On the Cation-Exchange Properties of Humic Brown Coals and the Possibility of Their Use for Water Softening

I. V. Gebler and K. K. Stramkovskaya

The growth of socialist industry, the adoption and introduction of the most modern production methods and units, the need to preserve equipment, and the struggle for high product quality impose special requirements on the quality of the water consumed.

Water intended for the following purposes must be softened: 1) for domestic and drinking needs; 2) for cooling various types of equipment (chemical apparatus, blast furnaces, open-hearth and other furnaces, cylinders of internal-combustion engines, condensers of steam engines, turbines, etc.); 3) for feeding steam-power installations (thermal power stations and district power stations, locomotives in transport); 4) for supplying industries in which water enters into the composition of the product or comes into direct contact with it.

The purpose of water softening is the complete or partial removal from water of hardness cations. This can be achieved, first, by converting soluble hardness salts—salts of $\mathrm{Ca}^{++}$, $\mathrm{Mg}^{++}$, and $\mathrm{Fe}^{+++}$—into sparingly soluble compounds, as a result of which a large part of the calcium, magnesium, and iron contained in the water precipitates; second, by replacing calcium, magnesium, and iron with other cations whose presence does not make it hard. The exchange of $\mathrm{Ca}^{++}$ and $\mathrm{Fe}^{+++}$ dissolved in water for $\mathrm{Na}^{+}$ or $\mathrm{K}^{+}$ is achieved by filtering the water to be softened through cation exchangers capable of cation exchange.

At the present time, three principal methods of water softening have found practical application: 1) the method of precipitating hardness salts by reagents; 2) the method of cation exchange (cationization); 3) the thermal method.

The cationization method is the most widely used.

Materials Capable of Ion Exchange

A very large number of solid, practically insoluble substances are known that possess the ability to undergo ion-exchange reactions. All these substances are divided into inorganic and organic ones, and, according to the nature of the ions exchanged, into cation exchangers and anion exchangers.

Among inorganic substances capable of cation exchange, the following may be noted:

1) a group of natural minerals united under the name of zeolites;
2) aluminosilicates of the kaolin group. These include bentonite clays, marine clays, and some others.

However, all these substances have not found practical application as cation exchangers, chiefly because of their low exchange capacity.

Among other minerals capable of cation exchange, glauconite sand, or glauconite, has found wide application; it is a

a particular ferro-aluminosilicate. It occurs in nature in the form of glauconite sand, or small grains 0.2–0.6 mm in size [2].

For purposes of water softening, glauconite sands from only certain deposits are used, since the grains of not all glauconite sands are water-resistant. In comparison with other cation exchangers, glauconite has a high specific gravity (2.2–2.85). The bulk density of glauconite ranges from 1.35 to 1.55 t/mm³. In view of the fact that glauconite has insignificant porosity, cation exchange proceeds on it predominantly extramicellarly, i.e., on the outer surfaces of the grains. In practice, under production conditions, the exchange capacity of glauconite does not exceed 200 g-eq./m³ [2].

A major drawback of glauconite is its peptizability, i.e., destruction of the grains with the release of aluminum and silicon compounds in colloidal form. This phenomenon is especially pronounced after regeneration of the glauconite.

In addition to glauconite, volchonskoite also has exchange capacity; it is an aqueous chromium-magnesium silicate of composition MgOCr₂O₃nSiO₂mH₂O. In terms of exchange capacity, volchonskoite ranks first among minerals, giving 3000–3700 g-eq./m³.

Volchonskoite has not found practical application because of the insufficiency of its explored reserves. In addition to these natural ion exchangers, artificially prepared aluminosilicates capable of ion exchange, the so-called “permutites,” are also known.

However, up to the present time the principal consumer of ion exchangers has been thermal power engineering, where enrichment of the treated water with silicic acid, as occurs in softening water with aluminosilicate cation exchangers, is entirely inadmissible. At present there is no industrial production of these cation exchangers in the USSR.

The second group of substances capable of the process of ion exchange consists of organic substances. Some of them are prepared artificially, while another part consists of natural substances. The group of artificial cation exchangers is represented by:

1. Sulfocoal. The starting material for preparing sulfocoals is preferably low-ash, caking hard coal.

Preparation of the starting material before sulfonation consists in crushing and screening out “grains” (particles 0.3–1.5 mm), which are subsequently subjected to sulfonation with oleum containing 20% free SO₃. The amount of oleum consumed is four to five times the weight of the coal. The reactions occurring with the organic matter of coals under the action of oleum are complex and have not been studied. The acquisition of cation-exchange properties, apparently, must be explained by enrichment of the substance with complex carboxylic acids, as well as sulfonic acids. The hydrogen of these active groups subsequently determines the capacity of the resulting materials for cation exchange [4].

