46. Cement and lime, gypsum

Contents

1. Scope

2. Environmental impacts and protective measures

2.1 Air

2.1.1 Waste gases/flue gases
2.1.2 Dust

2.2 Noise
2.3 Water
2.4 Soils
2.5 Workplace
2.6 Ecosystems

3. Notes on the analysis and evaluation of environmental impacts

4. Interaction with other sectors

5. Summary assessment of environmental relevance

6. References

1. Scope

Companies in the cement, lime and gypsum industries produce mainly powdery products which are mouldable when water is added to them and set after a certain reaction time. The following production stages are required to manufacture the products:

- Extraction: Transport, crushing, dosing of additives, storage, dressing of the raw materials;
- Burning;
- Storage and crushing of the burnt products;
- Addition of additives: e.g. gypsum in the case of cement or water in the case of lime;
- Packing and dispatch

In the cement industry there are essentially two production processes which are used to dress and burn the raw material, the so-called wet process and the dry process. In most cases the raw material consists of a mixture of limestone and clay in the ratio of approximately 4:1.

- In the wet process the raw material is ground, with the addition of water, to form a sludge which contains 35-40% water. During burning the water evaporates. The amount of energy required for this is 100% greater than in the dry process. Because of the conditions of the wet process the specific waste gas flow rate is higher. New furnaces for the wet process are now only being constructed for extreme raw material conditions, whilst older plants are being converted increasingly to the energy-saving dry process.
- In the dry process the raw material is crushed whilst being dried, preheated by the counterflow process in a heat-exchanger by the hot kiln waste gases and in most cases burnt in a rotary kiln at the required sintering temperature of approx. 1400°C. Some of the modern plants have capacities of over 5000 t/day, whilst the capacity of the wet kilns rarely exceeds 1000 t/day. Shaft kilns are only used occasionally in special cases where market or raw material conditions dictate, and for the most part their capacity is less than 200 t/day.

In the lime industry both shaft and rotary kilns are used for burning the limestone, the combustion temperature being 850-1000°C. In some cases ring kiln and similar internally developed shaft kiln processes are still used. Compared with the cement industry the capacities of the lime kiln plants are lower, rarely above 1000 t/day. Small producers with simple shaft kilns having a capacity of only a few tonnes per annum are commonly found in many countries.

Gypsum is dewatered at temperatures of 200 - 300°C max. and converted from dihydrate to hemihydrate. Direct current rotary kilns, calcining mills or calciners and boilers are used for burning. The capacities of modern gypsum works are between 600 and 1100 t/day, but some of the plants still have relatively low capacities.

Anhydrite accompanied by gypsum is found in nearly all gypsum deposits. Anhydrite is an anhydrous form of calcium sulphate (CaSO₄) which, after crushing and classifying, can sometimes be used as a quick binding agent without prior thermal treatment.

2. Environmental impacts and protective measures

2.1 Air

2.1.1 Waste gases/flue gases

No waste gases are produced in the extraction and crushing of cement, lime and gypsum raw materials (principally limestone, gypsum and anhydrite), processes which are mainly carried out in the quarries.

The cement raw materials are frequently dried during dressing and crushing so that the moisture produced can be driven off as harmless water vapour. During the burning of the raw materials for cement production, calcium carbonate is converted to calcium oxide when the carbon dioxide (CO₂) contained in the limestone is driven off. Sulphur compounds (mostly in the form of SO₂) and nitrous oxides (NO_x) may also be contained in the waste gas. Chlorine and fluorine gas and vapour emissions are prevented in the normal process by the fact that these impurities are deposited in the burnt product.

Water vapour and CO₂ emissions are process-related, whilst the occurrence of sulphur compounds can be greatly reduced by the use of suitable raw materials and fuels and control of the burning process. Up to certain limits, sulphur components are bound by the cement clinker during burning. Only under extraordinary operating conditions, e.g. where there is an excess of sulphur in the raw material and fuel, or in the case of reducing burning, will there be occasional short-term emissions of appreciable quantities of SO₂.

The flame temperature at which cement is manufactured may be as high as 1800°C, with the result that more nitrous oxides are formed by oxidation of the atmospheric nitrogen than in lime burning.

The NO_x values of 1300 - 1800 mg/Nm³h permitted in the waste gas in Germany (TA-Luft - Technical Instructions on Air Quality Control - Table 1) will probably become subject to more stringent requirements in the next few years. At the present time, possible ways of reducing the NOx values are the subject of large-scale trials, and there currently appear to be four potential methods:

- non-catalytic combustion;
- plants with activated carbon filters;
- optimisation of the burning operation;
- conversion of plants to a two-stage calcining installation (oxidising, reducing).

