Technology

Historical limestone quarries or limestone mining sites

The local limestone dates back more than 400 million years, to the warm and tropical climate that prevailed during the Silurian period; at that time, the island of Saaremaa shared the same latitude as what is now Australia. Therefore, our limestone has travelled across the equator and reached the northern hemisphere. Limestone can be found inside the earth as strata, deposited on each other. All the strata are characterised by different thickness and properties. Not all the layers are suitable for manufacturing lime and the raw material requires proper sorting to give us high-quality lime. Two main types of lime are manufactured in the world: quicklime, which is manufactured using pure limestone, and hydrated lime, which is made from limestone that contains clay. Quicklime is mainly manufactured in this specific region; however, hydrated lime has also been manufactured, although in limited quantities. Quicklime petrifies when left in contact with air as lime reacts with the carbon dioxide present in the air, and hydraulic lime or natural cement hardens, like cement, when water is involved in the process.

Mining
The first stage of lime production involves the extraction or mining of limestone from quarries or mines. The process starts with the removal of soil from limestone, followed by breaking limestone into smaller lumps. However, the very first kilns were probably used to burn pieces of limestone that were collected from fields. Wedges, crowbars and stone hammers were used to break and extract stones from mines. Wedges are first struck between two strata of limestone to edge them upwards and then smashed into lumps that are suitable for burning using large hammers. The presence of cracks and micro-cracks in the stone is observed for the purposes of splitting limestone as this helps the extraction process considerably. Limestone lumps of a suitable size are then graded according to their quality and burnt separately where appropriate, whereas strata not suitable for lime burning processes will be graded and used to re-cultivate the quarry. Early spring is the best time for limestone extraction – the stress caused by the cold is still present in limestone strata and this makes the breaking and extraction process easier.

The limekiln and preparing

Usually, two separated chambers can be found inside the kiln – a burning chamber and a stone chamber. The burning chamber or furnace is the place where firewood is fed during the burning process. It is separated from the stone chamber by sidewalls made of limestone and on top by an arch built of large pieces of limestone; therefore, the stone chamber is placed on top and along the sides of the burning chamber (see the drawing). After limestone is crushed, the kiln is prepared for the burning process. First of all, square stones of equal size, as smooth as possible, have to be collected to build the wall and burning chamber that will be later used to burn the wood. The burning chamber will be made of limestone, extracted from a special layer (so-called burning chamber layer), the lumps being as flat and rectangular as possible. The burning chamber occupies approximately 2/3 of the width of the kiln and its height, depending on the size of the kiln, is approximately 80-150 cm. Smaller and bigger openings are left in the walls of the burning chamber for a draft, which ensures a high-quality lime burning process (in the depicted kiln, the openings can be seen in the rear wall of the kiln). This kiln allows workers to observe the burning chamber, which is built without using vault stones.
Building the upper part or arches of the burning chamber or furnace is the most important stage of the building process – the arch must be strong enough to take the weight of the stones placed in the kiln without collapsing. The arch stones could never break or collapse as otherwise the lime burning process would fail, either in part or completely. Apart from taking the weight of the stones, the arches played another important role – the correct choice and fitting of the arch stones had to ensure uniform distribution of the flame all over the kiln. The arch stones are located in the spot where the heat is the strongest and applied for the longest time; therefore, the stones that are used must be absolutely fireproof and are usually picked from the same strata, called the arch stone stratum. There are different methods available for fitting the arch stones; however, in the region concerned, it was common to place larger stones edgewise and crosswise across the burning chamber, wedged into place with smaller or wedge stones to ensure a well-built, sturdy structure.
Once the arch stones are properly fitted, the filling of the kiln begins; the process involves loading limestone lumps of suitable size onto the arch – until the kiln is full. Depending on the size, the capacity of one kiln is between 10 and 40 tons of limestone. Limestone lumps must not piled into the kiln in a regular order; instead, the stack has to be irregular to leave enough space between the lumps – such a pattern ensures a good, constant draught inside the kiln and, therefore, uniform ‘completion’ of the stones. Sometimes, vertical wooden rods were placed between the stones; the rods burned up and created ‘flues’ or ‘draught ducts’ between the stones.

