In order to improve the reaction an extra step can be introduced in which the reactive compound is extracted from the matrix. The reactive compound can relatively easily be carbonated in a separate step. Many indirect carbonation routes using various minerals have been proposed.
HCl extraction route
The process was originally developed during World War II as an alternative route producing magnesium. First, the magnesium is extracted from the mineral with the help of HCl. The use of an excess HCl results in an acid solution in which the magnesium dissolves.
The HCl is recovered by heating the solution from 100 to ±250°C. During this temperature increase, the MgCl2•6H2O first loses its associated water, resulting in MgCl2•H2O and, finally, HCl separates instead of further water release.Overall, the process step can be given as:
Process flow diagram based on Newall et al.,1999
In order to recover successfully the HCl, the formation of soluble chloride has to be avoided. Alkali metal chlorides are soluble and alkali metals should therefore be absent in the feedstock. The serpentine feedstock should preferably contain far less than 1wt% alkali metals.
Because solid wastes in general contain more alkali metals, this route seems not to be useful for alkaline solid waste mineral sequestration. The loss of hydrochloric acid would be too great. Another undesirable side-reaction could be the extraction of iron from the feedstock that is present in significant amounts in serpentine.
In the process, iron is extracted when HCl is used, but precipitates simultaneously with the silica when the pH of the solution is raised. The thermodynamics of the process have been studied in detail by Wendt et al. (Wendt et al., 1998a). The third step of the process in which Mg(OH)2 is formed has a positive Gibbs energy change and therefore the energy consumption of the process is considerable
Molten salt process
A first approach towards lowering energy consumption is the use of a molten salt
(MgCl2•3.5H2O) as an alternative extraction agent. The salt is recycled within the process. There are two options. In the first process, Mg(OH)2 is produced and carbonated separately. In the second process, the steps are integrated into one step.
Option A: MgCl2•3.5H2O is used as solvent to produce Mg(OH)2. First, the serpentine is dissolved in the molten salt (T=±200°C).Then the silica is precipitated (T=±150°C), water is added and Mg(OH)2 precipitates. The MgCl2 is partially dehydrated in order to recover the solvent (T=±110-250°C). The magnesium hydroxide is separated and carbonated.
Mg(OH)2 (s) + CO2 (g) MgCO3 (s) + H2O (g)
Option B: The carbonation takes place directly in MgCl2•3.5H2O(l). The overall reaction is:
Mg3Si2O5(OH)4 (s) + 3CO2 (g) Τ 3MgCO3 (s) + 2SiO2 (s) + 2H2O (l)
The CO2 pressure is about 30 bar. Important drawback of this route is the corrosive nature of the solvent. This causes construction and operational difficulties. Furthermore, in spite of recycling, make-up MgCl2•3.5H2O is needed. Based on a detailed assessment, Newall et al. concluded that a commercial supply of MgCl2 of this scale is probably unrealistic and if at all possible, unaffordable.
Route via calcium hydroxide from calcium-rich silicate rock
The same principle as used in the HCl extraction route can be used with CaSiO3 as feedstock (Haywood et al., 2001). The process remains principally the same. First, calcium is extracted from the wollastonite rock and CaCl2 is formed. The CaCl2 is converted to Ca(OH)2 by precipitating Ca(OH)2, being less soluble than CaCl2, and separating the HCl by heating the solution. The solid calcium hydroxide is carbonated:
Ca(OH)2 (s) + CO2 (g) Τ CaCO3 (s) + H2O (l)
Wollastonite carbonation using acetic acid
A second approach towards the reduction of energy consumption is the use of other acids than HCl. Kakizwawa et al. selected acetic acid to extract calcium ions from wollastonite. The route consists of two steps. First, wollastonite is treated with acetic acid. Then the calcium is carbonated and the acetic acid recovered in a combined step.
So far, this route has not received much attention in the literature. The main advantage of the process is the ability to speed up the carbonation process by extracting reactive compounds from the matrix without using hydrochloric acid.
Process scheme of acetic acid route (Kakizawa et al., 2001)
Dual alkali approach
The dual alkali approach is based on the Solvay process in which sodium carbonate is produced from sodium chloride using ammonia as a catalyst (Huang et al., 2001). The process is given below.
2NaCl (s) + Ca(OH)2 (s) + 2CO2 (g) Τ CaCl2 (s) + 2NaHCO3 (s)
For two reasons the Solvay process is ineffective in sequestrating carbon dioxide. The first is that large amounts of energy are consumed. The second is that one mole CO2 is produced for every two moles sequestrated because Ca(OH)2 is used to recycle the ammonia. Slaked lime (Ca(OH)2) is produced by calcination of limestone:
CaCO3 (s) Τ CaO (s) + CO2 (g)
CaO (s) + H2O (l) Τ Ca(OH)2 (s)
Thus for every mole CO2 sequestrated by the Solvay process, one mole is indirectly produced. In order to improve energy efficiency, research on the modification of the process has been done. In the modified process HOCH2CH2(CH3)NH is used instead of ammonia (Huang et al., 2001).