These processes require different levels of investment and they all presuppose continuous operational monitoring.

In the cement industry oils, solvents, paint residues, old tyres or other combustible waste materials are frequently used as additional fuels. Some of these waste products introduce contaminants which are normally bound by the clinker and do not reach the waste gas. If such fuels are used, the process must be monitored by special safety inspections to prevent the emission of additional contaminants.

In lime burning, which takes place in much smaller plants than in cement production, CO₂ is also emitted with the flue gas, but the quantity of waste gas is much smaller than in cement works because of the size of the plant and because of the lower combustion temperatures in the process.

In lime slaking calcium carbonate is converted to calcium hydroxide with the addition of water, some of the water added being discharged again as water vapour, since the process is exothermic. However, this water vapour is harmless.

In gypsum burning water vapour and small quantities of flue gas are discharged into the atmosphere. Since the combustion temperatures of 300-400°C are not very high, and since in most cases the mass flows are very low, these burning plants only cause slight environmental pollution.

Anhydrite from natural deposits is only crushed before use, but anhydrite from phosphoric acid production must be dried before further use, in which case water vapour will be given off. However, this anhydrite is rarely suitable for industrial use, because it is often toxic.

2.1.2 Dust

During the extraction and further processing of cement, lime and gypsum dust is produced in various stages of the work due to process conditions. In the case of cement this dust is a mixture of limestone, calcium oxide, cement minerals, and sometimes even completely burnt cement, whilst in the case of gypsum the dust contains anhydrite and mainly calcium sulphate. With the exception of the pure CaO dust, which is produced during lime burning, the dust is harmless, but on the other hand it does give rise to considerable nuisance. In the case of the individual production units and conveying installations of a cement works 6-12 m³ of spent air and waste gas per kg of material have to be extracted and dedusted. The major sources of dust in a plant include:

- crushing and mixing of the raw material;
- burning of the cement;
- crushing of the cement (clinker + gypsum);
- slaking of the lime.

The proper use of high-performance extraction plants and dedusting installations, such as electrostatic separators, fabric and gravel bed filters, and often cyclones used in conjunction with these, is essential, otherwise correct process management cannot be guaranteed, costs due to machinery wear rise disproportionately and high dust levels impair working conditions, simultaneously causing loss of production.

The separated dusts are mainly recycled, provided no enrichment of heavy metal components such thallium is expected in the waste gas. Only under unfavourable raw material and fuel conditions will it perhaps be necessary to separate and eject partial quantities of the dust because of the excessive concentration of detrimental components in the product, e.g. alkaline chlorides. Occasionally the use of these dusts is possible in other branches of industry. If the dusts are dumped, the groundwater protection requirements must be met due to the water solubility of individual components.

In lime production the quantity of dust produced is smaller because a powdery product is only involved during the slaking, packing and loading of the lime. In the gypsum and anhydrite industry the amount of dust produced is also small.

High quality filters (electrostatic or fabric filters) now make it possible to achieve a dust concentration of less than 25 mg/Nm³ in the spent air in the cement, lime and gypsum industry. At present, values of below 25 mg/Nm³ are being discussed by the European authorities for new plants, whereas the German TA-Luft (Technical Instructions on Air Quality Control) still requires 50 mg/Nm³.

2.2 Noise

Cement works emit far higher noise levels than lime and gypsum works, but the latter also have production areas giving off considerable noise.

In the extraction of raw materials, noise and associated vibrations may occur as a result of blasting, but such noise emissions can be substantially reduced by means of suitable ignition processes. Moreover, the machines used for mining can be soundproofed to such an extent that they meet the requirements of the German TA-Lärm (Technical Instructions on Noise Abatement).

During dressing, noise pollution is liable to occur e.g. through the use of rebound crushers and mills for the crushing of hard materials. These crushing installations and the adjoining dressing installations can be enclosed in such a way as to protect the environment from oppressive noise. The noise generated by the majority of rock- and cement-crushing plants is so intense that they have to be installed in soundproofed premises in which personnel cannot work on a permanent basis.

Burning plants require numerous large fans which generate extremely penetrating noise, with the result that noise protection measures, e.g. in the form of enclosures, are also necessary.

