Lime Manufacturing Process (exracted from ELA data)

The lime making process consists of the burning of calcium and/or magnesium carbonates at a temperature between 900 and 1500 oC, which is sufficiently high to liberate carbon dioxide, and to obtain the derived oxide (CaCO3 ® CaO + CO2). For some processes significantly higher burning temperatures are necessary, for example dead-burned dolomite.

The calcium oxide product from the kiln is generally crushed, milled and/or screened before being conveyed to silo storage. From the silo, the burned lime is either delivered to the end user for use in the form of quicklime, or transferred to a hydrating plant where it is reacted with water to produce hydrated or slaked lime.

Lime processes mainly contains the following basic steps

Winning of limestone

The raw material for lime production is limestone or, to a lesser extent, dolomite or dolomitic limestone. Dolomite and dolomitic limestone are mixtures of calcium carbonate and up to 44% magnesium carbonate. While limestone deposits are relatively abundant in many countries, only a small proportion are suitable for commercial extraction.

High purity limestone or dolomite is quarried, crushed, and in some cases washed. It is then screened and transported to the kiln. Limestone is normally obtained by surface quarrying, generally adjacent to the lime plant, but in some cases sea dredging or even underground mining are used. A typical mining process includes:

 Limestone preparation and storage

Limestone is crushed to the appropriate size range, which is normally 5 to 200 mm depending upon the kiln used. Primary crushers receive quarry rocks as large as one metre in diameter and reduce their size down to 100-250 mm. Crushed stone from the primary crushers is transported via conveyors to vibrating screens, where large pieces are separated and recycled while those passing through are used as kiln charge, or may be fed into the secondary crushers located further down the process line.

Secondary crushers yield pebbles of 10 to 50 mm, which after screening are transported on belt conveyors and/or bucket elevators to limestone storage silos or compartments for storage prior to feeding the dryer or the lime kiln.

 Overview of a lime manufacturing process

[ EuLA]

Depending on the nature of the rock (hardness, lamination, size etc.) various types of primary crushers are used, such as: jaw crushers, gyratory crushers and impact crushers. As the kiln charge does not have to be very fine, jaw and impact crushers are also often used as secondary crushers, as are hammer mills. Sometimes crushing plants are located at the quarry and are mobile.

The particle size distribution must be compatible with the requirements of the kiln. This generally requires the stone to be positively screened to give a size distribution of, ideally 2 to 1, or at least 3 to 1.

Washing is sometimes used to remove natural impurities such as silica, clay and the very fine particles of limestone. This washing aids the burning process by leaving free space between the stones for combustion air circulation, thus reducing the amount of excess air and saving electrical energy. Techniques for piling the limestone, for better cleaning, have been developed.

Screened sizes of limestone are stored in bunkers and in outdoor stockpiles. Fine grades are usually stored in sealed bunkers.

In a very limited number of installations (for example, where the calcium carbonate is in the form of a sludge or filter cake), it is necessary to dry the feed material. This is generally done by using the surplus heat from kiln exhaust gases.

Fuels, storage and preparation

In lime burning, the fuel provides the necessary energy to calcine the lime. It also interacts with the process, and the combustion products react with the quicklime. Many different fuels are used in lime kilns. The most common in the EU is natural gas, but coal, coke and fuel oil are also widely used. Table 2.6 shows fuels used in lime-burning. Most kilns can operate on more than one fuel, but some fuels cannot be used in certain kilns. Fuels markedly affect the heat usage, output and product quality. Some fuels require a special refractory kiln lining.

Type of fuel

Widely used

Sometimes used

Rarely used

Solid

Bituminous coal
Coke

Anthracite
Lignite
Pet coke

Peat
Oil shales

Liquid

Heavy fuel oil

Medium fuel oil

Light fuel oil

Gaseous

Natural gas

Butane/propane

Town gas

Producer gas

 

Unconventional Wood/sawdust, Used tyres, Paper, Plastic, etc.

Biomass, Waste liquid and solid fuels

Table 2.6: Fuels used in lime-burning

[EuLA]

The choice of fuel(s) for a lime-burning operation is important for the following reasons:

a) the cost of fuel per tonne of lime may represent 40 to 50% of the production cost,

b) an inappropriate fuel can cause major operating costs, and

c) the fuel can influence the quality of the lime, notably the residual CO2 level, the reactivity, and the sulphur content.

In addition, the choice of fuel can affect the levels of emissions of carbon dioxide, carbon monoxide, smoke, dust, sulphur dioxide and oxides of nitrogen, all of which have an environmental impact.

The fuel should be prepared as required for the injection system, which can be of direct or indirect firing type. In the case of solid fuels, this involves delivery at the appropriate particle size for the installed handling system. In the case of liquid and gaseous fuels, the required pressure and (as appropriate) temperature need to be maintained.

 Calcining of limestone

The lime burning process typically involves:

providing sufficient heat at above 800 oC to heat the limestone and to cause decarbonation, and holding the quicklime for the requisite time at a sufficiently high temperature (typically in the range 1200 to 1300 oC to adjust reactivity.

A large variety of techniques and kiln designs have been used over the centuries and around the world. Although sales of lime kilns in recent years have been dominated by a relatively small number of designs, many alternatives are available, which may be particularly suitable for specific applications. Stone properties such as strength before and after burning, dust generation and product quality must be considered when choosing kiln technology. Many lime producers operate two or more types of kiln, using different sizes of limestone feed, and producing different qualities of lime. The main characteristics of some types of lime kiln are summarised in Table 2.7.

Kiln type

Fuelsa

Output range (tonnes/day)

Range of feed  stone size (mm) 

Shaft

Mixed feed

S

60-200

20-200

Double-inclined

G,L,S

10-160

20-100

Multi-chamber

G,L,S

40-225

20-150

Annular

G,L,S

80-600

10-250

Parallel-flow reg. (standard)

G,L,S

100-600

25-200

Parallel-flow reg. (finelime)

G,S

100-300

10-30

Other shaft

- central burner

G,S

40-80

40-150

- external chambers

G,L

40-120

80-350

- beam burner

G,L,S

50-800

20-175

- internal arch

G,L,S

15-250

25-120

Rotary

Long

G,L,S

160-1500

dust-60

Preheater

G,L,S

150-1500

0-60b

Other kilns

 

 

 

Travelling grate

G,L,S

80-130

15-45

"Top-shaped"

G,L,S

30-100

5-40

Fluidised bed

G,L

30-150

0-2

Flash calciner

G,L

300-1500

0-2

Rotating hearth

G,L,S

100-300

10-40

a G=Gaseous, L=Liquid, S=Solid

b cyclone preheater 0-2 mm, shaft preheater 10-60 mm, grate preheater 10-50 mm

Table 2.7: Characteristics of some types of lime kiln

[EuLA], [UK Report 1996]

Since the "oil crisis" of 1972, there have been a number of pressures on lime producers to replace existing kilns, in particular:

a) variable and, at times, high fuel prices,

b) fierce competition (arising from spare capacity), which has forced down the market price of lime,

c) a preference for quicklime with more consistent quality and particularly with high reactivity, low CaCO3 and low sulphur content, and

d) increasingly stringent environmental standards both for the workplace and for atmospheric emissions.

