Expanded Perlite
Industrial Minerals magazine, December 2006. issue No. 471
Perlite is a multi-talented mineral and expanded it opens up a plethora of varied end use applications from acoustic tiles to fruit juice filtration. Graham Ellicott takes us through the key markets for expanded perlite.
EXPANDED PERLITE has a wide range of end uses which take advantage of its many useful qualities, including construction, horticulture, and filtration applications. Perlite can even act as a filter. Its beneficial properties include a low density, good insulating characteristics, inertness, non-combustibility, and good absorption capacity.
CONSTRICTION: ACOUSTICALLY SOUND
In North America and Europe perlite's main use is in the production of products for the construction industry.
The major construction use for perlite in North America is in the manufacture of 'formed products' such as acoustic ceiling tiles, pipe insulation, and insulation boards.
Acoustic tiles containing expanded perlite are typically produced using one of two methods. The first method is the starch production process, which combines expanded perlite, mineral wool, and fillers such as clays and starch to make slurry. This in turn is dewatered and pressed using a proprietary process such as the Fourdrinier process.
In the second production process the expanded perlite, mineral wool and fillers are mixed with a metal silicate to achieve an 'earth damp' consistency. This mixture is then hydraulically pressed into shape in a mould. The resulting shaped product is then demoulded and either heat or catalytically cured.
Acoustic tiles produced using these manufacturing techniques are used to control echo and reverberation in enclosed areas such as offices, schools, and institutional buildings. The acoustic performance of the system is affected by the construction, for example, the thickness of the tile and the presence (and depth ) of any air gap behind the tile. It is possible to achieve noise reduction coefficients ( NRC) of 0.85 ( 85% of the noise is absorbed ) at certain frequencies with acoustic tiles. The sound energy is turned into heat.
In contrast, in Europe the major use of expanded perlite for the construction market is in the production of general building plasters. These products can be applied by hand or via a spray. The expanded perlite aids coverage, ease of handling, fire resistance, and resistance to chipping, cracking and shrinkage.
Perlite may be used on its own in plaster applications in additions of approximately 25 % by weight, but in some cases it is combined with other aggregates such as vermiculite. Proprietary perlite-base plasters are often used on substrates with moderate suction, whereas the vermiculite counterparts are used on surfaces which have a lower suction and are, thus, more difficult to adhere to.
FITRATION
Although European expanded perlite consumption in building plasters is fairly large, it is only about two-thirds of the usage in filter aids by volume.
Solids suspended in liquids can be removed by filtration, and in this process the solids are retained by a medium that con easily accommodate the flow of the liquid. The liquid that passes through the filter is the filtrate and the material retained by the filter is known as the residue. In general, there are four different types of filter media for industrial filtration, these are: granular ( for example sand and charcoal), cloth or wire meshes, papers, and porous solids.
Industrial filtration processes serve a very broad range of markets. They are used in the production of drinking water, sugar beer, wine, fruit juice, oils, chemicals, resins, waxes, and swimming pool water, as well as for the ' clean-up' of industrial effluent.
It is very important that industrial filter aids be both chemically and physically inert during the filter operation. In addition, they should not taint the liquids that are being processed. Expanded perlite fulfils these needs as it has been manufactured at high temperature. An added bonus is that it is much lighter than some of the other competitive flirtation materials.
CREATING THE VOID
The filter medium that retains the residue consists of the septum, which is typically made metal or cloth, and the filter aid. The design of the septum can have a signification effect on performance, as it has to allow the formation of a pre-coat filter cake, while also allowing the liquid to pass through with the minimum of resistance.
Pre-coat filter cakes are composed of irregularly shaped particles that 'fit' together so that a high percentage, typically 85 % or higher, of voids are formed. When added together these voids from billions of microscopic pathways between the filer aid particles. Expanded perlite aid purposes has to he carefully produced and classified so that the microscopic pathways produce the desired levels of clarity and flow rate.
HORTICULTURE
Expanded perlite is widely used throughout the world for horticultural applications. The properties that make it an attractive material for the horticulturalist include its porous structure, which enables it to improve soil aeration and drainage, its ability to absorb many times its own weight of liquid, and its inertness. The latter property of expanded perlite is very important to the horticultural market, as this means the perlite is free of seeds, pests and germs. In addition to this the particles have a reasonably stable pH of 6.5-7.5.
Expanded perlite is inorganic and, thus, its particles do not deteriorate.
Mixtures of expanded perlite and peat, or other compassed materials, can result in an ideal growing medium for plant propagation for the professional and amateur grower. The first uses of expanded perlite in the horticultural sector used medium and coarse grades of the products. However, fine grades are now growing in use, especially where higher levels of water retention are required.
Mixtures of expanded perlite and peat are usually referred to as soil-less mixes. Proprietary soil-less mixes have the advantage that are consistent from one batch to the next. In addition, the absence of soil means that the risk of contamination by disease, insects and herbicide residues is greatly reduced.
Another horticultural application for expanded perlite is in seed germination. Fine grades of the products are normally used for this end use. The mineral's ability to maintain reasonably constant moisture levels; its insulating nature; and its ability to reflect sunlight due to its white color are the important factors in its use for seed germination.
The major competitor to expanded perlite in many sector of horticultural market is exfoliated vermiculite. In most cases, the choice between the two minerals is often decided as much upon price and availability as it is upon their relative qualities.
HYDROPONIC PROPAGATION
The tern hydroponics is derived from the Greek words meaning water and work. The hydroponics propagation of plants is achieved by growing them in containers filled with a sterile growing medium, such as expanded perlite. The nutrients that plants would normally derive from soil are supplied to the roots through a water-nutrient mixture.
Mineral wool is the market-leading hydroponic growing however, it is claimed that expanded perlite is the better material when rooting is considered. This is due to the product's ability to maintain a constant level of nutrient and water. The strong capillary action of perlite replaces 'lost' water and nutrients as the plant absorbs them.
HIGH TECH PERLITE
A ' higher tech ' use of expanded perlite is as a filler in the production of adhesives, car repair compounds, artificial marble ( e.g. in ceiling roses and bath or sink surrounds ), coatings, stucco, and wall patching materials. In may cases these products use ' perlite bubbles', which are produced by milling the unexpanded mineral and then classifying it obtain a size fraction of typically 25-30u for subsequent expansion.
Contributor: Graham Ellicott, Independent Industrial
Kuwait Industry