Silica and the silica industry
WHERE SILICA OCCURS
Crystalline silica, in the form of the mineral quartz, is found in many different materials – with sandstone being almost pure quartz. Other forms of silica occur but are of little importance occupationally. The table below gives an indication of typical levels of “free” crystalline silica in certain mineral sources, but it must be noted that these figures do vary.
| Mineral sources | Percentage of crystalline silica |
|---|---|
| Aggregates | 0 to 100% |
| Ball Clay | 5 to 50% |
| Basalt | Up to 5% |
| Natural Diatomite | 5 to 30% |
| Dolerite | Up to 15% |
| Flint | Greater than 90% |
| Granite | Up to 30% |
| Gritstone | Greater than 80% |
| Iron Ores | 7 to 15% |
| Limestone | Usually less than 1% |
| Quartzite | Greater than 95% |
| Sand | Greater than 90% |
| Sandstone | Greater than 90% |
| Shale | 40 to 60% |
| Slate | Up to 40% |
Source: HSE brochure, Control of respirable crystalline silica in quarries.
ACTIVITIES INVOLVING USE OF CRYSTALLINE SILICA CONTAINING MATERIALS
Aggregates
Aggregates are a granular material used in building and infrastructure construction. Nearly 3 billion tonnes of aggregates are produced and used in Europe annually. However, a majority of operators in the sector are small and medium sized enterprises. A typical site provides direct employment for 7 to 8 persons. The aggregates industry operates in around 23,000 extraction sites with over 130,000 employees in the EU.
The most common natural aggregates are sand, gravel and crushed rock, from rocks of different geological origin, and with a wide range of free silica content (from 0% to 100%).
Subject to the individual risk assessments to be carried out under this Agreement, the sites with deposits with a high content of crystalline silica are more relevant. Nevertheless even in such cases, the risks of respirable crystalline silica exposure for workers are normally low, with low number of cases of silicosis reported.
The content of crystalline silica in recycled and manufactured aggregates varies depending on the composition of the material they are produced from.
Calcium Silicate Masonry Units
Calcium silicate masonry units are produced by mixing sand, lime and water. This mixture of natural ingredients is moulded into shape by mechanical or hydraulic presses. After moulding, the “green” material is hardened in an autoclave. In these autoclaves steam is introduced at pressures of 8 to 16 bars to raise the temperature up to approximately 200°C. After some hours of autoclaving the units have developed their final properties, especially the strength, and are ready for packing and dispatch. Dust generation can mainly occur in raw material handling and shaping mechanical treatments.
120 plants in 7 European countries produce calcium silicate masonry units.
Cement Industry
Cement is a powdered substance mainly used as the binding agent in the making of concrete. It is produced through several stages, basically made up of the following essential phases:
- manufacture of a semi-finished product, so-called “clinker”, obtained from the calcination in a high-temperature kiln (1,450°C) of a “raw mix” made up of a mixture of clay, limestone, and several other additives.
- manufacture of cement as a finished product, obtained by the homogeneous mixture of the ground clinker and calcium sulphate (gypsum).
- depending on the type of cement – one or more additional components: slag, fly ash, pozzolana, limestone, etc.
In 2017, the cement production of the current 28 Member States of the EU was 175 million tons, about 4% of the total world production (4.1 billion tons).
There are nearly 226 installations in the EU. The cement industry directly employs about 47,000 persons among CEMBUREAU’s members.
Ceramics Industry
The ceramics industry uses silica principally as a structural ingredient of clay bodies and as a major constituent of ceramic glazes. The principal ceramic products containing silica include tableware and ornamental ware, sanitary ware, wall and floor tiles, bricks and roof tiles, refractories etc.
Around 2,000 companies produce ceramics in the EU, of which 80% are small and medium size enterprises. The number of employees in the EU ceramics industry is estimated at around 200,000. The ceramic industry is present in virtually all EU Member States.
Engineered Stones
According to the European Standard EN -14618, Engineered Stone is called Agglomerated Stone.
Agglomerate Stones are the evolution in the tradition of the old “Terrazzo Tiles”.
Today, Agglomerate Stone is industrially manufactured by means of different moulding technologies, through vibration and simultaneous compression under vacuum; chemical additives functional to the process, pigments and a binder, generally polyester resin, added in the minimum quantity just to assure the complete bonding between filler and particles.