Heat transfer in lime burning can be divided into three stages:

a) Preheating zone. Limestone is heated from ambient to above 800 °C by direct contact with the gases leaving the calcining zone (i.e. products of combustion, excess air and CO2 from calcination).

b) Calcining zone. Fuel is burned in preheated air from the cooling zone and (depending on the design) in additional "combustion" air added with the fuel. This produces a temperature of over 900 °C and causes dissociation of the limestone into quicklime and carbon dioxide.

c) Cooling zone. Quicklime leaving the calcining zone at 900 °C is cooled by direct contact with "cooling" air, part or all of the combustion air, which in return is preheated.

These zones are illustrated in Figure 2.4 for a vertical shaft kiln

Figure 2.4: Vertical shaft kiln.

[VDI Draft Guidelines 2583, 1984]

Most of the kilns currently used are based on either the shaft or the rotary design. There are a few other kilns based on different principles. All of th

ese designs incorporate the concept of the three zones. Whereas shaft kilns usually incorporate a preheating zone, some other lime kilns, namely rotary and fluidised bed kilns, are nowadays operated in connection with separate preheaters. Two main types of preheaters are used; vertical shaft and travelling grate.

idth (or diameter) to 2 m. Uniform heat release can be achieved in larger shafts by:

Most kiln systems are characterised by the counter-current flow of solids and gases, which has implications for the resulting pollutant releases.

 Shaft kilns

Figure 2.4 shows a schematic diagram of a shaft kiln. The major problem with traditional shaft kilns is obtaining uniform heat release and movement of the burden across the shaft. Fuel injected at a wall usually does not penetrate more than 1 m into a packed bed. This limits the kiln w

In general, shaft kilns have relatively low heat use rates because of efficient heat transfer between the gases and the packed bed. However, they retain most of the sulphur in the fuel so low-sulphur fuel is required to produce a low-sulphur product. Older designs tend to produce quicklime with a low to moderate reactivity and a relatively high CaCO3 content. Modern designs incorporate features which enable highly reactive lime to be produced with low CaCO3 levels.

Before describing specific designs of vertical shaft kilns, it is appropriate to consider three important features which are common to all designs, namely charging, drawing and combustion.

Charging of raw material

Single point charging of lump raw material, especially to shaft kilns, can lead to problems in kiln operation. Larger stones tend to roll down the conical heap towards the walls, while the smaller fractions concentrate along the axis of the kiln. As a result, there is a gradation in the resistance to flow of kiln gases from a high level around the central axis to progressively lower levels towards the walls. This results in a greatly reduced flow of gases through the central part of the kiln and as a result part of the burden tends to be under-calcined.

A variety of devices have been developed to mitigate this effect and to minimise the asymmetry of the charging system. In the fixed plate and cone arrangement, the position of the cone and strike plate, relatively to the feed chute and to each other, can be adjusted to produce a more-or-less uniform profile around the kiln. Inevitably, fines tend to concentrate on either side of the feed chute centre line, but the effect on the kiln operation is small. The rotating hopper and bell system is more sophisticated and produces both a more uniform profile and a better dispersion of fines in an annular ring around the kiln.

For mixed feed kilns it is essential that the fuel is dispersed uniformly across the kiln. Therefore rotating hopper and bell systems are used, in which the bell may be fitted with extensions, which typically consist of four quadrants, one deflecting part of the charge towards the centre of the kiln, a second deflecting it further out, and with the third and fourth quadrants deflecting it progressively further away from the axis of the kiln. After each charge, the hopper and the apron are rotated by a fraction of revolution so that, on average, a uniform distribution is obtained.

Drawing of lime

In most cases the drawing system determines the velocity at which the limestone burden descends through the kiln. The drawing system should produce a uniform movement of the burden. Simple systems, using a single discharge point and a conical table, work satisfactorily while the burden moves freely. However, when there is a tendency for part of the kiln to stick or when fused lumps of lime bridge between the table and the wall of the cooling zone, lime is preferentially drawn from the free-flowing parts of the kiln, resulting in further over-heating in the problem area.

A better system uses four discharge points without a central table. If there is a tendency for part of the kiln to stick, the feeder(s) under that part can be operated at a faster rate than the others to help re-establish free movement. Similarly, if one feeder becomes blocked, appropriate action can be taken. Multiple discharge points can also assist with diagnosing problems within a kiln. By operating each in turn, the lime from different segments can be tested separately to identify if a particular segment is under or over-burned.

Still more sophisticated drawing mechanisms are used, such as;

a) hydraulically driven quadrants,

b) a rotating eccentric plate, and

c) a rotating spiral cone with steps and a slope designed to take lime uniformly across the shaft. This design is used on some mixed-feed kilns.

Combustion

In all combustion processes there is an optimum air to fuel ratio which gives the highest efficiency of combustion. A ratio lower than optimum results in incomplete combustion and increased levels of carbon monoxide, while a higher ratio results in the products of combustion being diluted and cooled by the additional quantities of air.

Combustion within the packed bed in vertical lime kilns is particularly problematical as mixing of gasified fuel and air under these conditions is more difficult. From the viewpoint of combustion efficiency, the fuel and air should, ideally, be distributed uniformly across the shaft. However, regardless of the firing system, variations in the air-fuel ratio occur.

Various techniques have been used to moderate temperatures in the calcining zone. Use of an overall deficiency of air is effective but increases fuel usage and can cause the emission of dark smoke. Recirculation of kiln gases is practised with some kilns to moderate kiln temperatures, particularly at the walls. In the annular shaft and parallel-flow regenerative kilns part or all of the combustion gases pass down part of the shaft in co-current flow with the lime. This results in a comparatively low temperature in the finishing section of the calcining zone.

Mixed-feed shaft kiln

Modern mixed-feed shaft kilns use limestone with a top size in the range of 50 to 150 mm and a size ratio of approximately 2:1. The most widely used fuel is a dense grade of coke with low reactivity and low ash content. The coke size is only slightly smaller than that of the stone so that it moves with it rather than trickling through the interstices. The stone and the coke are mixed and charged into the kiln in such a way as to minimize segregation.

The quality of the quicklime tends to be moderate, with the reactivity being considerably lower than that obtained by rotary kilns at the same CaCO3 level. The retention of sulphur from the fuel is high.

