Treatment of Wastewater for Phenol Removal Using Itat Brown Coal and Coke Derived from It

K. K. Stramkovskaya

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

The main harmful impurities in wastewater formed during the thermal processing of solid fuels are tar and phenols. Discharge of such waters into public water bodies causes great damage to the national economy.

At present, a large number of methods have been proposed for wastewater treatment, described in an extensive literature. However, many methods are not free of significant drawbacks. For example, the most widespread and most effective method for rendering phenolic waters harmless—extraction with butyl acetate—does not provide the required degree of purification. The residual concentration of phenols, depending on the composition of tar waters, usually ranges from 0.4 to 3 g/l [1]. Further treatment of such waters is usually carried out by a biochemical method; in this case, when biological filters are used, the initial concentration of phenols must not exceed 100 mg/l, and for aerotanks, 500 mg/l. Under industrial conditions, dilution of phenolic waters to the required concentrations is associated with additional capital expenditures and is not always rational.

A high degree of wastewater purification can be achieved by using adsorbent substances, for example, activated carbon, silica gel, organic ion-exchange resins, and other inexpensive sorbents [2].

For purification of tar water from phenols, we used brown coal from the Itat deposit and coke from this coal, obtained at a temperature of 850°C. The characteristics of these sorbents are given in Table 1. They were tested at various degrees of grinding, from the 0–0.25 mm class to 1–10 mm. The sorbent-to-water ratio used was 1:5, 1:10, and 1:16. The contact time with water varied from 3 to 30 min. Dephenolization was carried out by shaking the water with coal in a flask and, in addition, in some experiments a laboratory flotation machine was used to bring the water into contact with the sorbent.

The phenol content in wastewater was determined by the bromination method [3].

For treatment, water obtained by us in the process of thermal decomposition of Itat brown coal in the installation described earlier [4] was taken. Analysis of these water samples is given in Table 2.

Experiments on water treatment with a very fine fraction (coke 0–0.45 mm) showed (Fig. 1) that a high degree of purification, 88%, at an initial phenol concentration in the water of 2.63 g/l, was achieved at a sorbent-to-water ratio of 1 : 5 and a contact time of 10 min. Under these conditions, however, a very large amount of coke must be consumed (one ton of coke for every five tons of water), and separation of the fine sorbent fraction from the water is very difficult.

Table 1

Characteristics of the sorbent

Treatment of water with sorbents of various particle sizes was carried out at a coke-to-water ratio of 1 : 10. The results obtained are shown in Fig. 2. With increasing sorbent particle size, the degree of purification decreased very sharply. However, even the coarse size classes absorbed a large amount of phenols in 10 minutes. At the end of mixing, they separate rapidly from the water and can be removed.

Table 2

Characteristics of the tar water

Fig. 1.

A high sorption capacity for phenols is shown by the original brown coal dried to an air-dry state; under identical conditions with coke, it absorbs only 9–10% less phenols.

The data in Fig. 3 indicate that phenols are sorbed by coke slowly. To achieve a high degree of purification, it is necessary to provide the appropriate time.

To determine the capacity of coke for phenol, the same charge was used several times to purify a fresh portion of water. Coke, at a ratio to water of 1 : 100, after ten minutes of shaking, was separated—

was filtered off from the preceding portion of water and, while wet, was shaken with fresh water in the same ratio. After fivefold contact, five grams of coke, particle size 0.25–0.5 mm, absorbed 0.66 g of phenol, which constituted a phenol capacity of this sorbent of 13%.

Fig. 2.

Next, the phenol-saturated coke was regenerated by contact with one-, five-, or ten-percent caustic soda, after which the sorbent, washed with water, again absorbed phenols, giving a degree of purification of 30–33%.

Fig. 3.

In order to determine the possibility of achieving the most complete purification of tar water from phenols, one and the same portion of water was stirred for 10 min three times with a fresh portion of sorbent in a ratio of 1:10. The results are given in Table 3.

These experimental data show that supernatant water with an initial phenol concentration of 3.54 g/l, after threefold contact with fine coke, can be purified to a residual phenol content of 30–40 mg/l. A high degree of purification was also achieved when fine fractions of dry brown coal were used.

Table 3

Results of purification of supernatant water from phenols

The technological process for purifying wastewater with the indicated sorbents can be carried out in equipment used for coal flotation. Five-minute contact of coal with water in a laboratory flotation machine, into which air was supplied, made it possible to purify the supernatant water with the fine coal fraction by 77% at a sorbent-to-water ratio of 1:10.

Thus, the use of existing standard equipment (flotation machines and settling tanks) will make it possible to purify wastewater with a cheap sorbent by a continuous and highly productive method. In addition, the flotation method used at the oil-shale chemical combine in Kohtla-Järve made it possible to remove resin present in wastewater effectively [5].

Apparently, the use of brown coal and coke may be economically expedient for purifying wastewater with a low phenol concentration, i.e., for its final treatment. In this case, the spent coal can be used as fuel.

Conclusions

1. The experiments carried out showed that brown coal from the Itat deposit and coke made from it have a high sorption capacity and can be used for purifying phenolic waters.

2. Coke with a particle size of 0.25–0.5 mm absorbs 13% phenols by weight. The use of such a sorbent at a ratio of 1:10 made it possible to purify the supernatant water from pyrolysis of Itat brown coal to a residual phenol content of 30–40 mg/l.

References

1. B. I. Ivanov, K. A. Galushkina. Treatment of phenolic waters from the thermal processing of shales by condensation with formaldehyde. Chemistry and Technology of Fuel and Products of Its Processing. Issue 8, Gostoptekhizdat, 220, 1959.

2. A. Drikhs, R. Kubichko. Phenols and Bases from Coal. Gostoptekhizdat, 48, 1958.

3. A. M. Kunin, M. N. Derbaremdiker. Technological Control of Gas Production. Gostoptekhizdat, 273, 1958.

4. K. K. Stramkovskaya, A. A. Yarkin, and V. K. Oplyshko. Pyrolysis of volatile products of thermal decomposition of Itat brown coals. Coke and Chemistry, No. 3, 1966.

5. E. Madison. Wastewater treatment by the flotation method. Oil Shales, No. 2, 23, 1961.