Recycling in the production of mineral wool

. The main heat-insulating material used in Russia is mineral wool. One of the problems of waste reclamation of mineral wool products is returning of chips and globules back to the solid fuel furnace. To solve the problem, it is proposed to briquette waste before feeding it into the furnace. The author suggests using alumina cements of Russian production as a binder. The choice of the binder is conditioned by the chemical composition of the cement. The second factor that was considered when choosing the binding material was its strength in the early stages. Samples of a mixture of waste and binder were pressed into cylinders and then heat treated for an hour. As a result of the study, the dependences of the impact strength of the samples on steam curing temperature and the humidity of the samples have been obtained. It has been revealed that an increase in humidity has a greater effect on the rise in impact strength than an increase in temperature. Results: the dependence of the impact strength of briquettes made of the mineral wool industry waste and alumina cement on humidity and curing temperature has been revealed, which will make it possible to select optimal conditions for returning of waste to the furnace.


Introduction
Half of total production of thermal insulation materials is a mineral wool. It is used for heat and sound insulation in civil buildings, industrial buildings and equipment. The main devices for melting raw materials are cupola furnaces, which represent shaft melting furnaces of a continuous operation, where heat exchange is based on the counterflow principle.
Performance of the cupola furnaces depends on type and granulometric composition of raw material, uniform loading of raw material, intensity of blow and consumption of coke (when working with solid fuel). It should be noted that thermotechnical calculation of thermal devices will be complicated or impossible if these parameters will fluctuate.
Studies of the cupola furnace (when working with solid fuel) have shown that the heat transfer process is limited by internal heat transfer when heating a material with low thermal conductivity. Heat exchange can be activated by increasing the temperature difference between the heat carrier and the surface of a layer of charge, by increasing the volume of blown air into the cupola. [1] Vanyushkin N.M. and Semushkin A.V. [2] use methods of mathematical modeling to analyze the thermal unit. They point out, however, that in determining heat transfer in the upper layers of charge, the most important characteristic is the volumetric heat-transfer coefficient. The coefficient is determined by the formula: ; (1) where: W0 -average gas velocity over the entire section of charge, m/s; T-average charge temperature in the shaft, К; d -average diameter of pieces, m; μ-coefficient, (depends on the porosity of layer), in other words, guarantees the division of gas over the cross-section of the layer.
According to the abovementioned dependence (1), in addition to those factors, the influence of which was confirmed in [1] and the optimal solution was found, (increase in temperature is achieved due to the blow increase and a decrease in the amount of loaded coke), an additional factor is the decrease in the average diameter of pieces of raw materials. However, this indicator can be reduced only to a certain level, since raw material loading with a diameter of less than 20 mm (for high-melting rocks) and 40 mm (for slag and low-melting rocks) dramatically increases the aerodynamic resistanсe. All this leads to a decrease in characteristics that affect the heat transfer coefficient, mentioned in the numerator of formula (1).