Double-inclined shaft kiln

The double-inclined kiln is shown in Figure 2.5. It is essentially rectangular in cross-section, but incorporates two inclined sections in the calcining zone. Opposite each inclined section, off-set arches create spaces into which fuel and preheated combustion air is fired via combustion chambers.

Figure 2.5: Double-inclined shaft kiln.

Based on figure from [Ullmann’s, 1990]

Cooling air is drawn into the base of the kiln where it is preheated, withdrawn and re-injected via the combustion chambers. The tortuous paths for both the gases and the burden, coupled with firing from both sides, ensures an efficient distribution of heat. A range of solid, liquid and gaseous fuels can be used, although they should be selected with care to avoid excessive build-ups caused by fuel ash and calcium sulphate deposits.

The kiln can produce a reactive low-carbonate product.

The multi-chamber shaft kiln

This is a development of the double-inclined kiln. It consists of 4 or 6 alternately inclined sections in the calcining zone, opposite each of which is an offset arch. The arches serve the same purpose as in the double-inclined kiln.

Cooling air is preheated by lime in the cooling zone and is withdrawn, de-dusted and re-injected via the combustion chambers.

A feature of the kiln is that the temperature of the lower combustion chambers can be varied to control the reactivity of the lime over a wide range. The kiln can be fired with solid, liquid and gaseous fuels (or a mixture).

Annular shaft kiln

The major feature of the annular shaft kiln Figure 2.6a, is a central cylinder which restricts the width of the annulus, and together with arches for combustion gas distribution ensures good heat distribution. The central column also enables part of the combustion gases from the lower burners to be drawn down the shaft and to be injected back into the lower chamber. This recycling moderates the temperature at the lower burners and ensures that the final stages of calcination occur at low temperature. Both effects help to ensure a product with a low CaCO3 level and a high reactivity. the annular shaft kiln can be fired with gas, oil or solid fuel. The ehaust gases hav high CO2 concentration.

 

Figure 2.6: a) Annular shaft kiln; b) Parallel-flow regenerative kiln.

Based on figures from [Ullmann’s, 1990]

 

Parallel-flow regenerative kiln

The parallel-flow regenerative (or Maerz) kiln is shown in Figure 2.6b. Its characteristic feature is that it consists of two interconnected cylindrical shafts. Some early designs had three shafts, while others had rectangular shafts, but the operating principles are the same.

Batches of limestone are charged alternately to each shaft and pass downwards through a preheating/regenerative heat exchange zone, past the fuel lances and into the calcining zone. From the calcining zone they pass to the cooling zone.

The operation of the kiln consists of two equal periods, which last from 8 to 15 minutes at full output.

In the first period, fuel is injected through the lances in shaft 1 and burns in the combustion air blown down this shaft. The heat released is partly absorbed by the calcination of limestone in shaft 1. Air is blown into the base of each shaft to cool the lime. The cooling air in shaft 1, together with the combustion gases and the carbon dioxide from calcination, pass through the interconnecting cross-duct into shaft 2 at a temperature of about 1050 °C. In shaft 2, the gases from shaft 1 mix with the cooling air blown into the base of shaft 2 and pass upwards. In so doing, they heat the stone in the preheating zone of shaft 2.

If the above mode of operation were to continue, the exhaust gas temperature would rise to well over 500 °C. However, after a period of 8 to 15 minutes, the fuel and air flows to shaft 1 are stopped, and "reversal" occurs. After charging limestone to shaft 1, fuel and air are injected to shaft 2 and the exhaust gases are vented from the top of shaft 1.

The method of operation described above incorporates two key principles:

a) The stone-packed preheating zone in each shaft acts as a regenerative heat exchanger, in addition to preheating the stone to calcining temperature. The surplus heat in the gases is transferred to the stone in shaft 2 during the first stage of the process. It is then recovered from the stone to the combustion air in the second. As a result, the combustion air is preheated to about 800 °C.

b) The calcination of the quicklime is completed at the level of the cross-duct at a moderate temperature of about 1100 °C. This favours the production of a highly reactive quicklime, which may, if required, be produced with a low CaCO3 content.

Because the kiln is designed to operate with a high level of excess air (none of the cooling air is required for combustion), the level of CO2 in the exhaust gases is low -about 20% by volume (dry).

The kiln can be fired with gas, oil or solid fuel (in the case of solid fuel, its characteristics must be carefully selected). A modified design (the "finelime" kiln) is able to accept a feedstone in the range 10 to 30 mm, provided that the limestone is suitable.

Other shaft kilns

This group includes a number of designs not described above. In these designs fuel is introduced through the walls of the kiln, and is burned in the calcining zone, with the combustion products moving upwards in counter-current to the lime and limestone. In some designs, the fuel is partially combusted in external gasifiers. In others, it is introduced via devices such as a central burner, beam burner or injected below internal arches.

2.2.4.2 Rotary kilns

Long rotary kiln

The traditional/long rotary kiln consists of a rotating cylinder (up to 140 m long) inclined at an angle of 1 to 4 degrees to the horizontal. Limestone is fed into the upper end and fuel plus combustion air is fired into the lower end. Quicklime is discharged from the kiln into a lime cooler, where it is used to preheat the combustion air. Various designs of lime cooler are used, including "planetary" units mounted around the kiln shell, travelling grates and various types of counter-flow shaft coolers.

Many kilns have internal features to recover heat from the kiln gases and to preheat the limestone. These include:

a) chains (in kilns fed with calcium carbonate sludge),

b) metal dividers and refractory trefoils, which effectively divide the kiln into smaller tubes,

c) lifters which cause the stone to cascade through the gases, and

d) internal refractory dams, which increase the residence time of the burden.

The design of burner is important for the efficient and reliable operation of the kiln. The flame should be of the correct length, too short and it causes excessive temperatures and refractory failure, too long and it does not transfer sufficient radiant heat in the calcining zone with the result that the back-end temperature rises and thermal efficiency decreases. The flame should not impinge on the refractory.

Rotary kilns can accept a wide range of feed stone sizes from 60 mm down to dust. An interesting feature of the tumbling bed in the kiln is that larger stones migrate towards the outside of the bed, while smaller stones concentrate at the centre of the bed. This results in the larger stones being exposed to higher temperatures than the smaller stones, with the result that over-calcination of the finer fractions can be avoided. Indeed, it is frequently necessary to incorporate "mixers" or steps into the refractory lining to mix the bed and to ensure that the finer fractions are fully calcined. Because of the ease with which they can be controlled, rotary kilns can produce a wider range of reactivities and lower CaCO3 levels than shaft kilns. The variability of reactivity, however, tends to be greater than that of shaft kilns. Relatively weak feedstones, such as shell deposits, and limestones that break up, are unsuitable as feed to shaft kilns but may prove to be suitable for rotary kilns.

