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From the experiments of Phase II, Part I the optimum parameters of aluminum recovery from sludge were obtained. These parameters include a hydrochloric acid concentration of 2.0 N, an acid to sludge ratio of 1.83 on a mass basis, a mixing speed of 80 rpm, at a temperature of 40 °C and a mixing time of 40 minutes. The use of this combination of optimum values resulted in a higher recovery of aluminum from the sludge, which reached 82.4%. Table 3 shows the metals' content in the recovered coagulant solution compared with the commercial alum. It is obvious from Table 3 that the metals' concentrations in the commercial alum are higher than that in the recovered coagulant. This assures that the main source of metals in the recovered coagulant is the commercial one. In addition, the reduction in metals' concentrations of the recovered coagulant against the virgin solution is explained through two points: first, not all the metals are precipitated and removed in the sludge; and second, the leaching process did not perform 100% recovery for all the metals in the sludge.

Table 3

Comparison between the metals' analysis of the recovered coagulant solution from the alum sludge and commercial alum (all metals are expressed in mg/L)

  Ba Co Cr Cu Fe Li 
C.A.S 0.06 0.01 0.08 0.03 13.31 0.01 
R.C.S 0.06 0.01 0.06 0.02 10.70 0.00 
  Mn Ni Pb Sr V Zn 
C.A.S 1.08 0.02 0.52 0.10 0.07 0.07 
R.C.S 0.87 0.02 0.42 0.05 0.06 0.05 
  K Na Ca Mg     
C.A.S 0.782 0.882 14.52 4.78   
R.C.S 0.487 0.731 10.54 2.462   
  Ba Co Cr Cu Fe Li 
C.A.S 0.06 0.01 0.08 0.03 13.31 0.01 
R.C.S 0.06 0.01 0.06 0.02 10.70 0.00 
  Mn Ni Pb Sr V Zn 
C.A.S 1.08 0.02 0.52 0.10 0.07 0.07 
R.C.S 0.87 0.02 0.42 0.05 0.06 0.05 
  K Na Ca Mg     
C.A.S 0.782 0.882 14.52 4.78   
R.C.S 0.487 0.731 10.54 2.462   

C.A.S: commercial aluminum sulfate; R.C.S: recovered coagulant solution.

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