricalcimn phosphate, and sometimes iron and aluminum phosphates. The mode of formation of these rocks has been a much-disputed question, but they are now generally regarded as of organic, and probably animal origin. The beds are filled with fossil remains of land and marine animals and fishes. A nodular variety found in England was erroneously supposed to be fossil reptilian excrement, and was called" Coprolites." Some phosphorites are compact and hard to grind, as is the Spanish variety, but the American rock is softer and porous. In the United States there are two varieties, "land rock" and" river rock. "
Land rock occurs in heds ~weraging from 10 to 12 inches in thickness, and from 2 to 40 feet below the surface of the ground. These beds are sometimes composed of loose pebbles or gravel, but frequently these have been compacted into solid layers having a laminated structure; 01' they may form great boulders or conglomerate masses. The beds are often continuous over a large area, hut "pockets" or isolated beds are frequently found. Good rock will average from 75 to 80 per cent of tricalcium phosphate (CaaP20B)' In some cases the land rock is hard, dense, and nearly pure (hard consistency, and usually containing rather a large proportion of iron and aluminum.
Land rock is mined by stripping off the overlying earth, and digging out the phosphate rock with pick and shovel. It has been found practical to use steam shovels and dredges for "soft phosphate" and" pebble" deposits. In compact rock, blasting is necessary. '1'he work is done in open pits, tunneling not having proved successful. The depth of overburden which may be profitably removed, depends upon the thickness and purity of the deposit, but about 20 feet is the limit, except in the case of very thick beds of high grade ore. For ordinary rock, the limit is about 10 or 12 feet. [n a few cases hydraulic mining has been employed to wash away the overburden.
After mining, the rock is put through a "breaker," and reduced to lumps about 4 inches in diameter. These go to the "washer" which consists of a long, semicircular trough, set at a slight incline, in which there is a revolving shaft, carrying teeth or blades about 9 inches long, and arranged around it in the form of a spiral screw, having a pitch of about 1 in 6. The trough is set in a tank of water, or a large stream of water enters at the upper end. The lumps of rock are fed into the trough at the lower end, and being caught by the teeth, arc forced along and up the trough, against the water. The rubbing against each other, and the action of the water, washes away the sand and clay, and at the upper end the clean rock falls on screens, which separate the several sizes of lumps. It is usually dried by piling it on racks of cord wood, which are then fired and allowed to burn out; or it may be piled over castiron pipes having numerous apertures, and through which hot air from a furnace is forced. The rock is then shipped to the makers of "superphosphate."
River rock is dredged or dug from the beds of rivers and streams, especially Peace River and its tributaries in Florida, and from the streams near Charleston and Beaufort, S.C. When the deposit is in the form of loose nodules and gravel, steam dredges or centrifugal pumps are used to raise it; but when it is compact rock, special forms of grips and dredges are necessary. In most cases, river mining is not carried on in water more than 30 feet deep. River rock is very similar in composition to land rock, but is darker in color, even black, and contains more animal remains and fossils. It is preferred by foreign superphosphate makers and is generally shipped abroad.
prepared by treating insoluble rock or bone'" phosphate, with sulphuric acid. By the action of the sulphuric acid, the insoluble tricalcium phosphate is converted into monocalcium phosphate (CaH4P20S), while in many cases some free phosphoric acid is also formed. The reactions involved are as follows: -
1) CasP20S+2 H2S04+6H20= (CaH4P20S+2H20) +2(CaS04. 2 H20).
2) CasP20s+3 H2S04+6 H20=2 HSP04+3(CaS04 0 2 H20).
3) CasP20s+H2S04+6 H20 = (Ca2H2P20S 0 4 Hp)+(CaS04 0 2 H20).
Heactions 1and 2 are the ones desired in fertilizer making, but if too little acid is used, reaction 3 takes place to a greater or less extent, forming some dicalcium phosphate, which is also insoluble. If too much acid is used, reaction 2 takes place to an undesirable extent, and the product contains an excess of free phosphoric acid, which attracts moisture from the air, making the fertilizer moist and lumpy. A small excess of sulphuric acid over the theoretical quantity needed is generally used to prevent "reversion" (po 144) as far as possible. The proper regulation of the amount of acid is a matter of great care, and must be controlled by analysis of the material. The acid employed is "chamber acid" of 1.54 to 1.60 sp. gr. Concentrated acid is not used, because water is necessary in order that a hydrated calcium sulphate as well as a hydrated monocalcium phosphate may be formed. The formation of the gypsum (CaS04 0 2 H20) greatly assists in the subsequent drying of the product.
Hydrochloric acid is unsuitable for fertilizer making, because of its expense, and the formation of calcium chloride in the product. The raw phosphate should be as free as possible from impurities, such as carbonates, iron oxide and alumina. About 3 per cent of Fe20S + A120s is the limit now allowed.
The phosphate rock, powdered to pass an SO-mesh sieve, is put into a lead-lined mixer, provided with effective stirring apparatus, and the required amount of acid is added. The mixing is complete in two or three minutes, and then the liquid charge is at once run into a brick-lined" pit," where the principal reactions take place. The temperature rises very rapidly to 1000 or 1100 C., and a great quantity of gases (HCI, HF, CO2, and SiF4) escape through the draught flue. As the reaction progresses, the mass becomes stiff and finally solidifies, forming a single cake, which is broken up, removed from the pit, and dried by steam heat, at a temperature of 1050 C. The mass is then powdered in a " disintegrator mill." Frequently the superphosphate is mixed with nitrogenous or potash materials in the disintegrator, to furnish a "complete" fertilizer. It is then packed in bags and is ready for market. If the phosphate rock contains much iron or aluminum oxide, or if the decomposition by acid has been incomplete, a series of secondary reactions ensues, when the superphosphate is stored. By these, a part or all of the monocalcium phosphate (CaII4PPs), and the free phosphoric acid may be converted into the insoluble dicalcium phosphate, or into insoluble phosphates of iron or aluminum.
This constitutes what is called" reversion," and the insoluble calcium or iron phosphates so formed are called "reverted phosphate." Since fertilizer is usually valued according to its percentage of soluble phosphate, reversion is a serious matter for manufacturer and buyer. Reverted phosphate is recognized as having a certain value for fertilizer purposes, but much less than superphosphate.
'When due to incomplete decomposition of the roch:, rcversion takes place according to the following reaction: - CaH41'208+CaslOg = 2 Ca2H21'20g.
'When the rock contains iron or alumina, the temperature of the reaction in the pit is kept as low as possible, to prevent combination between these oxirles and the free phosphoric acid formed. It is customary in this case to remove the superphosphate from the pit as soon as it solidifies, and to cool it by exposure to the ail'. A "double superphosphate" is also made, especially in Europe, and contains more soluble phosphoric acid than the ordinary superphosphate. A certain quantity of bones or phosphate rock is decomposed with sufficient dilute sulphuric acid to set free all the phosphoric acid and precipitate all the calcium as hydrated calcium sulphate. The precipitate is then filtered off by means of the filter press (p. 12), and the clear solution of phosphoric acid is concentrated by surface heating in learl pans, to a density of M;O Be., at which strength the solution contains nearly 45 per cent 1'205' During this concentration, the iron and aluminum phosphates separate and are removed. The strong solution of phosphoric acid is then trmLted with ground phosphate rock, in propel' quantity to form monocalcium phosphate, which is dried and disintegrated. The reactions involved are as follows:-
1) CaSP20S + 3 H2S04 + 6 H20 = 3 (CaS04 . 2 H20) + 2 HsPO •.
2) CaSP20g+ 4 HSP04 + 6 H20 = 3 (CaH4])20g + 2 H20).

