Microwave technology of vermiculite exfoliation

Vermiculite and its applianceVermiculite and its appliance

Vermiculite is a natural, eco-friendly material from a group of foliated hydrous micas. Due to its unique features it is widely applied in various fields of industry and agriculture. Notably, it is only exfoliated vermiculite that possesses such qualities and  is obtained through a special thermal processing of vermiculite concentrate taken from deep in the earth.  While being processed vermiculite grows 7 to 10 times larger in its size and becomes a “wormling”. That is why it is called vermiculite, which is translated from Latin as wormling. Exfoliated vermiculite is an excellent heat-insulating and fire-resistant material, it does not decay and is not decomposed, absorbs water very well and with that keeps its permeability to air, it contains a lot of microelements and is capable to ion exchange. These characteristics make it indispensable in construction industry, agriculture, metallurgy, and ecology. It is used as concrete filler, applied in fire-resistant and heat-insulating constructions, as sorbent and filtration media, as soil filter in agriculture and hydroponics, as supplementary feeding for live stock and so on.

Vermiculite structure

Due to its structure vermiculite has all the above stated features. A particle of vermiculite consists of a great number of closely situated mica layers with water between them. It can be compared to a cake having multiple flat layers. Every layer is a flat and extensive crystal of mica. There is hidden water, as if in a cake cream, between mica layers. Owing to such structure vermiculite is scientifically called hydrated mica i.e. mica enriched with water. It is the presence of water in between the layers, which allows to obtain exfoliated vermiculite from regular vermiculite: the water between the layers is superheated fast and is pressured by the created steam, so the mica layers get spread out. As a result, a long and extended “wormling” is created out of a small flat mica layer of vermiculite.

A chemical formula of vermiculite is generally written as (Mg+2, Fe+2, Fe+3)3 [(Al,Si)4O10]·(OH)2·4H2O.  This is just a simplified formula because vermiculite structure is firstly defined by its deposit where it is obtained. The most known deposits are located in South Africa, Zimbabwe, China, the USA, Australia, Brazil, Kola Peninsula (Kovdor), Siberia. In view of the fact that vermiculite was formed under specific geological conditions of its deposits, its chemical composition and structure  differ significantly. As a result, these differences influence both energy costs for the exfoliation process and its other consumer qualities (its exfoliating index, ability to absorb water, and so forth). For instance, Chinese vermiculite gets exfoliated very well and requires little energy costs but Brazilian vermiculite requires the most energy costs for exfoliation.

How vermiculite gets exfoliated

All present technologies of vermiculite exfoliation are based on the principle when water betweeen the layers evaporates and  the crystal layers spread out pressured by the steam. In order to get a successful exfoliation, heating must be fast. At a slow heating, evaporating water will get enough time to diffuse to the edges of a vermiculite particle and to come out. At a slow heating the steam comes out and does not manage to create enough pressure to spread out the crystal layers. The faster vermicuite is heated, the more pressure water steam creates, and the more efficiently vermiculite is exfoliated.

Conventional technologies of vermiculite exfoliation

At present, the most popular technology of vermiculite exfoliation is based on heating of vermiculite particals by means of incandescent gases, which come from burning of natural gas or diesel gas oil. In units, which apply this technology, vermiculite is blasted  by fuel combustion products with temperature from 600 to 1 000°С. This significant tempreture is required to obtain a sufficient heating rate of vermiculite. At lower temperatures, vermiculite is either not exfoliated at all or this exfoliation is unsufficient for its future use. Typical fuel costs for this kind of units amount to 100- 150 liters of diesel gas oil, or 100 m3 of natural gas for 1 ton of production.Plus points of this kind of equipment for vermiculite exfoliation are their simple construction and relatively low cost of the used diesel gas oil or natural gas.

However, its disadvantages are the following facts:

1. Low environmental compatibility of the technology due to the necessity of fuel burning and thus harmful gas emissions. Allocation of such equipment at production site  requires approval of government agencies and payments of environmental fines.

