Innovative filters made of polyoxadiazole fibers for industrial gas and dust emissions treatment

Introduction

There is an increasing demand, for the last few years, especially in ferrous and nonferrous metal industries, on new filtration materials for technological gases treatment. Reasons for this are much more strict legislative requirements to the emissions in the atmosphere and our social and moral responsibility for environmental preservation. Filtration material and filter construction must provide not only the “physical effect” of gas clearing from dust and aerosols to the necessary degree of purity but also to be economically effective.

Air and gases clearing from solid and liquid particles contained in them are necessary to prevent contamination of air pool with harmful substances. Anthropogenic emissions cause not only the change of physical composition of the atmosphere but also cause number of chemical reactions between the constituents of atmosphere and emissions. Atmosphere becomes a multicomponent system with variables as to composition and thermodynamics parameters, that is why it is practically impossible to predict all chemical processes (Pelyk, 2014c). One of the most dangerous air pollutants is high-dispersity heavy metals and their compounds. At present, the annual production of metals equals or exceeds their natural content in the annual increment of biomass. It violates the natural circulation of metals and causes contamination of air, water, and soils. The toxic property of heavy metals is determined by their ability to make complex formations. Particularly dangerous are metals that are not the part of biomolecules compositions, i.e. xenobiotics, cadmium, and lead, for example. Other heavy metals, such as Cr, Mn, Zn, Cu, are also referred to high toxic metals and therefore can be dangerous for human, animals, and ecosystems in whole. Excess content of cations of Mn²⁺, Cr³⁺, Cu²⁺, Zn²⁺, and of Pb²⁺ results in the substitution of other cations by them in the active centers of enzymes. Cations of Pb²⁺, Cd²⁺, Cu²⁺, and of Zn²⁺ form particularly stable compounds with the final thiogroups of proteins, with sulfur content enzymes donor groups in particular, displacing cations of less connections. Enzymes inhibit in such cases. These heavy metals are called thiol poisons.

To clean gas emissions of metallurgical industry enterprises, multilevel combined schemes of dry and wet treatment are used (Pelyk, 2014a). In combined schemes of dry cleaning it is more prospective to use a bag-type filter based on new effective high thermally and chemically resistant filtration textile materials. Polyoxadiazole fibers are of considerable interest among the high thermal resistant ones (Pelyk, 2014b).

Our aim is to study the possibility of using filtration materials based on arselone fiber for industrial gas–dust emissions clearing from high-dispersity metals and their compounds in thermal-oxidative atmosphere.

Experimental

A bag-type filter based on arselone fabric was set up at Aktiubinsk ferro-alloy plant (Republic of Kazakhstan), and a bag-type filter based on arselone unwoven material was set up on gas clearing constructions of Aksusk plant (Republic of Kazakhstan). Experiments of bag-type filters made of woven materials were carried out on bag filters of the open type with the system of regeneration—a reverse blowing, and for the unwoven materials—on bag filters with the impulsive system of regeneration.

Gas–dust mixtures of these ferro-alloy plants have almost identical composition. The temperature of gas–dust outside a bag filter was 180–220℃. Efficiency of clearing of filtration textile materials of the arselone fibers after 18 months of exploitation was examined for the purpose of removal of Cd, Pb, Mn, Zn, Cu, Cr, Al, Fe, Ca, and Mg compounds from gas–dust.

During the first stage of research, a loss, during frying of the amount of dust detained by filters, was determined. For this purpose, the amount of dust was fried in a drier at temperature of 105–110℃. The process of frying was carried out for several times until permanent mass was received and losses, during frying, were determined by mass differences. For further researches fried samples of dust that did not contain hygroscopic water were used. Identical amounts of fried dust (1.4201 g) taken from different filters were weighed on analytical scales. The amounts of dust were dissolved in the mixture of nitric and hydrochloric acids adding hydrogen peroxide at continuous heating on the sand bath.

During the second stage of the research, the analysis of the filtrates, received as a result of dust dissolution, was carried out to find out the content of Cd, Pb, Mn, Zn, Cu, Cr, Al, Fe, Ca, and Mg.

The concentration of chrome, iron and aluminium was determined by a photometric method using HACH DR/400 V spectrophotometer. Content of chrome, in particular, was defined using chromazurol S, iron–1,10-phenanthroline, aluminum–eriochrome cyanine R.

Content of cadmium, lead, manganese, zinc, copper, calcium, and magnesium was determined by an atomic absorption method using atomic absorption spectrophotometer AAS-1N made by Carl Zeiss Jena company (Germany). Wavelength of resonance radiation (λ) during atomic absorption determination of these metals was 228.8, 217.0, 279.5, 213.8, 324.9, 422.7, 285.2 nm, respectively. Determinations were conducted in a flame variant (propane–butane–air).

