STUDY OF THE PROCESS OF OBTAINING HUMIC ACIDS FROM PEAT

N. M. Smolyaninova, A. N. Moskalchuk

(Presented by the scientific-methodological seminar of the Faculty of Chemistry and Technology)

Humic acids possess a number of valuable and distinctive properties (the ability to sorb various substances, to reduce a number of compounds, to exert a stimulating effect on plant growth, etc.), which has led to their use in various branches of the national economy [1—9].

The importance of humic acids as a factor in increasing yields and as a plant-growth stimulant has been demonstrated in numerous studies by L. A. Khristeva [2, 10, 11, 12, 13], S. S. Dragunov [14, 15], and many other Soviet and foreign scientists.

The organization of industrial production of humic acids will give impetus to their more rapid introduction into various branches of industry and agriculture.

It is especially expedient to organize the production of humic acids and humates based on them in the areas of the largest peat deposits located in the territory of Tomsk and Novosibirsk oblasts.

The present work is devoted to studying the process of obtaining humic acids from peat of the Tagan deposit in Tomsk Oblast and to determining the optimal conditions that ensure their maximum yield.

The characteristics of the peat from the indicated deposit are given in Table 1.

The generally accepted method for extracting humic acids from various solid fossil fuels is their treatment with aqueous solutions of alkalis and salts of alkali metals. The use of organic solvents, such as aqueous dioxane, amines and aminospirits, urea, furfural, etc. [16, 17, 18], is impractical because of their high cost and the low yield of humic acids. Proceeding from this, and also taking into account the lower cost of caustic soda in comparison with caustic potash, we adopted as reagents for the extraction of humic acids aqueous solutions of caustic soda and sodium carbonate of various concentrations.

The acid yield is determined by the method of G. L. Stadnikov [19], by a single treatment of peat with alkali at 80°C for one hour, with the ratio of the weight of dry peat to the volume of alkaline solution being 1 : 400.

Fig. 1 shows the effect of the concentration and type of alkaline reagent on the yield of humic acids, from which it is evident that caustic soda gives a higher product yield than soda ash. An increase

Table 1

Characteristics of peat from the Tagan deposit

Group composition, %

Botanical composition, %

of the concentration of the alkali solution above 1–2% has practically no effect on the yield of humic acids. When sodium carbonate was used, a 2% solution proved optimal.

Further work was carried out with a 1% solution of caustic soda.

An important technological factor in the production of humic acids is the ratio of the volume of alkali to the peat sample. Table 2 gives data on the yield of acids at different reagent ratios, which varied from 1:25 to 1:400. It follows from the table that, in a single treatment of Tagan peat with a one-percent solution of caustic soda, the amount of alkali (within the investigated ratios) has very little effect on the yield of humic acids. As the optimum, we adopted a peat:alkali component ratio of 1:100, since reducing the volume of alkali causes difficulties in settling and filtering the concentrated peat-alkali suspension, while increasing it is inexpedient.

Fig. 1. Dependence of the yield of humic acids from peat on the concentration and type of alkali: 1 — caustic soda; 2 — sodium carbonate.

When determining the influence of temperature and duration of the process of treating peat with alkali on the yield of humic acids, treatment was carried out without heating the reaction mixture, and also at temperatures of 50°C, 80°C, and 100°C (with boiling of the suspension). The treatment time was varied from 5 min to 24 hours (depending on the temperature).

The data presented in Fig. 2 show that, with increasing temperature, the yield of humic acids rises sharply. Under conditions of boiling the reaction mixture for 30 min, the product yield is 5 times higher than during the same time at room temperature.

It should be noted that carrying out the process while boiling the reagents is the most effective, since this ensures a high yield of humic acids within a very short time. It follows from this that the temperature factor is of extremely great importance for this process, which is a typical heterogeneous chemical process with diffusion complications.

Table 2

Influence of the ratio of reagents on the yield of humic acids

Fig. 2. Dependence of the yield of humic acids on temperature:
1 — at treatment time 0.5 hour
2 — “—” 1.0 “
3 — “—” 3.0 “
4 — “—” 6.0 “

The duration of treatment (Fig. 3) has a significant effect on the yield of humic acids; however, the influence of temperature is more pronounced.

Thus, on the basis of the results presented, as optimal conditions for extracting humic acids from peat we adopt treatment with alkali for 1 hour at a temperature of 80°C and 0.5 hour when boiling the reaction mixture. However, the latter operation may cause a significant change in the humic acids due to their oxidation, which proceeds especially intensively in alka-

alkaline medium in the presence of air [16]. From this standpoint it is desirable to carry out this process at 80°C, intensifying it by better mixing of the reagents. When humic acids are obtained for practical purposes, the oxidation factor may be neglected [18].

