Occurrence And Separation Of Calcium And Iron In Fly Ash

There are few systematic researches on the occurrence regularity and separation methods of calcium and iron concentrations about 10～20%in the fly ash,as has severely limited and affected its resource utilization.This paper conducted a systematic study on the occurrence regularity and separation methods of calcium and iron concentrations in the fly ash from both circulating fluidized bed and pulverized coal furnace.The following improvements have been made:（1）The occurrence states of calcium and iron in both the raw coal and fly ash have been identified by various characterization analysis methods.In the raw coal from circulating fluidized bed,the calcium exists in the form of dolomite,and the iron in the form of iron sulfide.In the fly ash from circulating fluidized bed,the occurrence phases of calcium are calcium oxide,calcium sulfate,and anorthitel,while the ocurrence phases of iron are hematite and fayalite.Calcium oxide and calcium sulfate are mostly independent,anorthite mainly adheres to the surface of mullite and iron oxide particles.Iron oxide can be both independent or mixed with aluminum-silicate compounds.Fayalite is mixed with other minerals.In the raw coal of pulverized coal furnace,calcium exists in the forms of calcium carbonate and calcium phosphate,and iron exists in the forms of pyrite and hematite.In the fly ash from pulverized coal furnace,calcium exists in the forms of anorthite,calcium silicate,calcium aluminate,and gehlenite in the glass.Iron exists in the forms of iron microbeads,ferrous,hematite,and magnetite,in which the iron microbeads are independent,while the other three componets are mostly in the glass.Solute segregation analysis showed that calcium in both kinds of fly ash was found rich in-325 mesh,while iron was found rich in+48 mesh and 160-325 mesh.（2）Step-leaching and magnetic separation quantitative analysis method was established and quantitative analysis on different calcium and iron phases in fly ash was conducted by it.The results showed that calcium oxide,calcium sulfate,and anorthite（all in terms of Ca O）in the fly ash of circulating fluidized bed accounted for 38.88%,18.98%and 42.14%of the total calcium,respectively.The study also found that f-Ca O in both+250 mesh and 250～325 mesh were about 1%,which can be used in cement and concrete.Hematite and fayalite（both in terms of Fe₂O₃）accounted for 8.33%and91.67%of total iron,respectively.The various phases of calcium in the fly ash from pulverized coal furnace was difficult to quantitative analysis because of their mutual doping.Strongly magnetic iron（including magnetite and elemental Fe）,weakly magnetic iron hematite,and iron microbeads（all in terms of Fe₂O₃）in the fly ash from pulverized coal furnace accounted for 50.12%,9.80%and 40.08%of total iron,respectively.（3）The removal method of calcium was developed based on the occurrence regularity of calcium in fly ash.Calcium oxide and calcium sulfate in the fly ash from circulating fluidized bed can be removed through leaching with water solution or hyposulphite solution,but anorthite can not be dissolved with this method.It is difficult to separate calcium from the fly ash from pulverized coal furnace since it exists in the glass.The use of weak acid salt solution can promote the dissolution of the calcium phase with the dissolution rate raised by more than 25%.（4）The separation regularity of iron in fly ash was explored.The main occurrence mode of iron in circulating fluidized bed is fayalite,which exists in combined with other main components of fly ash and is difficult to be separated;based on the magnetic difference of the iron phase in fly ash from pulverized coal furnace,cascade magnetic separation with magnetic tube to separate the magnetic iron from the fly ash was adopted.The results showed that cascade magnetic separation can separate all the magnetic iron from the fly ash from the pulverized coal furnace,with the iron removal rate at 59.92%.The iron grade in non-magnetic coal ash was reduced from 9.68%to4.99%and the iron grade in separated magnetic substance（in terms of Fe₂O₃）was66.63%.