In order to avoid nuisance, plants in the lime, gypsum and particularly the cement industries must be erected at least 500 metres from residential areas. The immissions values for nearby residential areas should not exceed 50 to 60 dB(A) during the day, and 35-45 dB(A) at night.

2.3 Water

In the vicinity of pits in the German cement, lime and gypsum industry the wastewater may contain up to 0.05 ml/l of total suspended solids. To avoid exceeding this value the pit water produced must be discharged via stilling basins. Water used for washing limestone must always be discharged via sedimentation ponds, and the surface water produced in the area surrounding the pits must be discharged separately.

Some cement and lime works are major water consumers, but because of the process involved they cause no water pollution. In cement works approximately 0.6 m³ of water per tonne of cement is required to cool the machines. Most of this water is in circulation, thus only the water losses need be made up. In plants involved in the drying process, water is also used for cooling the kiln exhaust gases, resulting in a calculated net consumption of approx. 0.4 - 0.6 m³ of water/t of cement. In plants using the wet process an additional 1 m³ of water/t of cement or so is required for the sludge milling. This water is discharged again by evaporation.

In the lime industry water is required for slaking burnt lime (approx. 0.33 m³/t of lime). Some lime works consume an additional 1 m³ or so of water per tonne of lime for washing the raw limestone when extremely pure qualities are required. After use, this washing water is fed to settling basins or settling ponds where the fine particles are deposited and the residual water evaporated or partially re-used.

The gypsum industry requires relatively little water because the processes take place at low temperatures, with the result that no cooling energy is required. In plasterboard production, water is added to the raw gypsum and remains in the product to set the gypsum (conversion of hemihydrate to dihydrate).

Water demand can be reduced by increasing the proportion of circulating water or by minimising the water losses.

In dry areas the cooling water demand can be reduced by installing special electrostatic precipitators which are operational at higher exhaust gas temperatures.

Any sanitary water produced must be discharged and disposed of separately.

2.4 Soils

In the area surrounding cement, lime and gypsum works the soils may be impaired by falling dust where the dedusting plants are inadequately maintained.

Although potentially environmentally relevant trace elements can be introduced into the cement production process by special raw material components such as iron ore and, more recently, by the increased use of combustible waste materials, these hazardous substances are almost completely absorbed by the cement clinker in the molten state, chemically bonded and therefore rendered harmless. To rule out the possibility of adverse effects when using special raw material components or waste products from other industries as fuel from the outset, analyses must be carried out te detect environmentally relevant trace elements such as lead (Pb), cadmium (Cd), tellurium (Tl), mercury (Hg) and zinc (Zn), which are deposited in the filter dusts. If necessary, technical measures such as dust separation must be applied to prevent the accumulation of hazardous substances in the process.

2.5 Workplace

Numerous machines generating noise levels of 90 dB(A) are still operated in cement, lime and gypsum works, even with the present state of the art. Noise levels can generally be reduced by means of static devices. Permanent workplaces inside the plants, e.g. control platforms, must be soundproofed, but if continuous noise levels of 85 dB(A) are still produced, hearing protection must be made available. At noise levels in excess of 90 dB(A) this protection must compulsorily be worn to avoid hearing impairment. Even where personnel remain in high-noise process areas for short periods, hearing protection is recommended.

In exceptional cases, e.g. during repair work or when rectifying faults, personnel may be exposed to high temperatures and higher levels of noise and dust for long periods, and suitable protective devices and protective clothing must be provided for these tasks. Moreover, work in the danger area must be restricted and supervised.

2.6 Ecosystems

Cement, lime and gypsum works require raw materials close to the surface, thus interference with the surrounding landscape cannot be avoided in the extraction of raw materials. The environmental effects of extraction are described in the environmental brief Surface Mining.

When selecting locations for cement, lime and gypsum works, due consideration must be given to the environmental aspects. In the case of locations in areas previously used for agriculture, possibilities for alternative employment must be examined, particularly for affected women. Besides complying with the regulations concerning waste gases, dust, noise and water, the conditions as regards the building land, integration in the landscape, and the infrastructure of the location must also be examined. Infrastructural considerations include, amongst other things, the recruitment and housing of employees, transport systems and traffic density and the existing and planned industrialisation of the area.

Since the environmental impact is not limited to the factory area, the local population, including women and children in particular, should be given access to medical care.