Rotary kilns can be fired with a wide range of fuels. As heat transfer in the calcining zone is largely by radiation, and as the infra-red emissivities increase in the sequence gas, oil and solid fuel, the choice of fuel can have a marked effect on heat usage. Values as high as 9200 MJ/tonne quicklime have been observed for simple gas-fired kilns, while a similar coal-fired kiln may have a heat usage of 7500 MJ/tonne quicklime. The use of internal fittings can reduce those heat usages to below 6700 MJ/tonne quicklime. Radiation and convection losses from the kiln are high relative to other designs of lime kiln.

A feature of rotary kilns is the formation of "rings". These consist of an accumulation of material on the refractory in a part of the kiln which has the appropriate temperature for a semi-liquid phase to form. Such rings can form from ash in coal-fired kilns and from calcium sulphate deposits. Alkalis (sodium and potassium oxides), clay and lime can contribute to the build-ups, which can be troublesome. In the case of coal-firing, fine grinding of the fuel can significantly reduce the rate of build-up.

Another feature of rotary kilns is that sulphur from the fuel, and, to a lesser extent from the limestone, can be expelled from the kiln in the kiln gases by a combination of controlling the temperature and the percentage of CO in the calcining zone. Thus low sulphur limes can be produced using high sulphur fuels, subject to any emission limits for SO2 in the exhaust gases.

Preheater rotary kiln

Modern rotary kilns are fitted with preheaters, see Figure 2.7, and are generally considerably shorter than the conventional rotary kiln (e.g. 40 to 90 m). The heat use decreases because of reduced radiation and convection losses as well as the increased heat recovery from the exhaust gases. Thus, with coal firing, net heat uses of less than 5200 MJ/tonne quicklime are reported.

Figure 2.7: Preheater rotary lime kiln.

[Ullmann’s, 1990]

A number of preheater designs have been developed, including vertical shafts and travelling grates. The preheater should be selected on the basis of the size and properties of the feedstone. Most can accept a bottom size of 10 mm; some have used stones down to 6 mm, and some cannot tolerate weak stones or stone that is prone to breaking up.

While the elimination of sulphur is more difficult with preheater kilns, there are a number of ways in which it can be achieved:

a) establishing a purge of SO2 by taking some of the kiln gases around the preheater (at the cost of increased heat use),

b) operating the kiln under reducing conditions and introducing additional air at the back end (only works with certain designs of preheater), and

c) adding sufficient finely divided limestone to the feed for it to preferentially absorb SO2 and so that it is either collected in the back-end dust collector, or is screened out of the lime discharged from the cooler.

2.2.4.3 Other kilns

Various designs of lime kilns have been developed, based on the technology used in modern cement kilns. One of the driving forces for developing new designs of kiln is that substantial quantities of calcium carbonate are available in finely divided form. The sugar and paper/wood pulp industries, for example, produce a mixture of calcium carbonate and organic matter which can be calcined and recycled, and many

limestone quarries produce a surplus of fine stone, which, in principle, would be suitable for calcination (although it is often contaminated with clay).

Travelling grate kiln

For limestone in the size range 15 to 45 mm, an option is the "travelling grate" (or CID) kiln (developed in Germany). It consists of a rectangular shaft preheating zone, which feeds the limestone into a calcining zone. In the calcining zone the limestone slowly cascades over five oscillating plates, opposite which are a series of burners. The lime passes to a rectangular cooling zone. The CID kiln can burn gaseous, liquid or pulverised fuels and is reported to produce a soft-burned lime with a residual CaCO3 content of less than 2.3%. The four kilns built to date have capacities of 80 to 130 tonnes/day of quicklime.

The top-shaped Kiln

Another relatively new development, which accepts feedstone in the 10 to 25 mm range, is the "top-shaped" lime kiln (developed in Japan). This consists of an annular preheating zone from which the limestone is displaced by pushing rods into a cylindrical calcining zone. Combustion gases from a central, downward facing burner, fired with oil and positioned in the centre of the preheating zone are drawn down into the calcining zone by an ejector. The lime then passes down into a conical cooling zone. The kiln is reported to produce high quality quicklime, suitable for steel production and precipitated calcium carbonate. Kiln capacities are up to 100 tonnes/day of quicklime, heat use is 4600 MJ/tonne of lime. It is reported that, because of its relatively low height, the kiln can accept limestones with low strengths.

Gas suspension calcination process

Gas suspension calcination, GSC, is a new technology for minerals processing, such as the calcination of limestone, dolomite and magnesite from pulverised raw materials to produce highly reactive and uniform products. Most of the processes in the plant, such as drying, preheating, calcination and cooling, are performed in gas suspension. Consequently, the plant consists of stationary equipment and few moving components, as shown in Figure 2.8.

The amount of material present in the system is negligible, which means that after a few minutes of operation the product will conform to specifications. There is no loss of material or quality during start-up and shut-down so there is no sub-grade product. The GSC process produces a product with high reactivity, even when calcined to a high degree. The material to be processed in gas suspension must have a suitable fineness, practical experience has shown that 2 mm particle size should not be exceeded.

calcined by multiple burners as it rotates on the annular hearth. The combustion air is preheated by surplus heat in exhaust gases and/or by using it to cool the quicklime. Due to the reduced abrasion compared with rotary and shaft kilns, rotating hearth kilns produce a high proportion of pebble lime.

Figure 2.8: Gas suspension calcination process

Process diagram of Norsk Hydro’s GSC plant

A GSC plant for the production of dolomitic lime has been in continuous operation at Norsk Hydro, Porsgrunn, Norway, since August 1986. Some performance figures for the balanced operation of GSC and crushing/drying are presented below:

Plant capacity

430 tonnes/day

Fuel consumption

4800 MJ/tonne product

Power consumption

33 kWh/tonne product

Rotating hearth kiln

This type of kiln, now almost obsolete, was designed to produce pebble lime. It consists of an annular travelling hearth carrying the limestone charge. The limestone is

2.2.5 Quicklime processing

The objective of processing run-of-kiln (ROK) quicklime is to produce a number of grades with the particle sizes and qualities required by the various market segments. A number of unit processes are used, including screening, crushing, pulverising, grinding, air-classifying and conveying. A well-designed lime processing plant achieves a number of objectives, namely:

a) maximising the yield of main products,

b) minimising the yield of surplus grades (generally fines),

c) improving the quality of certain products, and

d) providing flexibility to alter the yields of products in response to changes in market demand.

The processing plant should include adequate storage, both for the products and intermediates, to provide a buffer between the kiln, which is best operated on a continuous basis, and despatches which tend to be at low levels overnight and at weekends.