1. Cation exchangers—artificial resins—are products of the condensation of aromatic sulfo or carboxylic acids (or mixtures of them), as well as phenols, with aldehydes. With such condensation, a water-insoluble resin-plastic is obtained. These include the sulfophenolic cation exchanger, resorcinol (Vofatit C), and Vofatits P Kₛ Kₙ C (Table 1).

Natural substances capable of ion exchange are humic substances, i.e., chiefly humic acids and their salts—humates—present in chernozem, peat, and brown coals.

At present there is no doubt about the proposition that in all coal cation exchangers the active part capable of cation exchange is humic acids and their salts. Moreover, cation exchange proceeds at the expense of the cations of the carboxyl groups, and, at elevated pH, of the hydroxyl groups of these acids [2].

Table 1

Technological characteristics of certain cation exchangers (1, 4, 5, 2)

The following were tested as cation exchangers:

1) humic acids, isolated from peat or brown coal. Although the exchange capacity of these cation exchangers is sufficiently high, they have not found practical application because of their low mechanical strength and some solubility in water;

2) peat from various deposits, crushed and screened into the required fractions, has the capacity for cation exchange; however, peat is insufficiently mechanically strong and has the ability to swell strongly in the filter, to the point that the passage of water through it becomes impossible;

3) brown coals, characterized by a high content of humic substances and sometimes called humic coals, were proposed as cation exchangers by V. P. Astaf'ev.

The quantitative technical indices of humic coals found by Astaf'ev are as follows [3]:

1. The exchange capacity of Moscow-region coals, with a grain size from 0.3 to 3 mm, ranges from 400 to 800 t·deg/m³, and with a grain size from 0.3 to 1 mm—from 600 to 1200 t·deg/m³. Coals from the Bashkir deposits show the same exchange value. The exchange capacity of coals from the Bogoslovsk deposit, with a grain size from 0.3 to 1.5 mm, ranges from 1200 to 2000 t·deg/m³.

2. The bulk density of Moscow-region coals ranges from 0.6 to 0.7 t/m³; of Bashkir coals, from 0.5 to 0.6 t/m³; of Bogoslovsk coals, from 0.7 to 0.8 t/m³.

3. The consumption of common salt for regeneration is 75 g/t·deg.

4. The average rate of water softening is 10 m/hour.

G. P. Sludskii [6], while checking the exchange capacity of Bogoslovsk coal under industrial conditions, selected a fraction of the required granulometric composition, using screenings of coal that had lain in storage for a long time and had undergone considerable destruction under the influence of natural factors. A coal fraction of 0.3—3 mm was taken for loading the filter.

The working exchange capacity for the Na⁺ cation, at a filtration rate of 8—10 m/hour, was established as 180—200 t·deg/m³ when the cation exchanger was operated until the hardness of the softened water reached 0.2°.

V. A. Klyachko, A. A. Kastalsky [1], for the humic coal of the Bogoslovsk deposit, indicate an exchange capacity under Na-cationization of 350 g-eq/m³ (fraction 0.5–1.5 mm).

Such contradictory information on the exchange capacity of one and the same Bogoslovsk coal, and at the same time the desirability of replacing expensive cation-exchange materials with cheap and readily available brown coals, made it of interest to check the cation-exchange properties in general of brown humic coals with a definite content of humic acids.

In ion exchange, processes of both a volumetric and a surface character are possible, as well as their combination. In this case there may be two cases:

1. The exchanging ions or adsorbed molecules are too large in comparison with the internal dispersity of the ion exchanger and therefore prove unable to penetrate into its pores.

In this case the exchange process proceeds only on the external surface of the ion exchanger and is called extramicellar or extracrystalline. Thus, the internal surface of the ion exchanger proves inaccessible to ions of the given size.

1. The volumes of the exchanging ions are so small in comparison with the internal dispersity of the ion exchanger that they are capable of penetrating into it. The ion exchanger, with respect to such ions, proves to be porous, and the exchange process proceeds predominantly intramicellarly or intracrystallinely.