In cement production approximately 1.6 t of raw material per tonne of cement and additional quantities of gypsum are required, bringing the total raw material requirement to approximately 1.65 tonnes. In lime production the raw material requirement of approx. 1.8 t per tonne of finished product is about 10% higher than for cement production. In calculating this raw material requirement, the over-burden, which varies considerably from deposit to deposit, is not taken into account. In Germany most of the gypsum requirement could now be covered by the gypsum produced in flue gas desulphurisation plants, so that producing this raw material would no longer affect the landscape.

It is advisable to build up financial reserves for the subsequent recultivation of a quarry, even while the quarry is operational.

3. Notes on the analysis and evaluation of environmental impacts

Limit values for exhaust gas, dust and water have been formulated for dischargers of wastewater in the provisions of TA-Luft and TA-Lärm (Technical Instructions on Air Quality Control and Technical Instructions on Noise Abatement), in the Guidelines adopted by the Association of German Engineers (VDI) and in the administrative regulations specific to the various industries. Similar values are being adopted by most European countries. The US regulations published by the Environmental Protection Agency (EPA) are frequently more stringent than the German regulations, particularly in California.

For countries without their own environmental protection laws, these values must be examined and adapted in the individual case, taking the prevailing environmental conditions into consideration. In exceptional cases, particularly for rehabilitation of plants, special regulations must be established, but new plants should conform to the European standard values for environmental protection.

The Compendium of Environmental Standards offers advice on assessing environmental relevance for individual substances.

Table 1 - Limitation of hazardous substances under TA-Luft (Technical Instructions on Air Quality Control) and the 17th Administrative Regulation according to § 7a of the Federal Water Act

In developing countries dust emissions of 100 mg/Nm³ of exhaust gas or spent air should on no account be exceeded. Higher dust emissions will cause both internal and external environmental burdens.

Similarly, wastewater disposal should meet the minimum requirements imposed by the regulations laying down limits for dischargers of wastewater into receiving bodies of water.

The noise problem is underrated in many countries, but constant noise can lead to permanent damage. Here too, therefore, the prescribed noise limits must be adhered to in the workplace and in the surrounding residential areas (Section 2.2), and encroachment on residential areas must be prohibited.

All parameters must be regularly checked by means of internal audits, for which purpose training must be given and personnel generally sensitised to environmental matters if necessary.

The use of land by the cement, lime and gypsum industry must be kept within definable limits by forward-looking and detailed planning covering the areas of mining, recultivation and water management. The high costs often mean that there is no money available for recultivation of pits, often resulting in direct or consequential damage that may be difficult to repair (see environmental brief Surface Mining)

3.1 Inspection and maintenance of environmental protection installations

A control centre independent of the production process must be established to comply with existing environmental protection regulations. The responsible personnel must be enabled to perform and monitor all inspection functions including measurements relating to environmental protection in the works. They should be available for consultation on investments and take charge of negotiations with environmental protection authorities. Moreover, this department is responsible for ensuring that all environmental protection installations are regularly maintained and upgraded. This internal environmental department is also responsible for staff training.

4. Interaction with other sectors

Cement production may touch on other project areas, particularly where additional raw material components are used. For example, use is made of materials produced in lime works with inadequate lime content, other waste materials such as crystallised calcium carbonate from the chemical industry or ferrous residues from sulphuric acid production. Up to 5% gypsum per tonne of cement is required to control the rate of setting in the cement, and a major proportion of this gypsum requirement is now met in Europe by gypsum from flue gas desulphurisation plants. Up to 85% of fly ashes from power station dedusters and slags can also be added to the clinker to produce cement varieties with special properties.

Because of the high temperatures and comparatively long holding times of the materials in the relevant areas, cement kilns in particular are ideal for disposing of combustible waste. This possibility is increasingly important in countries where large quantities of vegetable waste with high potential energy, such as rice chaff, are produced in the region.

In the cement, lime and gypsum industry, secondary activities such as quarries, fuel stores, workshops etc. also exert environmental impact.

Table 2 - Environmental impacts of adjacent project areas - cement, lime and gypsum

5. Summary assessment of environmental relevance

The environmental impacts of cement, lime and gypsum works are caused by exhaust gas, dust, noise and water. The following table assigns values to the individual process stages as regards the environmental burden which they impose.

Table 3 - Environmental impact of process stages (cement/lime/gypsum)

Key: 1 very slight; 2 slight; 3 moderate; 4 considerable
1) dry process only

Proven technologies have been available for a good many years to reduce pollutant loads. In new plants for the cement industry in the industrialised countries, the costs of environmental protection measures, in the widest sense of the term, already account for as much as 20% of the total investment cost, and in the future this proportion will increase still further.