Burning

Once the kiln was full, limestone burning started. It usually took 3-4 days to burn a load of limestone; the kiln had to remain burning all the time and every hour, approximately 1m3 of well dried, 2m pieces of firewood were added to the kiln. The firemen worked shifts – in the past, for 12 hours and today – for 8 hours. The temperature inside the kiln remains between 1000 to 1300 degrees Celsius. During the burning process, CO2 is emitted from limestone and the following formula can be used to describe the process:
CaCO3==>CaO+CO2 (CaCO3 – limestone, CaO – quicklime)
The surface of well-burnt limestone acquires a greenish-yellowish tinge and it will lose about 30% of its weight; however, the shape of lumps does not change. The volume of burnt limestone inside the kiln somewhat decreases and this will result in decreased burning intensity inside the kiln, which is one of the indicators to show that the burning process is about to end. Burnt limestone or quicklime is a product that is characterised by extremely high chemical activity and, when in contact with water, it will immediately start to hydrate and will rapidly become a powder or a paste.
The heating of the AS Limex lime kiln usually begins early Wednesday morning and finishes late on Friday night. The kiln is allowed to cool on Saturday and Sunday and is then emptied on Monday. Burnt limestone is extracted from the kiln using a special crate that is filled manually and lifted out of the kiln by a crane, loaded onto a truck and then delivered to the hydration site. The burnt stone must be protected from moisture as quicklime or burnt lime is loaded onto the truck, as otherwise the hydration (slaking) process will start and loading becomes height complicated and even hazardous for health. Burnt limestone or quicklime is chemically very active and the hydration or slaking process can be triggered by air humidity. In the past, the lime was usually hydrated at construction sites but these days lime is hydrated immediately after burning, using a special bath, and left to mature in pits dug into the ground. In farms, the lime-pit was usually located behind the sauna or granary and rainwater was mostly used to hydrate the lime.

Slaking
To hydrate lime, quicklime (burnt limestone) and water are mixed, followed by a chemical reaction, described by the following formula:
CaO+H2O ==> Ca (OH)2
The chemical compound obtained is called calcium hydroxide or slaked/hydrated lime. Lime is hydrated in a special bathtub, filled with 200-300 kg of quicklime and about 2.5 tons of water, followed by 15-20 minute reaction or hydration. The solution will then be mixed to allow maximum contact between the burnt stone and water. As a result of the reaction, the burnt limestone will “melt” or decompose and dissolve in water, and we obtain white, liquid “lime cream”, which is then sieved and charged into pits in the ground, also known as lime pits. In pits like that, excess water will be drained into soil in a couple of days or a week and we are left with a mass resembling toothpaste, containing about 50% water. Ex post hydration and maturing of lime takes place in these pits; this may take anywhere from 3 months to dozens of years. The longer the lime is allowed to mature, the smaller the lime particles are and the quality of lime improves.

We market lime as lime paste that has matured in the ground; lime used for paint has to remain in the ground for at least 3 years, while a maturing period of at least 6 months is required for lime that is used for white-washing. It is important to keep lime in the form of a paste that contains water, as lime will start to carbonise when it reacts with carbon dioxide (CO2) present in the air and will lose it properties as a binding substance. Water between particles of lime will prevent contact between lime and air, which will maintain the quality of the lime. Lime has many different applications in today’s world: sugar industry, metal industry, paper industry and, of course, the construction sector. In the construction sector, lime is mostly used to manufacture whitewash, grouts and plasters. Lime is a binder in the products described and a chemical process takes place when lime is used that involves hydrated lime or calcium hydroxide reacting with carbon dioxide, present in the air, forming calcium carbonate which is identical to limestone in its chemical composition. The process is described by the following formula:
Ca(OH)2 + CO2 = CaCO3 + H2O
Therefore, the use of lime as a building material involves a closed process or the end product is a compound, which is, by chemical composition, identical to the component that started the process or calcium carbonate. Figuratively speaking – the whitewash you see on the walls of houses is chemically identical to the limestone you find in the ground.
The lime that we produce is mostly used as a building material, as a binder in different plaster and grout products and to produce whitewash. As we use wood as environmentally sound and traditional material to heat our kiln, the lime is also highly valued for its limited content of various additives. In lime-based grout, which is traditionally used in Estonia, sand and lime paste are mixed at 3:1 ratio while the common whitewash is obtained by mixing water and lime paste at 2:1 to 5:1 ratio, depending on the treated surface and the whitewash/paint layer. We use natural earth pigments to tint the whitewash within our own production system. Lime from Lümanda has been used at many different sites in Estonia, Finland, Sweden, Latvia and Lithuania. Our lime has been used as a grout or whitewash to restore many medieval churches. The most well known objects are the building of the Riigikogu (the Parliament) in Tallinn, main building of Tartu University and Hugo Treffner Gymnasium in Tartu, Alatskivi Manor in Tartu county, Gediminas Castle in Lithuania and Helsinki Dome Church and Helsinki Customs School.