Positive effect of heat treatment of basalt melts on the quality of mineral wool

The melting of the basalt feed is one of the most important components in the production process of rock wool products due to the fact that the quality of the melt essentially determines the properties of obtained mineral fibre and the quality of the final product.

The quality of mineral wool can be described based on the following properties:

· thickness of fibre,

· length of fibre,

· elasticity of fibre,

· chemical stability of fibre,

· preservation of non-fibrous formations with crystalline structure (pearls),

· water resistance.

In a series of experiments, it was demonstrated that the temperature reached in the oven determines the viscosity of the melt and thus the thickness and length of fibre. These properties influence the basic properties of the finished material (thermal conductivity, strength). The studies conducted on this topic have defined the optimum temperature of the melt at the centrifuge. It should best be kept within the range of 1420 - 1490 °C (2). At the given temperature, the melt obtains an optimum viscosity, and the mineral fibres thereby obtain their amorphous structure, the required elasticity and the optimal geometric dimensions (thickness and length).

Another important feature of mineral wool is the presence (or rather the absence) of non-fibrous crystalline formations. Frequently, the finished mineral wool contains not only the amorphous mineral fibres but also a certain proportion of spherical formations of the crystalline structure, so-called "pearls". These particles practically function as "bridges" for heat transfer, thereby increasing the thermal conductivity of mineral wool. For this reason, the proportion of these particles in mineral wool should be minimized as much as possible.

Picture 1: Non-fibrous formations of crystalline structure (so-called pearls) in basalt mineral wool

For solving the above problem, it is important, firstly, to understand the origin of these formations. In this connection the following question appears relevant: Why is one part of the melt having the same chemical composition and the same casting temperature was spun to fibres, whereas another part remained in the shape of so-called "pearls"?

The answer lies in the formation of microstructure, in the presence of hard-to-melt phases and, especially, in local clusters of these phases. The term “phase” in this context should be understood as a thermodynamically homogeneous part of a system. In a multicomponent system, phases may have different composition and structure. During the investigation of the properties of basalt melt a phase analysis was performed, and a qualitative as well as quantitative composition of the phases was defined. It was observed that the structure of the tested basalt consisted of 6 phases (1).

During the study, the changes occurring during the melting process and during the heating of the resulting silicate melt were observed. Thus, up to the temperature of 1200 °C mainly crystalline phases were available. It was also observed that the sample, which was treated for 1 hour at 1200 ° C, contained highly crystallized phases. The most stable phases (the phases having a higher melting point) in the investigated basalt were those based on iron compounds (magnetite and hematite).

It should also be noted that a few published studies describe the influence of melting temperatures on the structure and physical/chemical parameters of the basalt melt (3). The tests were performed with differently prepared basalt glass. The research clearly demonstrated that the basalt glass made at 2000 °C was much less susceptible to crystallization (3). Furthermore, the same study proved a relation between the melting temperature (thermal treatment of the melt) and fibre quality. In particular, it was confirmed that the fibres of homogeneous high-temperature melts (2000 ° C) contained no non-molten charged particles, quartz inclusions and gas bubbles and had a practically defect-free surface (3). This means that with thermal treatment of the melt, firstly, the equability of properties of mineral fibers takes place and secondly, the minimization of the non-fibrous spherical formations of crystalline structure, the so-called pearls, occurs.

Thus, in order to ensure the quality of the melt, different thermal zones should be present in a melting device: firstly, the melting zone (the melting of the basalt rock takes place already at the temperature of about 1200 °C); secondly, the zone of the subsequent heating to the temperature of about 1700 - 1750°C (to obtain the homogeneous phase of the melt) and thirdly, the zone of cooling to the temperature of about 1450 - 1490 °C (temperature of optimum viscosity at the spinner). These are different processes, the implementation of which in a conventional oven (regardless of whether in a coke-operated cupola, gas oven or electric oven) is principally impossible. For a non-stop process with described thermal operations a tubular melting device, divided into thermal zones, is necessary.

Based on the above facts and considerations, the new technological solutions can be found in the application of inductive, conductive or combined melting methods.

Literature:

[1] IB Ingineering GmbH: Properties of Basalt Melt, Part 1; http://www.ibe.at/wp-content/uploads/2018/04/Properties-of-Molten-Basalt-2.pdf.

[2] IB Ingineering GmbH: Properties of Basalt Melt, Part 2;

[3] N.N. Khodakova, O.S. Tatarintseva, V.V. Samoilenko: Influence of the conditions for the extraction of basalt glass on their structure and properties, Polzunovskij Vestnik No. 4, T. 2, 2014; available online at http://elib.altstu.ru/elib/books/Files/pv2014_04_2/pdf/148hodakova.pdf.