Preconcentration of metals, when necessary, was achieved by the method of solid-phase extraction by means of concentration cartridges “DIAPAK IDK” of “ELSIKO” firm (Russia) and based on transcarpathian zeolites (Vasylechko et al., 1999a, 2006, 2003, 2009, 2008, 2011, 1999b). The characteristics of transcarpathian clinoptilolite and mordenite, in particular their composition, are described in Tarasevich et al. (1991) and Vasylechko et al. (1999a, 2008), respectively. In some cases, the method of evaporation for concentration of solutions was used.

Researches of thermal resistance of filtration materials were conducted by means of the derivatograph of Paulik–Paulik–Erdei Q-1500D system. Researches were conducted in the temperature interval of 20–700℃, speed of heating was 10 K/min. Incinerated at 1100℃ α-Al₂O₃ (corundum) was used as the standard. Mass of the sample, in all cases, was identical and equaled 100–105 mg. Researches were conducted in open corundum crucibles in air atmosphere. To reduce the influence of fluctuations of the temperature in a stove, a thermocouple together with crucibles was insulated from the atmosphere of the stove by means of quartz glass. The results of thermogravimetric researches were presented graphically as thermogram—the combination of curves that describes dependence of mass of the sample (TG curve), speed of mass loss (DTG curve), and differences of temperatures of the sample and the standard (DTA curve) from time and temperature.

Textile materials of polyoxadiazole fibers synthesized by polycondensation of terephthalic acid with hydrazine sulfate at heating in the environment of oleum were used in the research. The elementary building unit of polymer of polyparaphenilen-1,3,4-oxadiazole has the structure as shown in the below scheme:

( p , 100 % ) = " SG - ADTJ 170062 - ILF 01 1 " ( 1 )

Results and discussion

The main advantage of arselone fiber is its high thermal resistance that gives an opportunity to produce fabrics that are used at high temperatures and in aggressive environments. Arselone fiber loses 20% of its durability only at 350℃, and bag filters made of it can be used at the temperatures of 200–300℃ and maintain the temperature rise up to 400℃ (Grebennikov et al., 2008). The fiber does not melt; has high breaking force, wearing property, and durability to wearing; small change of linear sizes; is well painted in bulk; has high hygroscopic properties (humidity 12%); and is close to cotton fiber by its consuming properties.

It is necessary to maintain the required temperature of cleansing gases at the filter inlet and inside it for efficient working of the bag filters. The temperature increase of filtration leads to the increase of activity of filtration environment components and speed of reactions of their interaction with the bag filters material (Pelyk, 2013). This leads to the reduction of their life time.

Depending on fiber composition of filtration material and conditions of its usage, destruction of polymers, these materials are made of, occurs because of the following reasons:

– as a result of polymer’s interaction with the water steam at increased temperatures (hydrolysis of materials). Materials made of heterochain polymers (lavsan, arselone, nomex) hydrolyze. The process of hydrolysis considerably fastens in the presence of acid fumes and aggressive gas-like components, thus, it is ineffective to apply materials based on heterochain polymers for moist gases treatment;

Studying the influence of operating conditions on the change of physical properties of filtration materials, it was found out that that their thermal stability decreases during the exploitation. This is indicated by results of thermogravimetric analysis of the samples of filtration materials based on arselone fiber, which are practically identical (Figures 1 and 2). According to the TG and DTG curves, in the range of temperatures from 76 to 78℃ a small weight loss is observed, which obviously is connected with the evaporation of the moisture (Pelyk, 2011). OPEN IN VIEWER

OPEN IN VIEWER

Figure 2.

Thermogram of filtration unwoven material made of arselone fiber after operation.

At further heating of the investigated samples to a value of 400℃, the mass of the examined samples remains permanent, which proves high thermal resistance of filtration materials based on arselone fiber in a thermal-oxidative atmosphere. In the range of temperatures of 400–450℃, there is intensive practically symmetric DTG maximum on the DTG curves of the examined samples. Maximum speed of mass loss of the arselone fabric, taken after exploitation, is indicated on DTA curves at 432℃ and for the unwoven material at 424℃, whereas for initial samples the temperature of the most intensive weight loss T_max is equal to 484 and 472℃ for arselone tissue and nonwoven material, respectively. At further increase of temperature the rate of thermal destruction slows down significantly and in the range of 500–700℃ remains almost unchanged, which can be seen from the character of the DTG curves. The total weight losses of the arselone tissue and nonwoven material at the conditions of the dynamic heating to the value of 700℃ are equal to 73.1 and 82.4%, respectively.

All this indicate that during operation, at the temperature increase, the speed of thermodestruction significantly slows and the reduction of mass does not occur. Unwoven arselone material is characterized by higher thermal resistance than arselone fabric. It can be explained by the influence of polytetrafluoroethylene treatment applied on the unwoven arselone material, which reduced the influence of high temperatures during operation (Lavrenteva, 2010).

