The production of lime from Limestone
The process of the production of quicklime , also identified as calcium oxide is achieved through the
decomposition of naturally occuring limestone products (calcium carbonate, CaCO3) , to produce quicklime
(CaO) and carbon dioxide (CO2).The process of the production of lime from limestone was a method used
by civilisations such as Egyptians when building the pyramid of Cheros, the Greek civilisation when painting
murals and the Chinese Dynasty who used Lime to stabilise soil in order to build the great wall of
china.There is no exact inventor or invention date but the earliest noted use of lime was around 14,000 BC in
the Lascaux caves in France.If the conditions of the process of producing lime from limestone such
temperature, concentration and pressure are monitored then the maximum yield of quicklime will be
achieved.
CaCO3(s) → CaO(s) + CO2(g)
Process / Steps
The production of lime is typically comprised of four different steps.As limestone occurs naturally as it is a
sedimentary rock , typically the first step is to collect the Limestone.This is done through underground mines
and quarries. Secondly, as it comes naturally it be will mixed with other impurities as it is found in
substances such as chalk and marble therefore it must go
through steps of screen preparation where the limestone is
chemically washed and sieved.This Brings the limestone to
near purity and to the next step, calcination.Limestone
calcination is the process of heating pure limestone at
intensely high temperature (900 Degrees C +, usually
towards the 1300’s Degrees C) in a large rotary or vertical
shaft kilns to produce the product of Calcium oxide (Quicklime) and Carbon dioxide (CO2) as seen in figure
(1.1).A optional process would be to hydrate the lime which would create slaked lime which can be used for
mortar, plaster and cement for construction.
Lime Production: Le Chatelier’s Principle and the Factors that cause a shift in
Equilibrium
Le chatelier's principle states that if disturbance (Temperature, pressure, concentration
and catalyst ) is applied to a system the equilibrium will shift to counteract this
disturbance.
Concentration Kc = [CO2]1
The concentration of solids remains constant (Kp = pCO2 atm or Pa), meaning that the equilibrium is solely
impacted by the change of carbon dioxide as seen in
(figure (1.2)) (lime production is carbon intensive) or heat
(typically, 900 oC or 1000oC in a lime kiln). Furthermore,
according to the collision theory, when increasing the
concentration of reactants (calcium carbonate), there are
more molecules likely to collide, forming a reaction.
Temperature:
Since the forward reaction is endothermic, 178 kJ of heat energy as seen in (figure (1.3)) (enthalpy value )
is absorbed for every mole of calcium oxide formed. Therefore, it is evident that CaO formation is favoured
by high temperature as the reaction fails unless temp is at least 900 oC (at 1000oC in a limekiln the
equilibrium position is more likely to favour the forward reaction). Hence, Kp will increase with increase
in temperature. Furthermore, as the temperature is intensely high, the activation needed to decompose
calcium carbonate into carbon dioxide and lime is low as the kinetic energy is being increased (heating up
the system), overcoming the high activation energy and shifting the equilibrium to the right hand side of the
equation. This is because as stated by LCP, an increase in temperature is counteracted by the movement of
the equilibrium to the right (forward reaction will absorb the heat) to minimise the increase in temperature.
It is vital to note that increasing the temperature does not make increase the speed of the reaction but also
its direction (to the right).The fact that the entropy of the chemical process is 168.18 J/K, the process of
Lime production is increasing in entropy and therefore more randomness.Hence this chemical process
favours the forward reaction at T > 1173 K.
Pressure
In a ventilated lime kiln as seen in (figure (1.5)) at normal atmospheric pressure( 1 atm) , the pressure of
carbon dioxide is decreased and hence, the chances of a reverse reaction. Decreasing the pressure will favor
the increase in the moles of gas in lime production and this leads to the formation of more CO2 and
ultimately more CaO. Furthermore, according to the collision theory, this will decrease the likelihood of
collisions occurring between carbon dioxide and calcium
oxide, reversing the reaction to form limestone. One mole
of gas is formed in the process. Since there are no gaseous
reactants, there is a net increase in the moles of gas,
decreasing the pressure and favouring the production of more carbon dioxide molecules. As seen in (figure
(1.4))
This will encourage a shift in
equilibrium to the right hand side
(products), increasing the yield of
lime.Due to an increase in gaseous
particles this chemical process is
increasing in entropy as any chemical
equation with a increase in gas
particles will be have an increase in entropy.This is seen through the equation where there are no gases on
the reactants side and when the reaction occurs there is an increase in gaseous particles, in this case being
CO2.
Partial Pressure
In this reaction, there is a decrease in the partial pressure of carbon dioxide and thus, there is a decrease in
the likelihood of the system shifting to the reverse reaction. As the kiln is ventilated (not a closed system) to
allow the escape of air and carbon dioxide, a true equilibrium is never established because and the system
favours the forward reaction.
