Volume 234　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1974

PELLETIZING FERRUGINOUS SLUDGE WASTE FROM ANILINE DYE PRODUCTION

V. M. VITUGIN, A. S. BOGMA, V. V. SHVETSOV

(Presented by the scientific-methodological seminar of KhTF)

Ferruginous sludge, formed in the process of producing aniline by the method of reducing nitrobenzene with cast-iron turnings, is practically a waste product of aniline production. At the Kemerovo Aniline-Dye Plant (KAKZ), the daily output of sludge amounts to as much as 200 tons.

Studies have established that the sludge contains 75–80% magnetite, 5–7% free iron, 2.5–3.0% free carbon, up to 2% mechanical impurities, and 1.9–2.2% organic substances soluble in spirit-benzene. The sludge is an extremely finely dispersed material: fractions larger than 60 μm — 5.1%, 60–30 μm — 12.6%, 30–24 μm — 82%, 24–10 μm — 21.8%; less than 10 μm — 52.3%.

The high dispersion of the sludge, because of its low gas permeability, does not permit its effective use in layered metallurgical processes and, in particular, in the sintering process [1]. For such a material, pelletizing is absolutely necessary.

The effectiveness of the process of pelletizing dispersed materials depends on the ability of the material to aggregate, determined by the nature of the material, and on the technological regime. The aggregate of natural properties of materials being pelletized, as we have proposed [2], should expediently be evaluated by the pelletizability index. The maximum molecular moisture capacity of the sludge is 15.1%, and the capillary moisture capacity is 30.6%. According to calculated data, the pelletizability index of the sludge is 0.98, which is close to the limiting value. Accordingly, the optimum moisture content for pelletizing is close to the maximum molecular moisture capacity of the sludge and, according to calculation [3], is 15.5%. The moisture content of the initial sludge is 24–26%; therefore, before pelletizing the sludge must be dewatered.

It is impossible to dewater KAKZ sludge to the optimum pelletizing moisture content by common mechanical methods. Taking this into account, as well as the specificity of aniline production and the composition of the sludge, the most expedient approach appears to be pelletizing the sludge with additions of cast-iron turnings and common salt by the Yarkho method [4]. The essence of this method consists in the formation of a strong framework from the corrosion products of a mixture of iron-ore fines, cast-iron turnings, and common salt. The presence in the sludge itself of unreacted cast-iron particles will help reduce the addition of fresh turnings to the charge. Due to the heat released in the corrosion process, the moisture content of the charge will decrease.

The present communication gives the results of laboratory studies of pelletizing KAKZ sludge by the recommended method.

Experimental charges were prepared from sludge with additions of gray cast-iron turnings of size 3.0–0 mm (from 3 to 20%) and table salt (0.25–1.0%), which was introduced into the charge in the form of a concentrated solution. The freshly prepared charge mixture was first held in open boxes until the maximum temperature was reached at the center of the layer. The holding time was 90–120 min. During this time the first stage of corrosion occurred in the charge, and the charge mixture was thereby heated to 65–80°C. When the charge was pelletized without preliminary holding, the strength of the pellets was low.

Pelletizing of the charge was carried out on a pan granulator 1 m in diameter, with a rim height of 140 mm, a pan inclination angle of 45°, and a circumferential speed of 1.0 m/sec. Green pellets 20 to 30 mm in diameter were held (for hardening) for several days in a laboratory room at an average air temperature of 18°C and a relative humidity of 60%. At certain time intervals, a sample (10 pellets) was taken from a batch of pellets to determine their strength.

The investigations showed that the initial moisture content of the charge has a very substantial effect on the corrosion process of the turnings. Both an excess and a deficiency of moisture reduce the intensity of the oxidation process of the turnings.

The optimum moisture content in the charge is determined by the completeness of the corrosion process of the turnings and depends on the amount of turnings in the charge. As the addition of turnings is increased, the optimum moisture content of the charge also increases. As the experiments showed, at an optimum charge moisture content equal in value to the initial moisture content of the sludge (24–26%), it is necessary to add more than 20% turnings to the charge, which is economically inexpedient. With the addition to the sludge of 20% turnings and 1% salt, the charge was heated by only 4–5°C.

It was established that with the addition of 20% turnings, the optimum sludge moisture content, at which the corrosion process of the turnings proceeds most completely, is 17%. However, in this case, owing to the heat of the corrosion process, the moisture content of the charge mixture after one hour of holding decreased to 9.6%, which is considerably below the optimum moisture content for pelletizing a charge of this composition (13.1%); pellets from such a charge crumbled. When the deficient amount of moisture was added to the charge, the pellets obtained after hardening had high mechanical crushing strength (far exceeding the requirements imposed on them) (Table 1, series 1).

Reducing the addition of turnings to 15%, with a corresponding decrease in the moisture content of the initial sludge, sharply lowers the pellet-hardening effect. With an addition of 10% turnings, the pellet-hardening effect becomes practically unacceptable.

Analyzing the above results of the experimental study, the most realistic method of reducing the addition of turnings appears to be the use of the comparatively high heat potential of the charge (series 1, Table 1) after one hour of holding by adding initial sludge to it.

Table 1

As is evident from Table 1, by thus reducing the addition of shavings to 5% (series 3), the strength of the pellets after three days of curing becomes quite sufficient for processing them in an electric-smelting process.

With a further reduction in the amount of shavings to 3%, the pellets proved to be overwetted and stuck together.

Conclusions

1. The chemical and particle-size compositions of ferruginous sludge—the waste of the Kemerovo Aniline Dye Plant—have been studied.

2. The possibility of pelletizing the sludge by the Yarkho method for the purpose of using the pellets as raw material in the electric-smelting process has been demonstrated.

References

1. V. S. Abramov. Preparation of ores for smelting. In: Ferrous Metallurgy of Capitalist Countries, 3–12, Metallurgizdat, Moscow, 1957.

2. V. M. Vityugin, A. S. Bogma. Evaluation of the lump-forming ability of fine-grained materials. Proceedings of Higher Educational Institutions, Ferrous Metallurgy, No. 2, Moscow, 1969.

3. P. N. Dokuchaev. Dissertation, Tomsk State University, Tomsk, 1968.

4. I. G. Tyrenkov. Briquetting of Ores, 34, Metallurgizdat, Moscow, 1948.