טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentGreiserman Michael
SubjectRemineralization of Desalinated Water by Pellet and
Micronized Dolomite {CaMg(Co3)2}Dissolution
DepartmentDepartment of Chemical Engineering
Supervisors Professor Emeritus Raphael Semiat
Professor Emeritus David Hasson


Abstract

Since desalinated water is devoid of minerals, a certain degree of re-mineralization is necessary in order to meet health requirements and to make the water palatable and non-corrosive. Currently Israeli government regulations call for re-mineralization of desalinated water by calcium hardness and bicarbonate alkalinity. Dosage of magnesium ions is presently being considered and there is therefore interest in re-mineralization by dolomite {CaMg(CO3)2} since this double salt can provide all required regulatory ions.  

 The general aim of the present research was to develop design capabilities for dolomite re-mineralization of desalinated water acidified with either CO2 or H2SO4 by two alternate techniques:

      Dissolution of dolomite pellets held in a packed bed column by a flow of acidified desalinated water through the column

      Dissolution of a micronized dolomite powder flowing in a pipe with acidified desalinated water

The first technique adopts the conventional method for desalinated water re-mineralization by calcite pellets. The second technique is a novel method for which no kinetic data is available.

The packed bed design model for dolomite dissolution adapted the basic Yamauchi concept for calcite dissolution to the case of a double salt dissolution - CaCO3 and MgCO3. This required development of suitable expressions defining equilibrium conditions in a double salt system. The assumption of equal dissolution rates for both salts found full confirmation in the experimental results.

The micronized dolomite slurry dissolution model was based on developing segregation flow equations for the two salts. The advantage of the micronized flow system is the very rapid dissolution rates that can be achieved due to the very high surface area exposed to the acidified desalinated water. Analysis of the dissolution was made for the case of kinetic control and for the simplified case of equilibrium conditions. Verification of the developed model was based on extensive experimental data.

Dolomite dissolution in a packed bed was investigated in a packed column (10 mm diameter, 170 cm height) filled with 2mm dolomite pellets at flow rates of 1-3 L/min. The parameters investigated were the effects of inlet CO2 or H2SO4 in the range of 1.5-10 mM and temperature in the range of 11-39°C. The experimental results provided full confirmation for the developed model. The data obtained clearly indicate the viability of desalinated water re-mineralization by dolomite, with a clear advantage for H2SO4 dissolution in preference to CO2 dissolution. An interesting observation was the significant increase in mass transfer coefficient with H2SO4 dissolution due to the considerable enlargement of the dissolution surface area by the strong acid attack.

The system for investigating micronized dolomite dissolution consisted of a feed vessel connected to a recirculation pipe (50 m long, 1.25 inch diameter). Dissolution data were obtained using dolomite powder of average diameter of 1.9 micron at inlet flow rates in the range of 3.5-10 L/min. The parameters investigated were acid and inlet slurry concentrations in the range of 2-5 mM. As in the packed system experimental results fully confirmed the proposed models indicating that the micronized system is also a viable option for desalinated water re-mineralization.