During the elaboration of new filtration textile materials we kept to the technical requirements to the maximum possible concentrations of harmful substances in atmospheric air, set by the enterprises of metallurgical complex for bag filters installment. According to these requirements the general volume of gas–dust mixture outside a bag filter is 500,000 m³/h; bulk weight of dust is 1.13×10³kg/m³; the main chemical composition of dust is the following: Cr₂O₃—38.15%, SiO₂—10.90%, CaO—0.75%, MgO—23.73%, Al₂O₃—8.14%, FeO—9.50%, S—0.45%, C—5.66%; chemical composition of gas: CO—67 g/m³, NO—16 g/m³, NO₂—10 g/m³, SO₂—18 g/m³.

Conducted studies showed that losses, during frying, of the gas–dust detained on the filtration fabric and on unwoven filtration material differ among themselves more than two times. For dust collected from the filtration arselone fabric this index is 5.52%, and for dust detained by the filtration unwoven material made of arselone fiber it is 2.13 %. The loss during frying is undoubtedly connected with the loss of hygroscopic water by dust samples. Fried samples of dust that did not contain hygroscopic water were used in further research. The results of researches are presented in Table 1.

OPEN IN VIEWER

Table 1.

Chemical analysis of the gas–dust composition detained with arselone filtration textile materials.

Component	Type of bag filter
	Arselone fabric		Arselone unwoven material
	Mass (mg)	%	Mass (mg)	%
Amount of dust after frying (g)	1.4201		1.4201
Insoluble precipitation	559.31	39.38	1188.01	83.66
Ca	5.41	0.38	1.01	0.07
CaO	7.56	0.53	1.41	0.09
Mg	18.91	1.33	33.51	2.36
MgO	31.51	2.22	55.81	3.93
Cd	Not detected		Not detected
Pb	11.75	0.83	Not detected
PbO	12.66	0.89
Fe	19.22	1.35	25.91	1.82
Fe2O3	27.51	1.94	37.01	2.63
Mn	172.41	12.13	1.41	0.09
MnO	222.61	15.67	1.81	0.13
Zn	41.71	2.93	1.11	0.08
ZnO	52.01	3.66	1.41	0.09
Cu	0.15	0.01	0.075	0.005
CuO	0.19	0.013	0.094	0.007
Cr	0.51	0.04	24.71	1.74
Cr2O3	0.73	0.05	36.11	2.54
Al	13.65	0.96	4.02	0.28
Al2O3	25.81	1.82	7.59	0.53

In both cases the dust dissolved partially, while the dust collected from the arselone fabric dissolved much better. Insoluble precipitation received during dissolution of dust, collected from the filtration fabric and filtration unwoven material, was 39.4 and 83.7% accordingly from the mass of the fried dust. Insoluble precipitations are likely to contain sand (SiO₂), sulfides, and carbides that under such conditions do not practically dissolve (Bock, 1979).

During the second stage of research, the analysis of the filtrates, obtained as a result of dust dissolution, as to content of Cd, Pb, Mn, Zn, Cu, Cr, Al, Fe, Ca, and Mg was conducted.

Analyzing data of Table 1, it is possible to highlight that efficiency of gas emissions treatment by bag filters made of the filtration woven and unwoven material is different. So, arselone filtration fabric detains more effectively calcium 5.41 mg (0.38%), aluminum 13.65 mg (0.96%), and also such toxic heavy metals as lead 11.75 mg (0.83%), manganese 172.41 mg (12.13%), zinc 41.71 mg (2.93%), and copper 0.15 mg (0.01%). Bag filters based on the unwoven material clear gas emissions more from fine low soluble compounds (sand, sulfides, carbides), of magnesium 33.51 mg (2.36%) and iron 25.91 mg (1.82%). Filtration unwoven material was found out not to detain extremely toxic lead at all, but the amount of dust, collected from the arselone fabric, contains 0.83% of it. However, the filtration unwoven material detains toxic chrome more effectively.

High sorption capacity of transcarpathian zeolites, ability to absorb low as well as high concentrations of metals, and availability of effective desorbents of metals give reason to offer these sorbents for removal of Pb, Cr, Cu, Mn, Zn from the solutions obtained at dissolution of dust samples detained by filters. After desorption of metals concentrated on zeolites, their ionic forms can be reduced to metallic state by well-known methods and in future they can be used for production needs.

Conclusion

Conducted studies showed that efficiency of removal of components from gas–dust mixture of ferro-alloy plants by filters of textile materials made of arselone fiber is different. Bag filters based on the unwoven material clear gas emissions better from fine low soluble compounds (sand (SiO₂), sulfides, carbides) and also from compounds of Mg, Fe, and Cr. Bag filters based on arselone fabric detain more effectively compounds of calcium, aluminum, and also of such toxic heavy metals, as lead, manganese, zinc, and copper. Filtration materials based on arselone fiber show high thermal resistance (to 400℃) in a thermal-oxidative atmosphere, keeping high efficiency in relation to metals for 18 months.