2. Reduced quality of exfoliated vermiculite. Exfoliation of vermiculite in atmosphere of incandescent gases results in increased crispiness of vermiculite. Because of its crispiness, vermiculite breaks  and creates a lot of dust. It is also reduced to minute particles while being transported for a long time period. Moreover, vermiculite sorbs post-combustion products on its surface, which decreases its quality when used in agriculture, live stock breeding and as absorbent.

3. Stationarity of equipment. This equipment is fixed in a stationary mode, has high size and weight parameters because of formidable heat insulation and the necessity to create additional infrastructure for fuel burning and carrying off its combustion gases.

4. High energy loss. The actual technology has the highest energy cost because its most energy is not used but taken away by gas emissions. Besides the energy costs, one has to take into account electrical power costs to operate a vermiculite handling system, a fan group, and an air cleaning system.

Another technology of vermiculite exfoliation is based on heating in electric furnaces. Vermiculite is moved into a tunnel furnace that is heated up by electric heaters to the temperatures of 600-800°С. Going through the furnace, vermiculite is heated by the hot air and intensive infrared rays.

This technology has its advantages compared to the previous one: the infrastructure gets simplified essentially since there is no necessity to burn hydrocarbons and their end-product emissions; high environmental compatibility because end-product emissions are absent; lower size and weight parameters of the equipment.

However, there are still disadvantages of this technology:1. High cost of production. Production costs for vermiculite in the actual units are high due to their high electric power costs.2. High energy loss. The energy loss in this technology can be compared to the loss in the units based on gas and diesel gas oil burning. This is related to the fact that high temperature of ca. 800°С is required to heat vermiculite, which translates into big heat loss in such units.

Microwave technology of vermiculite exfoliation

Microwave technology of vermiculite exfoliation is presently the most long-range and interesting for industrial integration. Essentially, such heat carriers as incandescent gas or intensive infrared rays are not needed for heating vermiculite. Heating of vermiculite is conducted by microwave energy in a special chamber, a microwave reactor that can be compared to a houshold microwave oven. Microwave energy is taken by vermiculite particles, and this energy gives them heat necessary for exfoliation. Temperature of the technological process in the microwave units comes up to just 150- 200°С. This feature makes the actual exfoliation process the most economical. Absence of intermediary means for vermiculite heating such as incandescent gases or electrical heaters allows to reduce energy costs by far and abandon formidable heat-insulation in the units.

Despite all the advantages of microwave exfoliation of vermiculite there have been no commercial samples on the market recently which could carry out this technology. This is because the first efforts to create industrial equipment for microwave exfoliation of vermiculite failed. It happened that, to operate the equipment reliably and to process vermiculite of different fractions and deposits, one had to conduct scientific research and design specialized microwave reactors capable to create a microwave field with high intensity of electric field. Such research, which ended up with creation of the first prototype equipment, was conducted at two scientific centers from 2010 to 2015. The microwave processing of vermiculite was researched  at the University of Nottingham, in Great Britain. This work was financed well by the European Union and awarded for the creation of a new ecologically-friendly technology of vermiculite processing. During this work, a pilot unit with capacity up to 15 kW was produced, which confirmed a high efficiency of microwave field for vermiculite exfoliation. Still, it was not possible to integrate that technology into industry because of emerged technical problems. A bit earlier than our colleagues from Great Britain, the experts from our company in collaboration with the Department of Radio Engineering Electronics from Saint Petersburg Electrotechnical University started the study of microwave processing of vermiculite and development of industrial equipment. Having a great deal of experience in the field of equipment and technology development related to microwave processing of materials our experts succeeded in solving the problem at hand. Initially, the research was conducted and the prototypes of equipment were created, which showed a principal possibility to apply the equipment under industrial conditions.  Consequently, the development and integration of the industrial units were conducted in cooperation with the US and Russian productions. At present, our company have already created the third generation of units for microwave exfoliation of vermiculite. Our units have modest size, are the most economical in the world and allow to obtain high exfoliation index for vermiculite from different deposits, with fractions super fine, fine, medium, large.