Editing Calcium carbonate (section)

==Calcination equilibrium==
[[Calcination]] of [[limestone]] using [[charcoal]] fires to produce [[calcium oxide|quicklime]] has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825&nbsp;°C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and [[carbon dioxide]] at any temperature. At each temperature there is a [[partial pressure]] of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium {{chem2|CO2}} pressure is only a tiny fraction of the partial {{chem2|CO2}} pressure in air, which is about 0.035&nbsp;kPa.

At temperatures above 550&nbsp;°C the equilibrium {{chem2|CO2}} pressure begins to exceed the {{chem2|CO2}} pressure in air. So above 550&nbsp;°C, calcium carbonate begins to outgas {{chem2|CO2}} into air. However, in a charcoal fired kiln, the concentration of {{chem2|CO2}} will be much higher than it is in air. Indeed, if all the [[oxygen]] in the kiln is consumed in the fire, then the partial pressure of {{chem2|CO2}} in the kiln can be as high as 20&nbsp;kPa.<ref name="solvaypcc2007">{{cite web|title = Solvay Precipitated Calcium Carbonate: Production|publisher = Solvay|date = 9 March 2007|access-date = 30 December 2007|url = http://www.solvaypcc.com/safety_environment/0,0,1000044-_EN,00.html|archive-date = 19 October 2007|archive-url = https://web.archive.org/web/20071019031826/http://www.solvaypcc.com/safety_environment/0,0,1000044-_EN,00.html|url-status = live}}</ref>

The table shows that this partial pressure is not achieved until the temperature is nearly 800&nbsp;°C. For the outgassing of {{chem2|CO2}} from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of {{chem2|CO2}}. And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101&nbsp;kPa, which happens at 898&nbsp;°C.{{clear right}}

:{| class="wikitable"
|+ Equilibrium pressure of {{chem2|CO2}} over {{chem2|CaCO3}} (''P'') versus temperature (''T'').<ref name=crc>{{RubberBible86th}}</ref>
|-
!''P'' (kPa)
|0.055||0.13||0.31||1.80||5.9||9.3||14||24||34||51||72 ||80||91||101||179||901||3961
|-
!''T'' (°C)
|550||587||605||680||727||748||777||800||830||852||871||881||891||898||937||1082||1241
|}