0.01 M CuSO4 + 1.5 M H2 SO4 1.0808 1.197 6.28 1764

0.025 M CuSO4 + 1.5 M H2 SO4 1.0919 1.22 6.15 1817

0.05 M CuSO4 + 1.5 M H2 SO4 1.0933 1.26 5.93 1943

ion by electrical migration; i.e. mass transfer takes place only by convective diffusion mechanism (Selman and Tobias, 1978; Berger and Hau, 1977). Three different solution compositions were used in the present study as follow:

(i) 0.025 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 + 1 N NaOH.

(ii) 0.025 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 + 2 N NaOH.

(iii) 0.025 M K3Fe(CN)6 + 0.1 M K4Fe(CN)6 + 3 N NaOH.

The use of high concentration of K4Fe(CN)6 compared to K3Fe(CN)6 ensured that no oxygen evolution takes place at the anode during limiting current measurement to avoid its interference with the cathode reaction. Solution density and viscosity were measured by a density bottle and Ostwald vis- cometer, respectively (Findlay and Kitchner, 1965) diffusivity of ferricyanide was obtained from the literature and was cor- rected for the change in viscosity using the Stokes–Einstein equation (Berger and Hau, 1977). Table 2 shows the physical properties of the solutions used in data correlation at 25 ◦C. To test the applicability of the mass transfer equation obtained from the ferri/ferrocyanide experiments on copper deposition from acidified copper sulphate where three different solutions of acidified CuSO4 were used, namely: 0.01 M CuSO4, 0.025 M CuSO4 and 0.05 M CuSO4, in all cases 1.5 M H2SO4 was used as supporting electrolyte. Table 3 shows the viscosity and density of these solutions at 25 ◦C, they were obtained as  mentioned

before (Findlay and Kitchner, 1965), the diffusivity of Cu++ was obtained from the literature (Selman and Tobias, 1978; Wilke et al., 1953). To examine the effect of drag reducing polymer on the rate of mass transfer at the rotating impeller, Polyox WSR-301 powder was added to the solution in concentrations 100, 200, and 300 ppm. All solutions used were prepared of A.R. grade chemicals and distilled water, temperature was 25 ± 1 ◦C during experiments.

3. Results and discussion

3.1. Mass transfer data correlation

Fig. 4 shows a typical polarization curve with a well defined limiting current plateau, the limiting current obtained from these curves were used to calculate the mass transfer coeffi- cient according to the following equation (Selman and Tobias, 1978):

I

ZF  = KAC (1)

The mass transfer coefficient is related to other variables by the following functional equation:

K = f (µ, p, D, N, db, di) (2)

Fig. 4 –  Typical polarization curve at different impeller rotational speeds.

1.6 1.8 2 2.2 2.4 2.6 2.8

Log N (rpm)

Fig. 5 – Effect of impeller rotational speed on the mass transfer coefficient at different Sc.

Dimensional analysis leads to

Fig. 6 – Approximate flow pattern of the present impeller.