ROK lime is often screened (typically at about 5 mm) to remove a less pure "primary" fines fraction. If the ROK lime has a top size in excess of (say) 45 mm, it is reduced in size with the minimum production of fines. Jaw and rolls crushers are widely used for this task. The crushed ROK lime is then fed to a multi-deck screen, which produces a secondary fines fraction (e.g. less than 5 mm), and granular, or "pebble", lime fractions (e.g. 5-15 mm and 15-45 mm). Oversize lumps (e.g., greater than 45 mm) may be crushed in a secondary crusher and recycled to the multi-deck screen.

The products are stored in bunkers, from which they can be either despatched directly, or transferred to another plant for grinding or hydrating.

Production of ground quicklime

The demand for various grades and qualities of ground quicklime has grown rapidly ever since the 1950s. Particle size requirements vary from relatively coarse products used for soil stabilisation to very finely divided products for specialist applications.

The coarser products are produced relatively cheaply in a single pass through a beater mill fitted with an integral basket. Finer products are generally produced in tube mills and vertical roller mills. In the latter case, a variable speed classifier is fitted above the mill to control the grading of the product, and recycle over-sized particles.

In the late 1980s, the high pressure roll mill was developed for the cement industry and is increasingly being used for quicklime. The product is passed through the grinding rolls, which effectively produce a flake. It is then fed to a dis-agglomerator and an air classifier which removes particles of the required fineness and recycles the coarse fraction. The power requirements of this system can be less than half those of ball mills and less than 60% of the ring-roll mills.

2.2.6 Production of Slaked lime

Slaked lime includes hydrated lime (dry calcium hydroxide powder), milk of lime and lime putty (dispersions of calcium hydroxide particles in water).

Production of hydrated lime

The hydration of lime involves the addition of water in a hydrator (CaO + H2O ® Ca(OH)2). The quantity of water added is about twice the stoichiometric amount required for the hydration reaction. The excess water is added to moderate the temperature generated by the heat of reaction by conversion to steam. The steam, which is laden with particulates, passes through abatement equipment prior to discharge to atmosphere.

There are many designs of equipment but technically the hydrator, see Figure 2.9, consists of pairs of contra-rotating screw paddles which vigorously agitate the lime in the presence of water. A strong exothermic reaction takes place generating 1140 kJ per kg CaO. The average residence time of the solids in the main reactor is about 15 minutes.

The heat release causes a vigorous boiling action which creates a partially fluidised bed. Dust is entrained in the steam evolved during the process. If this dust is collected in a wet scrubber a milk of lime suspension is produced, which is normally returned to the hydrator.

After hydration the product is transferred to an air-swept classifier where the coarse and fine fractions are separated using a recycling air stream. Some or all of the coarse fraction may be ground and recycled. The fine fraction is conveyed to storage silos. From here it is either discharged to bulk transport or transferred to a packing plant where it is packed in sacks or intermediate bulk containers.

Figure 2.9: Flowsheet of a 3-stage lime hydrator

[EuLA, (Pfeiffer AG, Germany)]

Production of milk of lime and lime putty

Milk of lime and lime putty is produced by slaking of lime with excess water. Slaking is done in both batch and continuous slakers. The term milk of lime is used to describe a fluid suspension of slaked lime in water. Milks of lime may contain up to 40% by weight of solids. Milk of lime with a high solids content is sometimes called lime slurry. Lime putty is a thick dispersion of slaked lime in water. Putties typically contain 55 to 70% by weight of solids. Lime paste is sometimes used to describe a semi-fluid putty.

2.2.7 Storage and handling

Storage

Storage of quicklime

Quicklime is preferably stored in dry conditions, free from draughts to limit air slaking. Great care is exercised to ensure that water is excluded from the lime, as hydration liberates heat and causes expansion, both of which could be dangerous.

Air pressure discharge vehicles are able to blow directly into the storage bunker, which is fitted with a filter to remove dust from the conveying air. The filter should be weatherproof and watertight. The collected dust can be discharged back into the bunker. A pressure/vacuum relief device fitted to the bunker is a precautionary measure.

All storage containers can be fitted with devices which can positively seal the base of the bunker to enable maintenance work to be done on the discharge mechanism.

Where the amount of quicklime is insufficient to justify storage bunkers the product may be stored on a concrete base, preferably in a separate bay within a building to prevent excessive air slaking.

Storage of hydrated lime

Hydrated lime absorbs carbon dioxide from the atmosphere, forming calcium carbonate and water. Therefore, it is best stored in dry draft-free conditions.

Hydrate bagged in paper sacks is preferably stored under cover to avoid deterioration by moisture, and re-carbonation of the hydrated lime. When "Big bags" are used, they are also best stored under cover to prevent any damage. Pallets of bagged hydrate have been stored successfully out-of-doors; the pallet covered by a plastic sheet, the bags placed on the sheet and the pack shrink-wrapped.

Bulk hydrate is stored in silos, which must be completely weatherproof. The silo is vented via a bag filter, which should be weatherproof and be capable of handling the delivered airflow. Where the filter is fitted on top of the silo, the collected dust is discharged back into the silo. The silo top can be fitted with an inspection manhole and a pressure relief valve. A high level indicator or alarm can be fitted to prevent over-filling. It is recommended that the base of the silo be at an angle of at least 60° to the horizontal, the discharge aperture not less than 200 mm and that a positive cut-off valve is fitted to the outlet to permit equipment beneath the silo to be maintained.

Because hydrated lime is prone to "arching", suitable arch-breaking devices, such as aeration pads, vibrators and mechanical devices, are fitted to prevent this. Conversely, precautions need to be taken to prevent "flooding" of aerated powder.

Storage of milk of lime

Many customers requiring addition of slaked lime to their process have found that milk of lime is a convenient form to store and handle. Providing certain precautions are taken, it can be handled as a liquid.

Any storage and handling system has to pay proper attention to the fact that when milk of lime is diluted with water, or when hydrated lime is dispersed in water, any carbonate hardness in the water will be deposited as calcium carbonate. Unless appropriate action is taken, this will result in scaling on the walls of pipes and on the impeller and casing of pumps. Two approaches can be adopted. Either the system can be designed to cope with scale formation, or action can be taken to prevent or minimise scaling.

It is important to prevent settling in milk of lime systems as the resulting putty can be difficult to re-disperse. Storage tanks should therefore be agitated. The degree of agitation can be low and should avoid forming a vortex, which would increase absorption of carbon dioxide from the atmosphere.

The discharge pipe from a storage vessel inevitably constitutes a dead-leg and provision can be made for back-flushing with water to remove any blockages. The storage area should be suitably bunded.

Handling

Many types of equipment are suitable for transferring the product and new ones are continually being developed. The following items have been used successfully, but may not be suitable for all applications.

