Humates as Coagulants

I. V. Gebler

During the hydrolysis of certain salts in acidic solutions—for example, \(\mathrm{FeCl_3}\) (pH 3), \(\mathrm{Al_2(SO_4)_3}\) (pH 4), and especially salts of titanium and zirconium (pH 2–3)—the precipitates of hydroxides and basic salts that form as suspensions are very difficult to filter, proceed extremely slowly, and give turbid filtrates.

In some cases this is of such serious importance that, in practice, in order to avoid the difficult filtration of such precipitates (when it is also impossible to separate them on a centrifuge), it becomes necessary to change the very scheme of production, choosing another, even though in other respects more complicated, one. The use in such cases of carpenter’s glue or gelatin as a coagulant either does not achieve the aim, or very large quantities of these substances are required.

The author has found that water-soluble humates—sodium or ammonium salts of humic acids, obtained from peat or humic brown coal by treating them with a solution of caustic soda or ammonia—possess good coagulating properties for acidic suspensions.

Humic acids, which are present in considerable quantity (up to 50%, and sometimes more) in peats and humic brown coals, have a condensed-aromatic structure and are oxy-carboxylic acids of high molecular weight.

Besides carboxyl and phenolic groups, there are also other peripheral groups: methoxyls, carbonyls, and aliphatic hydroxyls.

The most mobile is the hydrogen of the carboxyl group, through which salts are formed; of these, only the alkaline salts are soluble, obtained by the interaction of humic acids with caustic or carbonated alkalis. With the latter, only the carboxyl hydrogen reacts, while with alkali hydrates the phenolic hydrogen also reacts; accordingly, humic acids have a dual equivalent weight: in the first case of the order of 330 (\(\mathrm{C\!bH{-}OdeH}\)), and when phenolic hydrogens are also replaced—147 [4].

Humic acids also decompose carbonates of alkaline-earth and heavy metals with the formation of the corresponding water-insoluble salts (scheme 1), and form complex esters with acids (scheme 4). Both the acids themselves and their salts have a permutite-like character, being capable of cation exchange (mainly through the carboxyl hydrogen) (see p. 47). The molecular weight of humic acids in aqueous solutions is taken to be 1000–1400, and their molecule contains four carboxyl and hydroxyl groups.

$$ \begin{aligned} 1.\quad &\mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COOH}\\ \mathrm{COOH}\end{matrix}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COOH}\\ \mathrm{COOH}\end{matrix}\right)_2 \end{matrix} +4\mathrm{CaCO}_3 \to \mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2 \end{matrix} +4\mathrm{H}_2\mathrm{O}+4\mathrm{CO}_2. \end{aligned} $$

$$ \begin{aligned} 2.\quad &\mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COOH}\\ \mathrm{COOH}\end{matrix}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COOH}\\ \mathrm{COOH}\end{matrix}\right)_2 \end{matrix} +4\mathrm{CaCl}_2 \rightleftarrows \mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2 \end{matrix} +8\mathrm{HCl}. \end{aligned} $$

$$ \begin{aligned} 3.\quad &\mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COONa}\\ \mathrm{COONa}\end{matrix}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COONa}\\ \mathrm{COONa}\end{matrix}\right)_2 \end{matrix} +4\mathrm{CaCl}_2 \rightleftarrows \mathrm{Hum} \begin{matrix} \big/ \\[-2mm] \big\backslash \end{matrix} \begin{matrix} \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2\\[2mm] \left(\begin{matrix}\mathrm{COO}\\ \mathrm{COO}\end{matrix}>\mathrm{Ca}\right)_2 \end{matrix} +8\mathrm{NaCl}. \end{aligned} $$

$$ 4.\quad \mathrm{Hum}(\mathrm{COOH})_4+4\mathrm{CH}_3\mathrm{OH}\to \mathrm{Hum}(\mathrm{COOCH}_3)_4+4\mathrm{H}_2\mathrm{O}. $$

The above reactions proceed in dilute solutions the more completely to the right, the lower the concentration of the solutions. At higher concentration, strong acids decompose humates with the liberation of free humic acid, i.e., for the second scheme the reaction proceeds only from right to left.

