*4.1. Solution Route (L-L)*

In this method, a solution containing calcium salt is mixed with a solution of carbonate salt [49]. When calcium chloride and potassium carbonate solutions are used, the reaction is as follows:

$$\text{CaCl}\_2 + \text{K}\_2\text{CO}\_3 \rightarrow \text{CaCO}\_3 + 2\text{KCl} \tag{1}$$

The total volume of the first solution used can be added immediately to the second solution [50,51] or it can be injected to another one with a controlled rate [52]. Different types of stirring are used to produce a homogeneous reaction mixture. Stirrers applied in laboratories are usually mechanical and magnetic, although other types are also proposed, e.g., ultrasound or microwave [13,53]. Recently, a "dropwise precipitation" has been adopted to calcium carbonate precipitation [54]. In this method, a calcium ion solution is added in very small portions to a carbonate solution.