gypsum or other sulphate, is obtained.. ~Ioreovcr, a low-grade phosphate rock can be used. for making the phosphoric acid, which would not furnish a strong fertilizer with sulphuric acid.. A small amount of phosphate rock is used. directly for fertilizer, without other preparation than fine grinding. But tricalcium phosphate, being very insoluble, is only slowly assimilated by plants, and its action is not very marked.. Several years are necessary for its complete decomposition.
Phosphatic slag is now used. to a considerable extent as a fertilizer, especially in Europe. In the process of making Bessemer steel by the Thomas and. Gilchrist method, pig iron from ores containing phosphorus is treated with an excess of lime in a Bessemer converter, lined with lime, while a blast of air is forced into the liquid mass. At the high temperature of the melted iron, the phosphorus is oxidized to pentoxide, which combines with the lime. The silica, alumina, lime, and magnesia unite to form a slag, into which the calcium phosphate produced also goes. By proper regulation of the charge, a slag containing about 17 per cent of pentoxide (P205) is obtained. The phosphate in the slag is supposed to be a tetracalcic phosphate (CaJ)20~), which is insoluble in water, but is much less stable than tricalcimn phosphate. When exposed to the weather in the soil, it decomposes, though somewhat slowly, and the phosphorus passes into a form which plants can assimilate. In order that this decomposition may take place, the slag must be ground very fine, so that 90 per cent of it will pass through a sieve with 100 meshes to .......p
FIG. 55.
the linear inch. The grinding is best done in a ball mill (Fig. 55), which consists of a cast-iron dl'1l111 (D), and containing numerous chilled iron or steel balls (B) of different sizes. The coarsely ground sbg is powdered by the rubbing and pounding of the balls as drum rotates. It then passes through the perforated plates (P) falls on fine brass sieves (S). The coarser particles, which callnot pass through the sieves, return to the interior of the drum, through the openings (C, C), for further grinding.
Slag fertilizer needs no further treatment than very fine grinding, but it is slow in decomposing, and its full effect is not obtained for two or three years. It decomposes more rapidly than ground phosphate rock, however, and is cheap. There has been considerable controversy among agricultural chemists as to the relative value of soluble and insoluble phosphates. Some hold that the soluble phosphate is at once converted into the insoluble form when it comes into contact with the alumina and iron in the soil; and that this insoluble phosphate dissolved or absorbed by the sap in the plant roots, the sap presumably having an acid nature. Other chemists claim that only soluble phosphate, as such, can be taken up by the plant. It appears from observed facts, however, that both soluble and insoluble phosphates are taken up by the plant, but the nature of the soil an important factor. On a soil poor in lime, and containing organic matter, insoluble phosphates produce their best results; if the soil contains much lime, then the superphosphate appears have the advantage.
The soluble character of the superphosphate permits its diffusion through the soil by rain, so that it is brought immediately to the roots of the plants. But the insoluble phosphate must turned under the soil, and the roots grow to it; then, too, when finely ground, it possesses but little value, owing to the slow decomposition; but when in a very fine powder it is taken up in some by the roots of the plant with fair rapidity.
The manufacture and sale of artificial fertilizers are, to a certain extent, under legal restriction in nearly all the states. 1'0 prevent fraud, manufacturers are required to take out a license, and to submit samples for analysis by state chemists; frequently a guarantee of the stated composition is required. The methods of analyses fertilizers are set forth in detail in the bulletins of the several state agricultural experiment stations and of the United States Department of Agriculture.* In general the matter determined by analysis lllay be summed up as: - (a) Water, both hygroscopic and combined .
(b) Total phosphoric acid.
(c) Soluble phosphoric acid.
(cl) Reverted phosphoric acid.
(e) Total nitrogen.
(1) Potash.

