Most of the concrete used in the construction of buildings, roads, etc is made f

Question

Most of the concrete used in the construction of buildings, roads, etc is made from Portland cement, The charge to a Portland cement rotary kiln (where the materials are pulverized to produce the Portland cement) contains 20% of a dried building clay, -- 72 wt% SiO_2, 16 wt% Al_2O_3 7 % Fe_2O_3, 1.7% K_2O, 3.3% Na_2O--, and 80% limestone (95 wt% CaCO_3, 5% impurities). When the solid temperature reaches about 900 degree C, calcinations of the limestone to lime (CaO) and CO_2 occurs. As the temperature continues to rise to about 1450 degree C, the lime reacts with the minerals in the clay to form such compounds as 3 CaO.SiO_2, 4 CaO.Al_2O_3.Fe_2O_3, and 3 CaO.Al_2O_3. The flow rate of CO_2 is 1250 m^3/h at 1000 degree C and 1 atm. Calculate the feed rates of clay and limestone (kg/h) and the weight percent of Fe_2O_3, in the final cement.

Explanation / Answer

The reaction of Calcination is CaCO3--->CaO+ CO2

P= 1 atm, V= 1000m3 =1000*1000L

Moles of CO2 can be calculated from gas law n= number of moles= PV/RT = 1*1000*1000 L/(0.0821*(1000+273)=44616.4 moles

Moles of CaCO3 required = 44616.4moles = 44.62 kg moles, mass of CaCO3 required = moles* molar mass= 44.616*100=4462 kgs. since CaCO3 is only 95% pure, Lime stone required = 4462/0.95=4697 kgs. This correspond to 80% lime stone and rest 20% is clay.

hence amount of clay= 4697*0.2/0.8 = 1174 kg

Clay components (kgs) : SiO2= 1174*0.72= 845, Al2O3= 0.16*1174=188, Fe2O3= 0.07*1174=82 , K2O= 1174*1.7/100 = 20 and Na2O= 1174*3.3/100 = 39

moles of CaO formed = 44.62 kg moles, Mass of CaO formed= 56*44.62 =2499 kgs

all the Fe2O3 is in 4CaO.Al2O3. Fe2O3.

Moles of Fe2O3= 82/160= 0.5125 kg moles

4CaO.Al2O3. Fe2O3 ------------> 4CaO+ 4Fe2O3 + 4Al2O3.

Moles of CaO= 0.5125 kg moles, mass= 0.5125*56 = 28.7 kgs and moles of Al2O3= 0.5125*102 =52.5 kg

% Fe2O3 in CaO.Al2O3.Fe2O3= 100* 82/(82+28.7+52.5)=50.25

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