The more sophisticated the dedusting method, the greater the importance of systematic monitoring and maintenance for the continuing reliability and efficiency of the plants. Besides dedusting plants, changes in burning technology are becoming increasingly important for reducing NOx values.

Catering for the needs of the environment when planning and erecting cement, lime and gypsum works can also save money. The dusts generated are mainly preliminary, intermediate or end products which can reduce the direct production costs if recycled and returned to the process. Reduced ejection of dust also reduces wear on machines, thereby increasing their availability and saving repair costs.

The cement industry is becoming increasingly important as a recycler of waste materials such as food, waste oil or rubber tyres, thereby reducing the need for dumping. The initial fear that this disposal might lead to an increased emission of environmentally relevant trace elements has been allayed by measurements carried out during operation. When the materials are burnt, particular attention must be paid to correct firing, design and monitoring of the plants. Therefore the regulations concerning waste gas emissions and monitoring of such plants have been made more stringent.

The designers of a new plant must consider what environmental protection measures are necessary and appropriate as early as the planning phase. Suitable guidelines must also be established during the planning phase for countries which do not have their own regulations in this area.

Early involvement of neighbouring population groups in the planning and decision-making processes will enable measures to be devised to deal with any problems arising.

6. References

Erste Allgemeine Verwaltungsvorschrift zur Reinhaltung der Luft -TA-Luft - GMBl (joint ministerial circular) Nr. 24.

Allgemeine Verwaltungsvorschriften über genehmigungsbedürftige Anlagen nach § 16 der Gewerbeordnung GeWO (technische Anleitung zum Schutz gegen Lärm - TA-Lärm) verschiedene Ausgaben.

Allnoch, G. et. al.: Umweltverträglichkeitsprüfung von Entwicklungshilfeprojekten - Erstellung eines Kataloges von Emission- und Immissionsstandards, im Auftrag der GTZ Eschborn, 1984.

Betriebswacht, Datenjahresbuch 1991: Berufsgenossenschaft der keramischen und Glasindustrie, Würzburg.

Emissionsminderung Zementwerke VDI-Richtlinie 2094, Entwurf May 1981.

Entwurf zur Abwasserverordnung, Deutscher Industrie- und Handelstag, Anhang 17, Sept. 21, 1990.

Environmental Protection Agency: New source performance standards - Clean Air Act (USA).

Environmental Assessment Sourcebook Nov. 1990, Worldbank Draft Part 9.3-1 Cement /93-101, Mining and Mineral Processing 31.10.1990.

Funke, G.: Immissionsprognosen für Genehmigungsverfahren Zement, Kalk, Gips 33, p. 15-23, 1980.

Göke H.: Grenzen des Umweltschutzes aus der Sicht der Tagebau- und Steinbruchindustrie, Zement, Kalk, Gips 31, p. 252 - 254, 1978.

Gesetz zum Schutze vor Umwelteinwirkungen durch Luftverunreinigungen, Geräusche, Erschütterungen und ähnliche Vorgänge. Bundesimmissionsschutzgesetz - BImSchG - dated 15.03.1974 - BGBl. I (Federal Law Gazette I), p. 721 - 1193.

Hinz, W.: Umweltschutz und Energiewirtschaft Zement, Kalk, Gips 31, p. 215 - 229, 1979.

Luftreinhalte-Verordnung (LVR) Switzerland, of 16.12.1985, edition of 1 July 1990.

Luftreinhalteplan bei der Basel, February 1990.

Schulze, K.-H.: Immissionsmessungen und ihre Fehlergrenzen Zement, Kalk, Gips 36: p. 7 - 11, 1980.

Technical note on best available technologies not entailing excessive cost for the manufacture of cement: Commission of the European Communities, Report EUR 13005 EN, 1990.

Umweltschutz in der Steine - und Erden-Industrie Zement, Kalk, Gips 31, p. 215 - 229, 1979.

Verein Deutscher Zementwerke: Forschungsbericht der Zementindustrie, Tätigkeitsbericht, 1978 - 1981 - VDZ Düsseldorf 1981.

Ditto Tätigkeitsbericht 1981 - 1984

Siebzehnte Verordnung zur Durchführung des Bundesimmissionsschutzgesetzes 1990, (Verordnung über Verbrennungsanlagen für Abfälle und ähnliche brennbare Stoffe, 17. BImSchV)

Zukünftige Probleme des Umweltschutzes in der Zementindustrie, Zement, Kalk, Gips 33: p. 1 - 9, 1980.

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