In connection with this, it is of some interest to check the effect of coal swelling on its exchange capacity, in which the possibility of ion exchange should appear both on its external and on its internal surface.

Experimental Part

Every ion exchanger is characterized by the following indices:

I. Physical:

1) bulk density of the air-dry substance;
2) bulk density of the moist swollen substance and the percentage increase of its volume due to swelling;
3) volume of pores between the grains of the ion exchanger;
4) sieve analysis.

II. Mechanical strength and chemical stability.

III. Hydraulic indices:

1) pressure loss during filtration;
2) loosening index.

IV. Exchange capacity.

For the investigation, a brown humic coal from the Irshinsk deposit, located in the Rybinsk coal-bearing region of the Kansk Basin, was taken. The age of the coal-bearing deposits is Jurassic; its proximate analysis is given in Table 2.

Table 2

The cation-exchange properties of the brown coal were determined by the standard method adopted for testing ion exchangers [2].

The physical characteristics of the coal under investigation are given in Table 3.

Table 3

Mechanical strength.
An important factor in evaluating technological qualities is their mechanical strength. It determines the wear of the material during operation of cation-exchange filters. Up to the present, however, sufficiently objective methods have not been developed for evaluating the mechanical strength of cation exchangers. The latter is judged from indirect indicators. Such an indirect indicator may be the coefficient of conditional wear. It characterizes the yield, in percent, of finer fractions of the cation exchanger in comparison with the original material after no fewer than 50 normal softening cycles have been carried out.

The mechanical strength of this coal was determined by the method proposed by Prof. I. V. Gebler. According to this method, the sample is placed in a crusher, consisting of a drum 200 mm in diameter and 75 mm wide. On the inner surface of the drum there are, symmetrically arranged on each quarter of the circumference, four iron strips bent in the form of a trough for gripping pieces of material. The rotor of the crusher consists of three hammers hinged to a shaft. The rotational speed of the crusher drum is 60 rpm. The angular speed of rotation of the rotor is 810 rpm; the peripheral speed of rotation of the hammers themselves is

\[ V=\frac{\pi n R'}{30}=\frac{\pi\cdot810\cdot0.08}{30}=6.8\ \text{m/sec} \]

for $R'=0.08$ m.

The pieces of coal to be tested are split with a chisel and hammer to such a size that they pass through a sieve with round openings 25 mm in diameter and are retained on a sieve with openings 15 mm in diameter.

Thus, for the mechanical test the 25–15 mm class is always taken in an amount of 100 g; these pieces are placed in the crusher, where they are subjected to crushing for 10 sec. After this the coal is screened on sieves with openings of 15, 10, 7, 5, 3, and 1 mm. Since the amount of the class larger than 15 mm is highly characteristic, the strength of the coal may be briefly expressed by the amount of this class in percent, which is taken as the coefficient of mechanical strength of the coal, $K$. An additional index characterizing the resistance of the coal to abrasion is the yield of the fine class passing through the sieve with 1 mm openings.

Various coals tested by this method gave the following values (Table 4).

In comparison with coking coals used for producing gas coals, this coal has a mechanical strength close to theirs.

As a hydraulic characteristic, the head loss during filtration was determined. The test was carried out in a standard tube of diamet—

Table 4

with a diameter of 25 mm and a height of 1350 mm. The tube was filled to a height of 250 mm with coal of the indicated sieve composition, previously swollen in water for 48 hours and regenerated with a five-percent NaCl solution, at the rate of 150 g per ton-degree of the expected exchange capacity. The following data were obtained (Table 5).

Table 5

When the obtained data are compared with those available in the literature for sulfocoal, a cation exchanger now widely used, it follows that, with respect to head loss during filtration, the brown coal of the Irsha deposit does not exceed it; on the contrary, it gives a low magnitude of losses.

Exchange capacity

After determining the head loss during filtration, the filter was charged to a height of 410 mm with coal already swollen in water and was washed with one-percent sulfuric acid to remove Al+++, Fe+++, and Ca++ ions bound in the salts of humic acids, until the reaction for the corresponding cation disappeared, and then with distilled water until neutral reaction to methyl orange.

After 30 minutes of loosening of the coal in the filter, cationization with respect to the H+ cation was carried out. The solution for determining the exchange capacity, with hardness 12.3°, was prepared by dissolving calcium chloride in distilled water. Filtration was conducted at a rate of 7.5 m/hour until the appearance in the filtrate of a residual total hardness of 0.15°.