The influence of the degree of peat comminution on the yield of humic acids was also studied.

The experiments were carried out with air-dry and moist peat: the first was ground until it passed through sieves with hole sizes from 5.0 to 0.15 mm; the second ($W_p = 88.76\%$) was subjected to different degrees of processing by passing it through a meat grinder 1, 3, 5, and 7 times.

The results presented in Table 3 show that, as the degree of comminution of air-dry peat increases, the yield of humic acids increases, while with an increase in the degree of processing of moist peat it decreases.

Fig. 3. Dependence of the yield of humic acids on the time of treatment of peat with alkali at different temperatures:
1 — 50°C; 2 — 80°C; 3 — 100°C

The literature indicates [16, 20, 21] that, when peat is dried to the air-dry state, a number of irreversible colloid-chemical transformations occur in it, as well as oxidation of the organic matter, in particular of humic acids and the nonhydrolyzable residue (“lignin”); this should not fail to affect the yield of humic acids. To determine the effect of drying on the yield of humic acids, peat with natural moisture content, passed once through a meat grinder, was placed in a tray at room temperature. As it dried, at certain time intervals a sample was taken for determination of moisture content and yield of humic acids. The results obtained are presented in Table 4.

Table 3

Effect of the degree of peat comminution on the yield of humic acids (T = 80°C; treatment duration 1 hour)

The data show that the initial peat (with natural moisture content) and the final peat (air-dry) practically do not differ from one another in the yield of humic acids. Of interest is the presence of a minimum yield at a peat moisture content of about 66%. Here, the process of oxidative polymerization of humic substances may occur, as a result of which their solubility in alkali decreased. Observed-

its subsequent increase in the yield of humic acids is apparently explained by the oxidation of certain peat components, for example, the nonhydrolyzable residue, with the formation of acidic products, similar in their properties to humic acids and soluble in alkali.

Thus, from the standpoint of the yield of humic acids, both air-dry peat and peat with natural moisture content can be used.

Table 4

Effect of the initial moisture content of peat on the yield of humic acids

In developing a technology for the production of humic acids, it is necessary to determine the number of repeated peat treatments. Our experiments showed that during the first treatment with 1% alkali at 80° for one hour, at a reagent ratio of 1 : 100, 30.24% of humic acids is extracted; during the second—6.0%; during the third—1.19%. Consequently, exhaustive extraction of the acids requires multiple treatment of the peat, while for practical purposes it is more expedient to carry out a single treatment.

On the basis of the experimental data presented above, the following conditions for carrying out the process may be recommended:

1. Degree of peat comminution—less than 1 mm.
2. Reagent—1% aqueous solution of caustic soda.
3. Ratio of the weight of peat to the amount of alkaline solution: 1 : 100.
4. Process temperature: 80–100°C.
5. The initial moisture content may correspond both to air-dry peat and to peat with natural moisture content.
6. Duration of the alkali-treatment process—0.5–1.0 hour, depending on the temperature.
7. Treatment—single.
8. Stirring intensity—as high as possible.

The yield of humic acids from air-dry Tagan peat under these conditions was 35.05%.

Table 5 gives the characteristics of humic acids obtained under optimal conditions from various samples of Tagan peat. The content of acidic functional groups was determined by the me-

Table 5

Characteristics of humic acids of Tagan peat

Elemental composition is given on a combustible-mass basis, %. Content of acidic groups is given in mg-eq/g.

… in the year by K. I. Syskov and T. A. Kukharenko [22]; the amount of readily hydrolyzable substances was determined according to S. S. Dragunov [20], and the yield of alcohol-soluble substances in a Soxhlet apparatus. The hymatomelanic acids soluble in alcohol, whose yield was 15%, and the insoluble humic acids were characterized in the same way as the humic acids.

From the data in Table 5 it can be seen that, in the process of natural drying of peat, a significant change in the humic acids occurs. This is evidenced by the large difference between samples 1 and 2 in carbon and hydrogen content, in the yield of readily hydrolyzable substances, and especially in the amount of phenolic hydroxyls.

Conclusions

1. The influence of various factors on the yield of humic acids from Tagan peat has been studied.

2. The optimum conditions for the process of obtaining humic acids from peat of the Tagan deposit have been determined. The results obtained may serve as the basis for the development of an industrial technology for this process.

References

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