A subsequent hardening phase, conducted at room temperature or at medium temperature in suitable kilns, allows the mixture to reach the final stone consistency. The products are realised in forms of blocks or slabs, which are transformed into finished slabs for counter tops, tiles for floorings and wall coverings and other architectural elements.
Agglomerate Stone can technically be defined as a composite material, because it is made up of several different raw materials; the composition of this product, in a simplified way, can be divided into four distinct categories: raw materials constitute the structure; powders fill the interstices (fillers), binders bind the product and additives of varied nature (pigments, for example) give technical or aesthetic performances.
The original raw materials for Agglomerate Stone are marble, granite, feldspar, or quartz, which can be found in large dimensions in nature and which can be crushed or have already been crushed by natural events.
This industry can also make use of marble and granite excavation refuse, which is an interesting contribution to solving the problem of the environmental impact of the stone processing industry.
Expanded Clay Industry
Expanded clay is a ceramic-based lightweight aggregate made by heating clay to approx. 1,200°C in a rotary kiln. The yielding gases expand the clay during heating, producing a honeycomb structure. Expanded clay pebbles have a round or oval shape and are available in different sizes and densities.
The clay is extracted from clay pits normally located close to the plants. Once transported to the plant the clay is pre-treated and processed in rotary kilns. After passing through the kiln, the now expanded clay is cooled. As the hot clay cools, cold air is warmed and this heated air is used to dry, heat and expand the clay in the kiln. Expanded clay is used in a variety of applications in the construction and green sector.
Some 13 companies manufacture expanded clay in 11 countries, operating 17 plants throughout Europe. Their annual production is approximately 4,500,000m3 of expanded clay and they offer direct employment to around 2,000 people.
Foundries
The foundry industry’s products are ferrous, steel or non-ferrous metal castings produced by pouring molten metal into moulds which are typically, in total or in parts, made of bonded silica sand. The foundry industry is an important supplier to the automotive industry, mechanical engineering and other industries. It is a branch of mostly small and medium sized companies: roughly 4,000 foundries with 300,000 employees are situated in the EU Member States.
Glass Industry
Silicon dioxide is the principal glass forming oxide and thus silica sand is the major raw material used in most glass types. The main glass products include packaging glass (bottles, jars etc.), flat glass (for buildings, mirrors, cars, etc.), domestic glass (tableware: drinking glasses, bowls, decoration, etc.), reinforcement glass fibres, glass wool (for insulation) and special glass (for tv, laboratory, optics etc.).
Currently, the EU-28 glass industry employs about 190,000 people (incl. processors who are not melting glass and are therefore not exposed to respirable crystalline silica). The number of workers involved in glass melting activities is estimated to be around 100,000.
After melting the raw material, there is no crystalline silica anymore in glass which is an amorphous material.
Industrial Minerals
Industrial minerals are commercially valuable minerals and rocks, which are used in the industries based on their physical and/or chemical properties.
Around 138 million tons of industrial minerals – bentonite, borate, calcium carbonate, diatomite, feldspar, kaolin, lime, mica, plastic clays, sepiolite, silica, talc, vermiculite – are extracted every year in Europe. Each of these industrial minerals have specific properties, which make them special and essential for some industrial applications. They are used in various markets such as glass, ceramics, industrial fluids, agriculture, construction materials, metallurgy, coatings, pet litter, plastics, paper, paints, electronics, detergents and other. Although not all do, industrial minerals may contain variable amounts of crystalline silica.
Silica is found commonly in the crystalline state but occurs also in an amorphous (non-crystalline) state. Crystalline silica is hard, chemically inert and has a high melting point. These are prized qualities in various industrial uses, mainly in foundry, construction, ceramic and chemicals industries.
Those industrial minerals are produced by 300 companies or groups operating about 810 mines and quarries and 830 plants in 21 EU Member States, and in Switzerland, Norway and Turkey. The industrial minerals industry employs about 100,000 persons in the EU.
Metal Ores
A wide range of metal ores are extracted within the EU and for some, such as, antimony, bauxite, chromium, cobalt, copper, gold, iron, lead, manganese, nickel, silver, titanium, the EU is a relatively significant producer. In some cases, the European producers rank amongst the first ten producers in the world.