Skip hoists can be used for all granular and lump grades but are more suitable for particles greater than 100 mm. Elevators - both belt-and-bucket and chain-and-bucket elevators have been used for all grades of quicklime. Drag-link conveyors are suitable for granular and fine quicklime. They are generally used for horizontal or inclined transfer. Conveyor belts are widely used for transferring lump and granular grades horizontally and on an upward slope. Screw conveyors are widely used for fine quicklime. Vibrating trough conveyors have been used for particle sizes up to 40 mm. They operate more successfully when there is a slight downward slope from the feed to the discharge point.

Pneumatic conveying can be used for products with a maximum size up to 20 mm and often has a lower capital cost than alternatives, but the operating costs are higher. The product is fed into a rotary blowing seal connected to a blower. The pipeline bore, and volume/pressure of the blowing air, is designed taking into account the size of lime being conveyed, the transfer rate and the length/route of the pipeline. The receiving silo is equipped with an air filter and a pressure relief valve.

2.2.8 Other types of lime

2.2.8.1 Production of calcined dolomite

Dolomite is calcined in both shaft and rotary kilns. Three qualities of calcined dolomite are produced - light-burned, dead-burned and half-burned.

Light-burned dolomite is generally produced in either rotary or shaft kilns. The principles of making light-burned dolomite are similar to those of making high calcium quicklime. Less heat is used owing to the lower heat of calcination and the lower dissociation temperature of dolomite (MgCO3).

Dead-burned dolomite is produced in two grades. A high purity grade, used for the manufacture of refractories, is made by calcining dolomite at temperatures of up to 1800 °C in either rotary or shaft kilns. A "fettling" grade is produced by the calcination of dolomite with 5 to 10% iron oxide at 1400 to 1600 °C, usually in a rotary kiln. The exhaust gases from both of these processes are at higher temperatures than from other lime kilns; they are generally cooled to below 420 oC using heat exchangers, tempering air or injection of atomised water.

Half-burned dolomite (CaCO3-MgO) is produced by the slow calcination of dolomite at about 650 °C. It is produced in relatively small quantities and Germany is the only country in Europe to manufacture it.

2.2.8.2 Production of hydraulic limes

Natural hydraulic limes are produced from siliceous or argillaceous limestones containing more or less silica, alumina and iron. Typical levels in the limestone are; SiO2: 4 to 16%, Al2O3: 1 to 8% and Fe2O3: 0.3 to 6%. The calcium plus magnesium carbonate content can range from 78 to 92%.

The limestone is generally calcined in shaft kilns which must be controlled closely to ensure that as much of the silica and alumina as possible reacts, without sintering the free lime. Typical calcining temperatures are 950-1250 °C: the required temperature rises as the cementation index increases (i.e. from feebly to eminently hydraulic limes).

The calcined lime is hydrated with sufficient water to convert the free CaO into Ca(OH)2. If the free CaO content is greater than 10 to 15%, the hard sintered lumps disintegrate into a powder. Otherwise, the lime must be ground before hydration. It may also be necessary to grind the hydrated product to achieve the required degree of fineness and setting rate.

"Special" natural hydraulic limes are produced by intimately blending powdered natural hydraulic limes with powdered pozzolanic or hydraulic materials. Artificial hydraulic limes are produced by intimately blending powdered hydrated limes with pulverised pozzolanic or hydraulic materials.

2.2.9 Captive lime kilns

2.2.9.1 Lime kilns in the Iron and steel industry

Most of the lime used in the iron and steel industry is for fluxing impurities in the basic oxygen furnace. Lime is also used in smaller quantities in the sinter strand process for the preparation of iron ore, in the desulphurisation of pig iron, as a fluxing agent in other oxygen steelmaking processes, in the electric arc steelmaking process and in many of the secondary steelmaking processes.

The lime kilns in the Iron and steel industry are mainly shaft kilns of different designs and capacities. They do not differ in consumption/emission patterns from non-captive lime kilns.

2.2.9.2 Lime kilns in the Kraft pulp industry

There are about 100 lime kilns in the European Paper industry. They are all rotary kilns with capacities between 30-400 tonnes of burned lime per day. Most of the kilns are long rotary kilns, but there are also some modern preheater rotary kilns.

The long rotary lime kilns are usually fed with a slurry of calcium carbonate with a water content of 30%. The basic fuel is normally natural gas or oil. In addition, non-condensable gases produced in several areas of the pulping process are usually burned, increasing the content of H2S, organic sulphur compounds and SO2 in the stack gases. In some cases sawdust and gases obtained by gasification of biomass are also used as fuel.

Venturi type wet scrubbers and electrostatic precipitators (for particular matter) are normally installed to clean the exhaust gases.

2.2.9.3 Lime kilns in the Sugar industry

Most of the lime kilns in the European Sugar industry are mixed feed shaft kilns. The majority of the kilns produce 50 to 350 tonnes of quicklime per day during the sugar campaign, which, in the 1997/1998 season, lasted between 63 and 170 days, with an average of 86 days.

The quicklime and the CO2 in the exhaust gas are both used in sugar factories. The gas produced by the kiln is captured and most of it is dedusted in a wet scrubber before use in the sugar process (carbonatation). Most of the CO2 recombines with the milk of lime in the limed juice to give CaCO3.

The most common fuel in sugar industry lime kilns is coke. This is mainly because the product gas contains more CO2 (40-42% CO2 by volume) than product gas from oil or gas fired kilns (28-32% CO2 by volume).

The consumption levels (limestone and fuel) for sugar industry lime kilns are about the same as for the same types of lime kiln in other sectors.

 

2.3 Present consumption/emission levels

The main environmental issues associated with lime production are air pollution and the use of energy. The lime burning process is the main source of emissions and is also the principal user of energy. The secondary processes of lime slaking and grinding can also be of significance, while subsidiary operations (namely crushing, screening, conveying, storage and discharge) are relatively minor in terms of both emissions and energy usage.

2.3.1 Consumption of limestone

Lime production generally uses between 1.4 and 2.2 tonnes of limestone per tonne of saleable quicklime. Consumption depends on the type of product, the purity of the limestone, the degree of calcination and the quantity of waste products (dust carried from the kiln in the exhaust gases, for example).

2.3.2 Use of energy

Calcining of limestone

Typical heat and electrical power use by various types of lime kiln are shown in Table 2.8. Energy use for a given kiln type also depends on the quality of the stone used and on the degree of conversion of calcium carbonate to calcium oxide.

The heat of dissociation of calcium limestone is 3200 MJ/tonne. The net heat use per tonne of quicklime varies considerably with kiln design. Rotary kilns generally require more heat than shaft kilns. The heat use tends to increase as the degree of burning increases.

The use of electricity varies from a low range of 5-15 kWh/tonne of lime for mixed-feed shaft kilns, to 20-40 kWh/tonne for the more advanced designs of shaft kiln and for rotary kilns.

