Describe the extraction of magnesium from seawater in the Dow process. Show all

Question

Describe the extraction of magnesium from seawater in the Dow process. Show all relevant equations involved in the process, starting with the precipitation of Mg^2+ as Mg(OH)_2 (s) using CaO(s) until the electrolysis of molten MgCl_2. If the concentration of Mg^2+ in seawater is 0.055 M, what is the minimum mass of calcium oxide. CaO, that must be added to 1.00 m^3 (1.00 times 10^3 L) of seawater to precipitate all of Mg^2+ as Mg(OH)_2? How many kilograms of magnesium can be produced in 1.00 hour by electrolysis of molten MgCl_2 using electrolytic cells that deliver a constant current of 2.5 times 10^5 A? What is the by-product in this electrolysis?

Explanation / Answer

a)
Magnesium forms stable compounds and reacts with oxygen and chlorine. The extraction of the metal from raw materials is an energy-intensive process Commercial production has two different methods:
Electrolysis of magnesium chloride
or
Thermal reduction of magnesium oxide, where power costs are low.
Electrolysis is the cheaper method and accounts for 75 percent of world magnesium production approx.

In the Dow process, magnesium hydroxide is precipitated from sea water by slurrying with calcined dolomite
and then converting it to magnesium chloride by reacting it with hydrochloric acid.
Mg + 2H2O -------------> Mg(OH)2 + H2

2 steps of Electrolysis of magnesium chloride :
1. The preparation of a feedstock containing magnesium chloride
2 .The dissociation of this compound into magnesium metal and chlorine gas in electrolytic cells.

Electrolytic cells are essentially brick-lined vessels with many steel cathodes and graphite anodes.
They are mounted vertically through the cell hood and partially submerged in a molten salt electrolyte
which consists of alkaline chlorides to which the magnesium chloride produced is added in concentrations
of 6 to 18 percent.
MgCl2 -------------------------> Mg2+ (liq) + Cl2 (gas)
In order to avoid impurities present in carnallite ores, dehydrated artificial carnallite is produced by
crystallization from heated magnesium- and potassium-containing solutions.  Seawater is mixed in a
flocculator with burned reactive dolomite. An insoluble magnesium hydroxide precipitates to the bottom
of a settling tank. When it is pumped as a slurry, filtered, converted to magnesium chloride by reaction
with HCl and dried in a series of evaporation steps content. Final dehydration takes place during smelting.

2 .The dissociation of this compound into magnesium metal and chlorine gas in electrolytic cells

In thermal production, dolomite is calcined to magnesium oxide (MgO) and lime (CaO),
and these are reduced by silicon (Si), yielding magnesium gas and a slag of dicalcium silicate
2CaO + MgO + Si --------------> 2Mg + Ca2SiO4     

b) The solubility product of Mg(OH)2 is 1.8*10-11

1.8*10-11 = [Mg+2] [OH-]2 = (0.055M)2 [OH-]2
[OH-] = 7.7138*10-5 M
  7.7138 *10-5 mol/lit = ( x g / 56.0774 g/mol) / 1*103 lit
x = 4.3257 gm of CaO is the minimum mass of CaO

c) Mg + 2e- -------------> Mg+2
for each mole of Mg 2 electrons are required
for (x gm / 24.31 g/mol) of Mg ---------? electrons are required
( 2x / 24.31 ) mol of electrons are required
Charge for 1 mole of electron is 96486 C
The charge of ( 2x / 24.31 ) mol of electrons is ?
( 2x / 24.31 ) * 96486 C
current , i = Coulombs / sec
2.5*105 =[ ( 2x / 24.31 ) * 96486 ] / 1*60*60 sec
x = 113379.1431 gms = 113.379 kg of magnesium can be produced
  

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