Metal ores are produced in 14 EU Member States as well as in Norway, Turkey, Kosovo and Serbia. In the EU, this section of the mining and minerals industry employs directly more than 20,000 people. There are about 90 metal mines operating in EU plus a number of exploration companies.
Although not all do, metal ores may contain variable amounts of crystalline silica.
Mineral Wool
Mineral wool has a unique range of properties, combining high thermal resistance with long-term stability. It is made from molten glass, stone or slag that is spun into a fibre-like structure which creates a combination of thermal, fire and acoustic properties, essential to the thermal and acoustic insulation as well as to the fire protection of domestic and commercial buildings or industrial facilities.
These properties derive from its structure, a mat of fibres which prevent the movement of air, and from its chemical composition.
Insulation manufacturers are developing to meet the growing environmental concerns of society, improving standards and regulations for the use of insulation materials.
Among mineral wools, only glass wool is of concern with regard to crystalline silica as glass wool is manufactured using sand, whilst stone wool is not. After melting the raw material for glass wool, there is no crystalline silica any more, as it becomes an amorphous material.
The mineral wool industry is present in all European countries and employs over 20,000 people across the EU.
Natural Stone Industry
Dimension stone exists in nature as an almost ready-made building material. Few realise, however, that it takes millions of years for this material to get to the point at which it can be easily produced and processed.
The industry consists only of small to medium-size enterprises of between 5 to 100 employees and is an essential supplier of the building industry. More than 40,000 companies exist in the EU, employing around 420,000 persons in the EU. Work with natural stones not only covers the production of stone in quarries, much more important is the processing of stones and the implementation of stones. Restoration and high-tech applications need qualified education and training which starts with stone workers up to high-tech stone engineers.
Mortar Industry
Mortar is a generic term comprising masonry & repair mortars, plaster & renders, adhesives, screeds as well as mortars for special uses, such as anchoring mortars. Mortars consist of aggregates, one or more binders, possibly additives and/or admixtures as well as, depending on the type of binder, water. Mortar is distinguished from concrete based on the grain size of the aggregates. By definition, mortars include aggregates within general <4mm grain size. However, in case of special decorative renders and in floor screeds grain sizes of up to 8mm are also common.
The factory-made mortar industry provides both dry-mixed products (predominantly based on inorganic binders) and ready to use mortar products (based on inorganic and/or organic binders). Besides factory-made mortars a large part of the sector also designs and provides thermal insulation composite systems (ETICS) for renovation and new buildings.
Based on an internal survey conducted 2019 among the members of the European Mortar Industry Organisation (EMO), there are approximately 280 mortar manufacturers (legal entities) within EU-28 with up to 840 production sites. According to this estimate and the NEPSI reported figures, the sector has more than 35,000 employees of which approximately 11,600 are exposed to respirable crystalline silica.
Precast Concrete Industry
Precast concrete is a factory-made building material widely used worldwide and available in all sizes and forms, from very small paving units to more than 50 metres long bridge elements.
Its production process consists of mixing cement, aggregates, water, additives and admixtures in different proportions, pouring them in moulds and letting them harden. The products are supplied to the market in a dust-free hardened state. Dust generation can mainly occur in raw material handling and post-manufacturing mechanical treatments. The industry is composed of small to medium-size enterprises, spread all over Europe. Estimated figures for the EU are: 10,000 production units, 250,000 workers and 300 to 400 million tons of products.
Ready Mixed Concrete
Ready mixed concrete is a mixture of cement, water, aggregates (sand, gravel or crushed stone), chemical admixtures, eventually additions (fly ash, silica fume, ground granulated furnace slags and others) entrapped and entrained air.
Dust generation can mainly occur in the plant where the aggregates are stored before being mixed: ready mixed concrete is manufactured in batching plants and mixed with either stationary or truck mounted mixers. Aggregates containing limited amounts of fines or dirt/clay are washed out. Ready mixed concrete is transported in closed truck mixers whereas the concrete is kept under continuous agitation until it is discharged for use: in this state, concrete does not produce any dust, neither during transport nor during dischargement.
Due to the wide range of applications, its ease of use, high quality, convenience and economy, ready mixed concrete is extensively adopted today, from pavements to high rise buildings and bridges.
The European industry consists mainly of SMEs – small to medium-size enterprises. There are (2018) more than 12,000 plants in Europe, with a production of 250 million of m3 and more than 44,000 employees.