Kiln type

heat use

(MJ/tonne lime)

kiln electricity use

(kWh/tonne lime)

Calcium quicklime, light- and hard-burned dolomite

 

 

Mixed feed shaft kiln

4000-4700

5-15

Double-inclined shaft kiln

4300

30

Multi-chamber shaft kiln

4000-4500

20-45

Annular shaft kiln

4000-4600

18-35

Parallel-flow regenerative shaft kiln

3600-4200

20-40

Other shaft kilns

4000-5000

10-15

Long rotary kiln a

6500-7500

18-25

Grate preheater rotary kilns a

5000-6100

35-100

Shaft preheater rotary kilns a

4800-6100

17-45

Cyclone preheater rotary kilns a

4600-5400

23-40

Travelling grate kiln

3700-4800

31-38

Gas suspension calcination

4600-5400

20-25

Fluidised bed kiln

4600-5400

20-25

Dead-burned dolomite

 

 

Mixed feed shaft kiln

6500-7000

20

Grate preheater rotary kiln

7200-10500

35-100

a) producing reactive calcium quicklime

Table 2.8: Typical heat and electricity use by various types of lime kiln

[EuLA], [UK IPC Note, 1996], [ Jørgensen]

Lime hydrating

The hydrating process is exothermic, so excess water is added to control the temperature in the hydrators. This excess water is converted into steam, which is discharged into the atmosphere, together with a small amount of air that is drawn into the hydrator to prevent moisture and dust from entering the plant/quicklime feed equipment, and to assist in the evaporation of excess water.

The energy requirements to operate the hydrators, air classifiers and conveying equipment amounts to approximately 5 to 30 kWh/tonne of quicklime.

Lime grinding

The energy use in lime grinding varies from 4 to 10 kWh/tonne of quicklime for the coarser grades (for example, those used for soil stabilisation) to 10 to 40 kWh/tonne of quicklime for the finer grades. The amount of energy required also depends on the equipment used. Fine impact mills can be used for the coarser products. Ball mills, ring-roll mills and high-pressure mills plus dis-agglomerators (with progressively lower specific energy use) are used for making finer products.

3a2 Nox

3a21 general

Shaft kilns generally emit less NOx than rotary kilns. This is because the temperatures in shaft kilns are usually below 1400 oC, so that the formation of thermal NOx (by reaction of nitrogen with oxygen) is comparatively lower. Additionally, the combustion processes usually produce relatively lower flame temperatures, and low-intensity mixing conditions, resulting in lower levels of fuel NOx. Where, however, shaft kilns are used to produce hard-burned calcium limes, or dead-burned dolomite, higher levels of NOx are produced.

In rotary kilns, the flame is better defined and flame temperatures are higher than in shaft kilns, which results in higher levels of fuel NOx. Moreover, because of the different heat transfer processes, the maximum temperature of the kiln gases is also higher, resulting in increased thermal NOx levels. The production of dead-burned dolomite in rotary kilns results in still higher NOx levels.

Typical emissions of NOx from various types of lime kiln are shown in Table 2.9.

Kiln type

mg NOx/Nm3 1

kg NOx/tonne lime 2

Calcium quicklime, light- and hard-burned dolomite

 

 

Mixed feed shaft kiln

< 300

< 1

Double-inclined shaft kiln

< 500

< 1.7

Multi-chamber shaft kiln

500-800

1.7-2.8

Annular shaft kiln

< 500

< 1.7

Parallel-flow regenerative shaft kiln

< 400

< 1.4

Other shaft kilns

< 300

< 1

Rotary kilns, soft burning

100-700

0.4-2.8

Rotary kilns, hard burning

400-1800

1.6-7

Travelling grate kiln

< 300

< 1

Dead-burned dolomite

 

 

Mixed feed shaft kiln

< 300

< 1

Rotary kilns

2000-5000

15-45

1) Emission concentrations are one year averages, and are indicative values based on various measurement techniques. O2 content normally 10%.

2) based on typical exhaust gas volumes (wet) of

Table 2.9: Typical emissions of NOx from some types of lime kiln

[EuLA]

SO2

  general

Typical emissions of SO2 from various types of lime kiln are shown in Table 2.10.

Kiln type

mg SO2/Nm3 1

kg SO2/tonne lime 2

Calcium quicklime, light- and hard-burned dolomite

 

 

Mixed feed shaft kiln

< 300

< 1

Double-inclined shaft kiln

< 500

< 1.7

Multi-chamber shaft kiln

< 500

< 1.7

Annular shaft kiln

< 300

< 1

Parallel-flow regenerative shaft kiln

< 300

< 1

Other shaft kilns

< 300

< 1

Rotary kilns, soft burning

< 800 3

< 3

Rotary kilns, hard burning

< 800 3

< 3

Travelling grate kiln

< 300

< 1

Dead-burned dolomite

 

 

Mixed feed shaft kiln

< 800

< 1.5

Rotary kilns

< 5000

< 42.5

1) Emission concentrations are one year averages, and are indicative values based on various measurement techniques. O2 content normally 10%.

2) Based on typical exhaust gas volumes (wet) of

3) May be higher with high-sulphur fuels.

Table 2.10: Typical emissions of SO2 from some types of lime kiln

[EuLA]

In the majority of lime burning operations most of the sulphur present in the limestone and the fuel is captured by the quicklime. In shaft kilns and fluidised bed kilns, the efficient contact between the kiln gases and the quicklime usually ensures efficient absorption of sulphur dioxide. This is also generally valid for rotary and other kilns, with packed-bed preheaters.

However, where low sulphur quicklime is produced in rotary kilns, and hard-burned calcium lime/dead-burned dolomite in either shaft or rotary kilns, part of the sulphur in the fuel and limestone is expelled as sulphur dioxide in the exhaust gases.

 Dust

  general

Calcining of limestone

Dust generation arises from finely divided particles in the limestone feed, from thermal and mechanical degradation of the lime and limestone within the kiln, and, to a lesser extent, from fuel ash. The levels of dust generation vary widely, depending on kiln design among other things, and range from 500 to over 5000 mg/Nm3, corresponding to approximately 2 to 20 kg/tonne of quicklime (based on 4000 Nm3/tonne lime). All rotary kilns are fitted with dust collection equipment, as are most shaft kilns.

Because of the wide range of exhaust gas conditions, a variety of dust collectors are used, including cyclones, wet scrubbers, fabric filters, electrostatic precipitators and gravel bed filters. After abatement, emissions typically range from 30 to 200 mg/Nm3, about 0.1 to 0.8 kg/tonne of quicklime (based on 4000 Nm3/tonne lime).