The action of a humate coagulant proceeds as follows. In an acidic suspension the humate decomposes and free humic acid is released in the form of a dispersed precipitate, with a large particle surface and, at the same time, a tendency toward flocculation; the larger aggregates thereby formed carry along small particles of cations bearing a positive charge and losing it under the influence of negatively charged humic-acid particles, which leads to coagulation of the precipitate; the latter becomes coarse-grained and readily filterable, giving a clear filtrate. Heating to 50–60° accelerates the coagulation process, but is not obligatory. It is necessary that, before and after addition of the humate to the suspension, the reaction of the latter remain acidic (\(\mathrm{pH}=1\)); this is achieved without difficulty because of the small required amount of humate, which amounts to 1 to 3% relative to the weight of solid in the suspension and depends on the character of the latter.

$$ 2\mathrm{TiO}(\mathrm{OH})_2 = 2\mathrm{TiO}^{+} + 4\mathrm{OH}^{-}. $$

$$ \mathrm{Hum} \begin{matrix} \big/ \mathrm{COOH}\\ \big/ \mathrm{COOH}\\ \big\backslash \mathrm{COOH}\\ \big\backslash \mathrm{COOH} \end{matrix} = \mathrm{Hum} \begin{matrix} \big/ \mathrm{COOH}\\ \big/ \mathrm{COOH}^{-}\\ \big\backslash \mathrm{COOH}^{-}\\ \big\backslash \mathrm{COOH}^{-} \end{matrix} +4\mathrm{H}^{+}. $$

$$ \mathrm{Hum} \begin{matrix} \big/ \mathrm{COOH}^{-}\\ \big/ \mathrm{COOH}^{-}\\ \big\backslash \mathrm{COOH}^{-}\\ \big\backslash \mathrm{COOH}^{-} \end{matrix} +4\mathrm{H}^{+}+2\mathrm{TiO}^{+}+4\mathrm{OH}^{-} = \mathrm{Hum} \begin{matrix} \big/ \mathrm{COOH}>\mathrm{TiO}\\ \big/ \mathrm{COOH}\\ \big\backslash \mathrm{COOH}\\ \big\backslash \mathrm{COOH}>\mathrm{TiO} \end{matrix} +4\mathrm{H}_2\mathrm{O}. $$

Sodium salts are removed from the precipitate when it is washed, and the organic substance of the humic acids burns off when the precipitate is ignited.

For coagulation, for example, of metatitanic acid, the process may be expressed as follows (by the carboxyl group) (see p. 47).

Preparation of a Humate Coagulant from Peat

The peat is dried to a moisture content of approximately 20–25%, then coarsely ground and analyzed for the content of “conditional” humic acid with an admixture of a small amount of resin acid, which is part of the peat bitumen.

From the coarsely ground and dried peat, an average sample of 200 g is taken and ground until it passes through a sieve with 1 mm openings. From this sample an average sample, exactly 1 g, is taken and placed in a conical flask of 750 ml capacity, into which 150 ml of distilled water and 50 ml of 0.1 N caustic soda are poured. The flask is placed in a boiling water bath and heated, with shaking, for up to two hours. After cooling, the contents of the flask are quantitatively transferred to a one-liter flask, diluted with water to the mark, and left at rest for 24 hours. After this, carefully, without disturbing the sediment, 50 ml of the solution is taken from the flask with a pipette and as much as necessary is poured into one of the cups of a Duboscq colorimeter; into the other cup is poured a standard solution containing 0.1 g of conditional humic acid per liter.

After the colors have been equalized, a reading is taken from the colorimeter scales, and the humate content \((\Gamma)\) in the peat under examination is calculated in percent by the following formula:

\[ \Gamma = c \cdot \frac{h_T}{h_x} \cdot 100\%, \]

where \(c\) is the concentration of the standard solution, g/l;

\(h_T\) is the height of the column of the standard solution on the colorimeter scale;

\(h_x\) is the height of the column of the solution under examination on the colorimeter scale.

Since the concentration of the standard solution is always equal to 0.1 g/l, the preceding formula takes the form

\[ \Gamma = 10 \cdot \frac{h_T}{h_x}\%. \]

Note 1. Instead of a Duboscq colorimeter, other colorimeters may be used, for example, an Eger colorimeter or Hehner cylinders.

Note 2. The standard solution of conditional humic acid is prepared in a similar manner, but the peat sample is taken as 10 g and the 0.1 N NaOH solution as 500 ml. From the one-liter flask 100 ml is taken, and in this solution the content of free humic acid is determined by the gravimetric method [5] and is then adjusted to a concentration of 0.1 g/l.

