Hot process softening is usually carried out under pressure at temperatures of 227-240 o F (108-116 o C). At the operating temperature, hot process softening reactions go essentially to completion. This treatment method involves the same reactions described above, except that raw water CO 2 is vented and does not participate in the lime. Lime Softening Reactions For Kids Lime softening, also known as Clark's process, 1 is a type of water treatment used for water softening which uses the addition of limewater (calcium hydroxide) to remove hardness (calcium and magnesium) ions by precipitation.
Softening refers to the process of removing hardness ions such as Ca and Mg from water. Lime softening, in particular, is the removal of Ca and Mg ions through the addition of addition of lime, Ca(OH)2. Interestingly, silica (SiO2), a major constituent of concern for RO membrane fouling, can also be removed through the series of precipitation reactions that occur with lime addition.
Lime Softening Reactions Reaction
The background to this post is that a colleague and I had come across some experimental data which showed that increasing lime dosage aided silica removal. In these experiments, CO2 was also dosed in conjunction with lime. But, we couldn’t get our heads around how lime addition was linked to silica removal (though we understood the silica was somehow precipitated out), and what purpose the CO2 addition served.
After some earnest googling, the said colleague came across this rather enlightening paper, and I thought I’s highlight some of the interesting things I learned (or in some cases, got re-inforced).
1. Removal of carbonate hardness
During lime softening, calcium bicarbonate is precipitated out as calcium carbonate. Magnesium bicarbonate is converted to magnesium carbonate at a pH of 9.4. However, magnesium carbonate is a soluble salt! Thus, excess lime needs to be added to precipitate it out as insoluble magnesium hydroxide at a pH of 10.6.
2. Removal of soluble and insoluble silica
Upon precipitation, magnesium hydroxide forms larger flocs which entraps and adsorbs in soluble silica particles in water. Thus, silica is removed by magnesium hydroxide by adsorption. The optimal pH for silica adsorption onto Mg(OH)2 is around 10-11. Solubility of Mg(OH)2 decreases close to zero at pH 9.5 and higher. Ionisation of soluble Si(OH)4 to silicate ions also increases with pH greater than 7. These silicate ions are adsorbed and can form silicate hydroxides bonds as shown by the chemical reaction below.
Lime Softening Reactions
3. Role of CO2 addition
Although no CO2 addition is mentioned in this paper, it gives us a clue for its role in the above mentioned experiments which started this whole discussion. If the lime added is in excess of the Ca and Mg to be removed (for example, because we need to raise the pH to remove silica), the excess lime could react with Na in the water forming soluble sodium carbonate. As sodium carbonate will not be precipitated out, it will add to the scaling potential of any downstream RO processes.
The excess lime can be neutralised by CO2 via the following reaction which forms the insoluble calcium carbonate and thus, is also precipitated out.
Concluding thoughts
Though this paper only mentions about the role of magnesium hydroxide in removing silicate, I wonder if some silicate will similarly adsorbed on to calcium hydroxide (lime) directly and be removed if calcium hydroxide is added in high enough concentrations. But, then again, adding excess lime may not be the ideal solution as it will increase the hardness of water (which we were trying to reduce in the first place)?
References