Lime hydrating

The gaseous effluent from hydrating plants is rather small in volume; levels are around 800 m3/tonne of hydrated lime, but it may contain 2 g/m3 of dust before abatement. Thus the generation of dust can be about 1.6 kg/tonne of hydrated lime. Both wet scrubbers and bag filters are used to de-dust the emission.

Emission levels after abatement range from 20 to over 200 mg/Nm3, corresponding to approximately 0.016 to 0.16 kg/tonne of hydrated lime.

Lime grinding

Air is drawn through all of the grinding equipment to remove ground lime of the required particle size. The product is separated from the air in bag filters, often preceded by cyclones. Thus, dust collection is an integral part of the process.

Emission levels typically range from 20 to 50 mg/Nm3, corresponding to 0.03 to 0.075 kg/tonne of lime (at a typical air flow of 1500 Nm3/tonne of lime).

Subsidiary operations

Subsidiary operations may include crushing, screening, conveying, slaking, storage and discharge. Dust emission is controlled by containment and, in many cases, by extracting air to keep the equipment under slight suction. The air is passed through bag filters and the collected dust is generally returned to the product.

Fugitive dust from, for example, stock piles of raw materials and solid fuels can cause problems.

CO2 general

The dissociation of limestone produces up to 0.75 tonne of carbon dioxide (CO2) per tonne of quicklime, depending on the composition of the limestone and the degree of calcination. The amount of carbon dioxide produced by combustion depends on the chemical composition of the fuel and on the heat use per tonne of quicklime, generally it is in the range 0.2 to 0.45 tonne CO2 per tonne of quicklime.

In recent years, the emission of carbon dioxide per tonne of quicklime in most countries has been reduced, mainly by replacing old kilns with more thermally efficient designs, and by increasing the productivity (reducing the amounts of waste dust). The German and French lime industries have entered into voluntary agreements to reduce CO2 emissions, and in the UK it has been calculated that the emission of CO2 per tonne of lime decreased by approximately 20% in the 15 years up to 1994.

 

Other Pollutants

3a5  CO

When resulting from incomplete combustion, carbon monoxide (CO) emissions generally represent a loss of efficiency. However, in some types of kiln, and when making certain products, controlled levels of carbon monoxide are necessary to produce the required combustion conditions and product quality.

Some limestones contain carbon, which can lead to higher CO emissions from the lime burning process.

Typical emissions of CO from various types of lime kiln are shown in Table 2.11.

Kiln type

g CO/Nm3 1

kg CO/tonne lime 2

Calcium quicklime, light- and hard-burned dolomite

 

 

Mixed feed shaft kiln

12-37

42-130

Double-inclined shaft kiln

< 1.4

< 5

Multi-chamber shaft kiln

< 1.4

< 5

Annular shaft kiln

< 1.4

< 5

Parallel-flow regenerative shaft kiln

< 1.4

< 5

Other shaft kilns

< 14

< 50

Rotary kilns, soft burning

1.2-12

5-50

Rotary kilns, hard burning

1.2-12

5-50

Travelling grate kiln

< 1.3

< 4

Dead-burned dolomite

 

 

Mixed feed shaft kiln

37-63

70-120

Rotary kilns

0.6-6

5-50

1) Emission concentrations are one year averages, and are indicative values based on various measurement techniques. O2 content normally 10%.

2) Based on typical exhaust gas volumes (wet) of

Table 2.11: Typical emissions of CO from some types of lime kiln

[EuLA]3a6

Volatile organic compounds

Emissions of volatile organic compounds (VOCs) may occur for short periods during start-up, or upset conditions. Such events can occur with varying frequencies: between once or twice per year for rotary kilns to once per 1 to 10 years for shaft kilns. In a very limited number of cases where the limestone contains a significant amount of organic matter, volatile organic compounds can be emitted continuously.

Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)

Raw materials or fuels that contain chlorides may potentially cause the formation of polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) in the heat (combustion) process of the lime kiln. Data reported in the document "Identification of Relevant Industrial Sources of Dioxins and Furans in Europe" indicate that lime production plants are of minor significance for the total PCDD/F emissions in Europe. [Materialien, 1997] Measurements collected by EuLA from 7 kilns, of which 4 are rotary kilns and 3 are shaft kilns, show dioxin levels below 0.1 ng TCDD-equivalents/Nm3. Measurements at 2 annular shaft kilns in Germany were all below 0.05 ng TE/m3 [ LAI, 1994] .

However, the scarcity of measurements means it can not be ruled out that individual plants may be found in Europe with a local impact [Materialien, 1997]. Significant levels of dioxins have been measured at 3 kilns, 2 rotary kilns and 1 shaft kiln, in Sweden. The measurements were made between 1989 and 1993 and the measured levels were between 4.1 and 42 ng/TCDD-equivalents (Nordic)/Nm3. All measurements of high dioxin levels have been explained either by the raw material and/or fuel content, or the less than optimum burning conditions, underlining the importance of controlling the kiln inputs and maintaining a stable kiln operation. Two of these plants use limestone with a natural content of tar, also causing high emissions of VOCs. One measurement of 12.1 ng/m3 was made at a rotary kiln after a change of fuel from coal to oil was carried out in a much shorter period of time than normal. The measurement of 42 ng/m3 was made at a rotary kiln during a full scale trial with waste oil as fuel. Because of the high dioxin value the kiln was not permitted to use this waste fuel. [ Branschrapport, 1994] [ Junker]

Metals

Little data is available concerning metal emissions. The high purity of most limestones used for the production of calcium and dolomitic limes means metal emissions are normally low. Measurements at different types of lime kilns, collected by EuLA, show levels of cadmium, mercury and thallium well below 0.1 mg/Nm3.

2.3.4 Waste

Early designs of shaft kilns often produce two types of inferior products; an impure fine fraction (possibly mixed with fuel ash) and a fraction consisting of under-calcined lumps.

Modern kilns make very little out-of-specification product. If such product occurs, it consists principally of dust collected from the exhaust gases, and typically amounts to 0-5% of the total, depending on the characteristics of the feedstone and the quicklime. Small quantities of partially calcined material are produced when the kiln is started-up from cold, and during shut down. Such events may occur at frequencies ranging from once per 6 months to once per 10 years.

Some hydrating plants improve the quality of hydrated lime by removing an inferior grade, consisting of a coarse, carbonate-rich fraction. These inferior grades of material are incorporated into selected products, wherever possible. Otherwise, they are disposed to landfill.

2.3.5 Noise

Charging lumps of limestone into lime kilns can result in noise at levels which require abatement. Chutes etc. may be lined internally, or externally with resilient material. Fans used to exhaust gases from the kiln and positive displacement blowers, which are sometimes used to supply combustion air, can produce pure tones which require silencing. Outlet silencers and lagging of ducting are used to achieve the required reduction.