The exchange capacity obtained with respect to the H+ cation in softening water with CaCl₂ was equal to 420 ton-degree/m³ of air-dry coal. After this the coal was regenerated with one-percent sulfuric acid and washed with water. In order to conduct a test for the Na cation through the coal, before its regeneration with NaCl solution, water with calcium bicarbonate of hardness 29° was passed through it until the hardness of the water flowing from the filter became equal to the hardness of the water entering the filter.

Then this coal was regenerated with a five-percent solution of common salt, taken in amounts of 150 and 70 g per ton-degree of hardness, calculated for an expected exchange capacity of 500 ton-degree/m³.

After loosening, water containing dissolved CaCl$_2$ with a hardness of 12.3 was softened at a filtration rate of 7.5 m/hour.

The following data were obtained (Table 6).

Table 6

The exchange capacity of the coal with respect to carbonate hardness and to the H-cation should be considerably greater. For the H-cation and carbonate hardness, the reaction should proceed to completion, owing to the fact that the CO$_2$ formed will be removed from the reaction sphere.

For this coal, in which the exchange capacity had previously been determined by Na-cationization, regeneration was carried out with a one-percent solution of H$_2$SO$_4$, it was washed with distilled water to a neutral reaction, and hard water with a carbonate hardness of 29.3° was softened.

Calcium bicarbonate was prepared by passing CO$_2$ through limewater until it cleared. The resulting solution was then filtered to remove traces of CaCO$_3$ precipitate.

The following results were obtained (Table 7).

Table 7

In order that the reactions proceed intermicellarly, the coal was treated, for the purpose of swelling it, in five-, ten-, and fifty-percent aqueous pyridine solution.

The increase in volume due to swelling was obtained as follows (Table 8).

Table 8

It should be noted that, when the coal was treated with a pyridine solution at concentrations of 5 and 10%, the extract was light; at a concentration of 50%, however, the extract was strongly colored.

Samples of coal swollen in pyridine, washed with one-normal sulfuric acid and distilled water, showed the following exchange capacity for the H-cation when softening water with carbonate hardness of 26° at a filtration rate of 6 m/hour.

Table 9

A lower filtration rate in this case was adopted because, with extramicellar exchange, the reaction proceeds more rapidly than with the presumed intermicellar exchange, since in the former case long diffusion paths are excluded for the exchanging ions. Hence it is evident that swelling of the coal in a pyridine solution greatly increases the exchange capacity. The pyridine needed for its swelling can be regenerated by salting out. The swelling process may be recommended to be carried out as follows. Dry coal is soaked with a pyridine solution, kept in it for 24 hours, and then the excess pyridine solution is drained off. Since pyridine dissolves well in water, the coal should be washed with a small amount of water. From the resulting solution the pyridine can be regenerated by salting out with common salt.

Conclusion

Coals of this type possess sufficiently high mechanical strength and a low head-loss index during filtration.

Their exchange capacity for the Na-cation and total hardness reaches up to 500 m-equiv./m³ of air-dry coal; for the H-cation and carbonate hardness it is about 600 m-equiv./m³ of air-dry coal. They may be recommended for H-cation softening of waters with high carbonate hardness, which usually predominates in natural waters.

Preliminary treatment of the coal with aqueous pyridine solutions, owing to swelling of the grains and an increase in the reaction surface as a result of the penetration of salt solutions into the coal particles, increases its exchange capacity.

LITERATURE

1. Klyachko V. A., Kastalsky A. A. Water Purification for Industrial Water Supply. pp. 177, 196, 199. Moscow. State Publishing House of Construction Literature, 1950.

2. Apel’tsin I. E., Klyachko V. A., Lur’e Yu. Yu., Smirnov A. S. Ion Exchangers and Their Application. pp. 72, 250. State Publishing House “Standartgiz,” 1949.

3. Astaf’ev V. P. and Astaf’eva M. F. Experience in softening with humic coals. For Fuel Economy, No. 1, pp. 32–34, 1946.

4. Prokhorov F. G. and Korsheva I. G. Characteristics of the principal organic cation exchangers. Proceedings of VTI, No. 6, 1947.

5. Demin V. S. Chemical water purification with “K” sulfocoal. For Fuel Economy, No. 4, 1949.

6. Slutsky T. P. Result of the industrial use of Bogoslovsk coal as a cation exchanger. For Fuel Economy, No. 3, 1947.