Another substance frequently sold as fertilizer is pulverized gypsum (CaS04 • 2 H20), which, when crushed to a fine powder, is brought into commerce under the name of "plaster." As a fertilizer it is of little value, except in soils poor ill lime or those containing "black alkali" (sodium carbonate). But it is also claimed to have a beneficial action in retaining nitrogen in the soil. The calcium sulphate is supposed to be decomposed by the ammonia and carbonic acid from the air and rain, forming ammonium sulphate and calcium carbonate. Ammonium sulphate furnishes nitrogen in a form which plants can assimilate.
Much attention has been devoted, especially in Germany, to methods of recovering fertilizing material from the sewage of cities. But when closets are flushed with water, the effluent is generally too dilute to be worth recovering. It is, however, used to some extent, in irrigating lands, generally those owned by the !11tmicipality. Sewage is often precipitated with lime or other substance, but this generally renders the sludge useless for fertilizing. The contents of dry vaults or cesspools are collected at regular intervals and used for fertilizing.
But sewage treatment of any kind is usually practised to prevent pollution and unsanitary conditions in the streams and water supplies, rather than for the utilization of the fertilizing materials to be obtained.

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

  • About Lenntech

    Lenntech BV
    Rotterdamseweg 402 M
    2629 HH Delft

    tel: +31 152 610 900
    fax: +31 152 616 289
    e-mail: info@lenntech.com