The equilibrium quotient (Q<K)
For this specific reaction, the equilibrium constant is substantially bigger than 1 (K>1), specifically
K= 1.7568809808x1023 . . This can aid in predicting the equilibrium shift, in this case it will be
favour the right hand side (the forward reaction), producing more lime and carbon dioxide. This is
because the ratio of products to reactants is less than when the system is at equilibrium, and since
the products are lower, to reach equilibrium, the system will favor the forward reaction. This
occurs in order to to use up the excess reactant to make more products.
Removing a product
An example of the yield obtained can be seen in scienceviews.com’s explanation of the yield of lime.It
states that at optimal temperature (1010-1345 Degrees C) the yield receive from 100 kg of limestone would
be 56 kg of quicklime and 44 kg of carbon dioxide.Therefore the percentage yield obtained from limestone
would be split 56 (CaO) : 44 (CO2).
Essentially, to maximise the production and yield of lime, it is essential that the reaction is manipulated and
continuously monitored to favour the forward reaction, and avoid the reverse. Furthermore, if the industry is
able to monitor the shift in equilibrium by flushing carbon dioxide from the mixture as it is released from the
lime kiln. Ultimately, this drives the reaction to the right hand side, as elaborated by the figure above.As seen
in ((figure 1.6))
The effects of monitoring the Process of the production of Lime from Limestone
Positives
Negatives
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In terms of Calcium Oxide only, when
monitored correctly there are few hazards to the
environment.
Lime has a variety of uses, this is a huge
positive as it can be used to boosted the
economy by creating more products.Products
such as steel are created, 45% of lime is used in
the steel industry. 80 kg of lime is used to
produce every 1000 metric tons of iron through
the process of removing Slag from
Iron.Aluminum can be created through the use
of Lime as lime removes silicates from
alumina.Lime also creates silicates that can be
used to create glass which is so important to
today's economy.
The abundance of limestone allows for great
economical gain as it can produced without fear
of being a limiting reagent.
Quarrying and mining boosts the economy
immensely as it provides high paying jobs and
raw materials.
Providing jobs around the area of the quarry is a
huge positive.
Over 22 million tons of limestone is produced in
the US every year, therefore we can see the
importance to the economy.
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Acidity levels of 12 pH are dropped down to 7.5
- 8.5 pH in the case calcium oxide .This is
negative to the environment as a decrease in pH
can lead to more pollution in the environment.
The process of the production of lime from
limestone is immensely negative to the
environment as the production CO2 must occur
in order for the process to occur.Data provided
by ScienceView.com outlines that 44% of
Carbon Dioxide must be produced for 56% of
limestone.Also, in order for the reaction to
produce more lime the carbon dioxide must be
let out following le chatelier's principle.The
effect this has on the environment ranges from
Air-Pollution to Climate change.This issue of
Carbon dioxide emission is the most negative
impact on the Earth's environment.
Limestone mines and quarries are extremely
negative to the environment due to the
environment being permanently damaged due to
the mining that occurs.This damage to the
environment destroys animal habitats and
homes.Also the noise created denies the chance
of birds and other wildlife to inhabit the area.
Socially the negative effects of Lime production
is that the noise that quarries and mining areas
collecting limestone create heavy amounts of
noise that damages people's quality of life.
Cement factories create huge amounts of dust
that affects people's breathing and damages their
quality of life.This is again a negative social
impact.
Many injuries have occured in the workplace for
Lime production due to the fact that the lime
Kilns are huge and contain heavy
equipment.This is a huge social effect and
ethical effect.
The process of the production of lime from limestone is positive to an extent in the criteria of the economy, society and
the environment.The Carbon dioxide emissions of this process is immensely negative due to the impacts result in air
pollution and climate change . The production limestone creates 44% of carbon dioxide that must be let out in order for
process to occur correctly.To add to this quarries and mines are extremely destructing to the environment not only due
to the fact that they damage the area they are based in but also due to being extremely noisy.This in turn does not allow
for animals, birds especially to live in that area. Also, the mines and quarries loud noise affects the quality of life for
those who live around the area, this is a huge social negative.Due to the heavy machinery used in the collection of
limestone and the production of lime man injuries and deaths occur.This is a huge social negative as injuries affect the
quality of life of the injury and also the deaths are a massive ethical issue.However the Calcium oxide itself is not
harmful when maintained correctly.Adding to this Calcium oxide is a huge benefit to the economy as it allows for
products such as glass, steel and silicon to be created.Another huge positive is the abundance of the Limestone that is
used in the process.Finally, the fact that the production of lime creates many jobs due to people in the area of the mines
and quarries are able to work is a huge positive economically and socially.