Preparation of the Working Coagulant Solution

Dried peat (with 20% moisture), when boiled with a dilute solution of caustic soda, absorbs a very large amount of solution and, swelling, does not release it on settling. Therefore, peat ground to 3–0 mm is placed in bags made of strong cotton-paper fabric of one size or another (depending on the required amounts of coagulant), in a volume of approximately 50% of the full capacity of the bag, in order to prevent strong compaction during swelling of the peat. The bags with peat are placed in a vertical position in a cylindrical boiler heated by an open flame, on a perforated false bottom of the boiler (Fig. 1). On

for each kilogram of humic acid contained in the peat, 20 liters of a 1% solution of caustic soda are poured into the boiler. The amount of caustic soda is taken as less than that required according to the equivalent weight of humic acid (147), in order to guarantee free alkali in the solution. Boiling is continued for 8–10 hours, with periodic addition of evaporating water so that the bags with peat are always below the level of the liquid.

After boiling is completed, the solution is discharged through the tap between the bottoms of the boiler into a settling reservoir equipped with a “floating tube” seal (Fig. 2), and as much water is added to the boiler, for washing the humate out of the sludge, as there had been caustic-soda solution; after one hour of boiling, the liquid is discharged into the same settling tank and joined to the first solution.

After settling for twenty-four hours, the liquid is drawn off from the upper level through a hose with a float, leaving in the settling tank the sludge that is to be discarded.

The concentration of the resulting solution, depending on the moisture content of the peat taken and on its humic-acid content, is determined colorimetrically against the same standard humate solution.

By evaporating the solution it may be brought to the desired humate concentration, or, after being evaporated to dryness and subsequently ground into powder, it may be used in dry form.

Evaporation of the humate solution should be carried out in a conical iron pan with steam heating, while the liquid is constantly stirred by means of a stirrer or a paddle with metal brushes.

Fig. 1. Laboratory boiler for 10 kg of peat with a moisture content of 20%.

Ammonium humate has the advantage that it is entirely removed when the precipitate is ignited, without washing out the cation, and by equivalent weight liberates 43% more humic acid than sodium humate; accordingly, less ammonium humate is required for coagulation of the precipitate.

Its preparation from peat is carried out as follows.

A prepared sample of peat (as described above), in an amount of 5 g, is placed on a watch glass with ground edges in a desiccator containing a 20% ammonia solution in its lower part; similarly, the same weighed portion of peat (5 g) is placed in a desiccator with water, and both desiccators are left side by side for 24 hours. After this, both glasses (covered with second glasses) are weighed; the difference in the weight of the peat samples from the desiccators with ammonia and with water shows the amount of ammonia absorbed by the peat, in percent.

This sample is transferred to a 750 ml conical flask, into which is poured—

Fig. 2. Settling tank with a “floating tube” gate.

Fig. 3. Tank for saturating peat with ammonia.

200 ml of distilled water is added. After heating for one hour in a boiling water bath, the contents of the flask are transferred to a liter flask and left for 24 hours. The humate content is determined, as indicated above, colorimetrically against an ammonium standard solution. The latter is obtained in an analogous way, but with gravimetric determination of humate, increasing, if necessary, the sample of peat placed in the desiccator.

Preparation of a Working Solution of Ammonium Humate

For this purpose a cylindrical vessel with a double bottom is used, of which the second is a grid with 10 mm openings (Fig. 3). Bags of peat are placed on this grid (particle size 3–0 mm). Aqueous 10-percent ammonia is poured onto the lower bottom of the vessel in an amount of 10 liters per 100 kg of peat. The peat remains in the vessel for 3–5 days to absorb the ammonia, after which it is boiled with water in the same boiler as described for boiling peat with caustic soda, and under the same process conditions as described above.

The concentration of the solution is also established colorimetrically against a standard solution of ammonium humate.

As reports have shown, the ammonium humate coagulant gives good results in filtering suspensions of hydrates of titanium, zirconium, and iron oxides formed during the hydrolysis of salts, at a consumption of 0.5–2.0% of the weight of solids in the suspension.

To the ammonium humate solution, in order to avoid the appearance of mold (especially in dilute solutions), a small amount (0.5 g/l) of phenol or thymol should be added.

REFERENCES

1. V. S. Veselovskii. The Chemical Nature of Fossil Fuels. Publishing House of the Academy of Sciences of the USSR, Moscow, pp. 145–155, 1955.
2. Sven Oden. Die Huminsäure, 1919.
3. Fuchs W. Die Chemie der Kohle, 1931.
4. G. A. Stadnikov. Chemistry of Coal, M.–L., 1932.
5. G. L. Stadnikov. Analysis and Study of Coals, 2nd ed., Academy of Sciences of the USSR, 1936.
6. Coagulants for the Purification of Drinking Water, edited by Prof. V. T. Turginovich, M.–L., 1948.