The lime cycle Tabitha L

(L6)

Chalk is correspondingly known as calcium carbonate, with a molecular formula of CaCO3, which is a form of limestone. It is part of the lime cycle, the process in which limestone is heated to produce quicklime, then can be hydrated to slaked lime and can be naturally converted back, by carbonation, to limestone. The cycle is one of the oldest chemical processes used on large scales and subsequently, lime products have many important uses in many industries, especially in the construction of materials. Conversely, lime products are not commonly directly sold to consumers but a study by EuLA shows that, on average, an EU citizen indirectly consumes around 150g of lime per day1. This exemplifies the wide range of lime consumption, but what opportunities and effects does it offer to a multitude of industries, contributing to the economy?

The lime cycle is composed of three main reactions, starting with limestone, which contains calcium which is the fifth most abundant element in the earth’s crust, and limestone forms about 20% of the world’s sedimentary rock. When burnt in a kiln at 900c, a thermal decomposition reaction occurs producing calcium oxide, CaO, also known as quicklime and producing carbon dioxide. The temperature at which limestone is burnt will affect its reactivity in the other stages of the cycle and the quality often depends on a multitude of factors including physical properties, reactivity to water and chemical composition. The quicklime produced can be highly beneficial due to its high reactivity and versatility, as quicklime can be modified to meet specific requirements for various applications like high surface area and lower reactivity, therefore making it a very useful product for many industries.

Quicklime can readily react with water, producing an exothermic reaction to produce slated lime, which is calcium hydroxide, Ca(OH)2 (s). It is estimated that about 10% to 15% of quicklime is converted into slated lime in developed countries, and comparatively, for countries which do not have a large steel industry, the estimated percentage is larger2. Here an excess of water can be added to slated lime to produce limewater, Ca(OH)2 (aq), which is used to test for the presence of carbon dioxide in experiments through carbonation. Slated lime can be carbonated by being exposed to the air for long periods of time, it will absorb water-soluble carbon dioxide from the air producing calcium carbonate (a white solid) and water, returning to the start of the cycle.

As demonstrated above, the lime cycle produces three main products of limestone, quicklime, and slated lime which have various applications for numerous industries: including construction, environment, agriculture, and manufacturing. The earliest use of lime dates to roughly 10,000 years ago. Lime products are a vital part of the construction industry, with lime products being used as a filler and bonding agent in building materials, for example in lightweight materials and materials with high thermal insulation capabilities like autoclaved aerated concrete or sand-lime mortar which were being used in Roman times. Here the re-carbonation of lime-based products can act as a benefit, for when it carbonates in lime mortar, it increases the strength of the final bond and acts as a self-healing solution if any cracking occurs. It is also a useful tool for the restoration of historical buildings, and thus preserving cultural heritage, but it is also used in modern buildings due to its beneficial properties like breathability and flexibility.

Furthermore, lime products are an essential material for the manufacturing industry, being especially fundamental to steel manufacturing: lime is used to remove impurities (a flux to remove phosphorus, sulphur, silica, and manganese) giving steel the correct chemical composition. While it is also used in plastic production, to remove water from material and rubber before setting, and used for fillers and coatings for paper. Additionally, both lime and limestone are essential components in glass manufacturing, as calcium is used as a stabiliser, in order to improve quality and appearance and quicklime is used to achieve higher efficiencies and outputs. Likewise, lime products play an important role in agricultural industries, lime can be added to soils to adjust PH; for when the soil is acidic, lime products are alkali so neutralise the soil and improve fertility. Similarly, lime products are used in soil treatments at civil engineering projects, as it improves the stability and durability of the clay in the soil. Moreover, for agricultural uses, lime products are used for animal hygiene, as used to sanitise farm environments, therefore, preventing outbreaks and the spreading of diseases. Another vital and essential use of lime is in environmental remediation processes like wastewater treatments, as it regulates PH levels and removes impurities from the water, improving water quality, in addition to its use in flue gas desulfurization, to improve air quality, having a considerable influence on our environment.

Complementary to this, as demonstrated, lime has many applications which have an effect on numerous industries, but how does this all contribute to the global economy? In 2018, lime was the world’s 578th most traded product, with a total trade of $1.1 Billion3. The production of lime increased strongly from 2008 to 2014, prompted by recovery after the economic downturn, but recently has been stable, with China being the largest producing country in 2021 with a production volume of 310 million metric tonnes4. Nevertheless, lime only represents 0.0057% of the total world trade, due to the fact that lime is readily available in all parts of the world and transport costs are a large portion of the production costs. Therefore, despite large producers of lime, like the Lhoist group and Carmeuse Group, local producers with large capacities also play an important role in local markets5.

Conversely, despite the environmental benefits lime can provide, for instance in treating water, lime production, especially in thermal decomposition from calcium carbonate to calcium oxide, contributes a substantial amount to greenhouse gasses. Due to the high temperatures required in the kiln, fossil fuels are burnt to achieve this, causing the combustion of fossil fuels leading to vast carbon dioxide emissions, and additionally, carbon dioxide is given off in this reaction as a product, with on average 0.785 tonnes of carbon dioxide per tonne of lime, having a harmful impact on the environment. In comparison though, lime is seen as a positive advantage compared to cement, considering both products give off carbon dioxide during manufacturing, however, lime reabsorbs carbon dioxide when it sets, being re-carbonated, while cement does not. Furthermore, limestone is burnt at around 900c compared to 1300c for cement saving on fuel. Therefore, it can be concluded that carbon dioxide emissions in the manufacturing of lime are a better alternative than cement. In addition, lime’s unique properties enable other industries to reduce their carbon footprint, as demonstrated by hydrated lime in asphalt which makes transport more energy efficient.

In summary, despite some of the negative effects of carbon dioxide emissions in the lime cycle, the multitude of positive benefits and uses in many industrial sectors can be argued to outweigh the negatives. Due to its unique and versatile characteristics, it has a large influence on our society and is therefore seen as an imperative and often inimitable component for many industries, contributing to the economy overall.

Notes:

1 EuLA,‘ A Competitive and Efficient Lime Industry’, published July 2014: https://www.eula.eu/wp-content/uploads/2019/02/A-Competitive-and-Efficient-Lime-Industry-Technical-report-by-Ecofys_0.pdf

2 British Lime Association, ‘Lime Cycle’ : https://britishlime.org/education/lime_cycle.php

3 The Observatory of Economic Complexity, ‘Lime’: https://oec.world/en/profile/sitc/lime

4 Statista, ‘Lime production by country in 2021’: https://www.statista.com/statistics/657049/production-of-lime-worldwide/

5 S&P Global Commodity Insights, ‘Lime/Limestone’, Published march 2019: https://www.spglobal.com/commodityinsights/en/ci/products/lime-chemical-economics-handbook.html