Experimental
The article proposes the use of Russian production alumina cement for optimizing the granulometric composition of raw materials in the production of mineral wool products. First, this choice is related to the quality of products. Based on the Russian state standard «Mineral wool» regulates the minimum value of the pH modulus of mineral wool 1.2. The pH modulus is determined by the formula: (2) where, in the numerator -the total value of silicon dioxide and aluminum as a percentage by weight; in the denominator -the total value of calcium oxide и magnesium as a percentage by weight.
The durability of mineral wool is improved by increasing pH modulus, due to increase in the chemical resistance of mineral wool and, in particular, water resistance. According to the information provided in the sources [3], the chemical composition of conventional aluminous cements fluctuate between: SiO2 9-12%; Fe2O3 1-5%; Al2O3 40-42%; CaO 39-42%. Thus, even in extreme negative values for pH, the minimum amount of oxides (SiO2, Al2O3) and the maximum amount of CaO, the value of the MK index will be ≈1.2.
Secondly, high strength characteristics are achieved already in the early periods of hardening. Krivoborodov Y.R. and Boyko A.A., by changing the content of oxides and the cooling rate of the slag (SiO2 and CaO), have obtained alumina cement with a strength of 34 MPa in a one day [4][5].
To study the possibility of using aluminous cement of the GTs40 and GTs50 grades of a Russian manufacturer, there was conducted a research of briquettes, which are mineral wool and non-fibrous inclusions. Briquettes were exposed to thermal and humidity treatment binder together with waste of mineral wool production [6].
In previous studies [7], there was determined an optimal ratio between weight of the binder component and waste, which is 4:1. The minimum effective heat treatment temperature was also determined as 50°C.
The briquette was produced according to the following scheme. First, mixing and moistening of aluminous cement together with waste. Second, dosing the resulting mixture and loading into the press-mold. Third, pressing with a force of 14 MPa. Fourth, cylinders taken out of the mold with a diameter of 25 mm and a length of 100 mm were sent to the steaming chamber, where the briquettes were there at a constant temperature and relative humidity ≈100% for an hour. Fifth, the samples were cooled down for 30 minutes after thermal and humidity treatment, and then tested for impact strength using the Charpy method.
Humidity is one of the most important factors influencing the formation of a briquette. There are general patterns of the influence of humidity on the compaction process: with increasing humidity, the density of the initial powder usually decreases; with increasing humidity, the intensity and magnitude of particles of powders increase at low pressing pressure; at low pressing pressure, wetter powders produce compacts with increased density. To establish the dependence of strength on humidity, the briquettes were made with different humidity (from 5% to 10%). At the same time, the test sample began to crumble when the humidity dropped below 10%. When the humidity was above 20%, water separation was observed in the sample during the pressing process.
The impact test results are presented in tables 1 and 2. The maximum and minimum values are the same when using GTs40 and GTs50. The minimum value of strength was obtained with humidity of 10% and a temperature of 50 °C, and the maximum value was obtained with humidity of 20% and a temperature of 90 °C. In general, based on the obtained values, it can be concluded that with an increase in temperature and humidity, the impact strength also increases.  In program STATISTICA 6, surface plots were built with a quadratic fit of the raw data ( Fig. 1 and 2). Surfaces have the following mathematical dependencies: • For the briquette using GTs50 • For the briquette using GTs40 A=1.2W2-0.3t2+0.9Wt-52W+47t-1353; (4) where, А -impact strength of the briquette, kJ/cm2; W -humidity of the sample,%; t -Temperature of thermal and humidity treatment, °С. From the presented equations (3), (4) the influence of the humidity of the briquette is greater than the influence of temperature. Thus, it is more effective to press waste together with cement on the verge of water separation. Moreover, low-grade cement gives higher strength at temperatures of 50 °C and 60 °C. This is due to the following parallel ongoing processes which run in the opposite direction: • The emergence of a significant number of new phases, which lead to the formation of much denser crystallization structure and increase in the strength of the contact layer at the border of the cement stone with fiberglass; • An increase in the porosity of the alumina cement stone as a result of the continued drainage of the mixture into the external environment after compaction, which contributes to a decrease in the strength of the contact layer.
One of the fastest minerals that undergoes hydration is C3A. After 10 minutes of its interaction with water, C2AH8 и Ca(OH)2 appear. After 1 day of hardening, the expansion was 0.42%. Recrystallization of metastable hexagonal primary crystals CAH10, C2AH8, C4AH14 into stable cubic crystal hydrate C3AH6 occurs in cement stone at temperatures above 25 °C. However, this takes time. The CAH10 formed at 21 °С almost completely converts C3AH6 into AH3 and at 60°С within 28 days.
The X-ray diffraction analysis of the samples showed that, at low values of humidity of the sample, hydrated phases are present as: β-C2AH8 (crystallizes in the form of colorless thin hexagonal plates, often spherulites); C4AH13 (has the appearance of hexagonal plates). At high moisture content are present C4AH11 and C2ASH8, which is formed in geothermal conditions. The solution obtained from СA quickly becomes supersaturated -САН10, but to a lesser extent -CAH8 or AH3. This situation changes with increasing temperature.

Conclusions
Thus, the use of alumina cement for optimizing the granulometric composition of raw materials in the production of mineral wool products in the most extreme negative ratio of oxides of the pH modulus will not affect the quality of the melt, otherwise it can improve. Since alumina cement doesn't break down at high temperatures, it will improve traction of the thermal unit. The obtained mathematical dependences of the impact strength on the moisture content of the samples and the processing temperature will make it possible to select the optimal ratios for practical application.