Recovery of nickel and copper from metal finishing hydroxide sludge by kinetic acid leaching

Corresponding author: email: salhiram@yahoo.fr Received date: June 06, 2016; revised date: December 03, 2016; accepted date: December 11, 2016 Abstract: The aim of the present work was the recovery of nickel and copper from metal finishing hydroxide sludge by kinetic acid leaching. The study was divided into two main parts: First, a kinetic model of precipitate leaching with the presence of acid was developed and subjected to theoretical approach so as to determine optimal leaching conditions (pH and time). Next, the study was conducted experimentally on pure metals hydroxides and then on electroplating sludges. The leaching operation was performed at atmospheric pressure with constant stirring. pH and solubility versus time at different temperature values and different amounts of sludge were carried out. The resulting filtrate was found to contain more than 99% of nickel and copper.


Introduction
The electroplating waste waters are generally detoxified (oxidation of cyanide and reduction of chromium) and then neutralized to precipitate metal hydroxides [1][2][3]..In the present work, the electroplating sludges are subjected to precipitation so as to get rid of hazardous metals.
Many studies reported that the sludges may present a great danger to the environment.The economic considerations are also worth considering because of the appreciable loss of metals.Since the eighties, the environment protection, the waste minimization and the metals recovery became a social and political issue in the U.S.A. [4][5][6][7] where the volume of sludges is so important owing to the large electroplating industries To combat this problem some solutions can be convenient to be implemented such as:  The clean technologies (membranes processes): The rinsing wastewaters are directly treated by electrodialysis, ultrafiltration, and solvent extraction.The metals are concentrated and reused in the electrodeposition bath.The water is also recycled.[8][9][10][11][12][13][14][15][16][17][18][19][20].However some economic, technical and psychological considerations hindered a rapid progress of this approach.
 The metal recovery from electroplating sludges: This solution is extremely necessary to treat, at least, the existing sludges.The American government through the EPA encouraged this solution and research projects were supported.
The economical recovery of metals with a sufficient purity is quite intricate and needs a big effort and hard research based on two main aspects: -First, it is necessary to develop simple and economical analysis methods to define the sludges composition.The simultaneous precipitation of many metals is always accompanied by secondary processes such as: coprecipitation, occlusion adsorption etc.The precipitate aging is also a problem.

Kinetic Aspect
The leaching of industrial waste is generally nonreactive.The reactions are most often carried out at high temperature or pressure and administrated by heterogeneous kinetic characteristics.In most cases, the rate of the overall reaction is characterized by diffusion of reactant and is therefore proportional to its concentration.Furthermore the overall rate increases with stirring owing to the slow diffusion in the electrolyte.In some cases, the overall dissolution rate may be affected by the transfer of the charged particles making up the initial crystal through a solid-electrolyte interface [40].
The most extensive study performed up to date on the leaching behavior of metal hydroxides and metal sulfide sludge may be the study reported by Hohman who developed a kinetic model of precipitate leaching [41].The leaching process was based upon an area-dependent mass transfer controlled process.The driving force for the leaching process was assumed to be proportional to the difference between the concentration of metal in solution (C) and the saturation concentration (Cs) predicted from precipitate equilibria consideration.The basic differential equation describing the leaching process is as follows [48, 49]: Where: Cs: saturation concentration of metal (moles/L) C: metal concentration in solution at time t (moles/L) A: area available for mass transfer, (cm 2 ) K: mass transfer coefficient (cm 2 .min - ) t: leaching time (min).and the fraction leached (α) of dissolved sludge depends on: r0: initial radius of precipitate particle (cm).Ksp: metal hydroxide solubility product (moles n /L n ).
WT: total weight of precipitate (g).

Vs: volume of solution (L).
For large particles (1mm < r < 1cm) the change in particle size influences the leaching behavior.The slow concentration rise indicates that the surface area decreases quickly, causing a rapid dropping in effective mass transfer coefficient.For small particles (1µm < r < 10µm), the driving force dominates, causing a rapid rise in the heavy metal concentration until saturation.Intermediate radius values show a balance of both effects.In the case of small molecules, it is considered that the specific surface area is constant, so the integrated equation (1) becomes: The consumable concentration of H + is: The remaining amount of H + in the solution at the time t is: This relationship is represented in Fig. 1. a.
where we note that when: t  to find the variation of pH with respect to time we replaced C of the equation ( 6) in the equation ( 7) we get: The concentration of the saturated solution is: We replaced Cs of the equation ( 9) in the Equation ( 8) we got: This relationship is shown in Fig. 1. b.
This difference relies on the structures of these hydroxides.In fact the structure of the nickel hydroxide is crystalline whereas that of copper is colloidal.The same behavior was observed in the leaching of the mixture of two hydroxides (CuO, Ni(OH)2) and three hydroxides (Fe(OH)3, CuO, Ni(OH)2) as shown in fig.11 and 12, as well as the hydroxide sludge as shown in fig 13,14,15, 16 and 17, where a higher leaching copper rate was recorded compared to that of nickel even if the amount of nickel in these samples ranged from 4 to 6 times the amount of copper.
On the contrary the iron hydroxide (III) does not begin to dissolve even at pH = 0.5.The lixiviation solutions of industrial sludge at pH=1.5 contain iron ions, which confirm that the sludge contains iron other than Fe(OH)3.
At pH = 0.0 the leaching of all metal components of hydroxide sludge is complete.Copper and Nickel can be separated from iron and chromium starting from initial pH of 1 over a period of 90 minutes and leveling up at pH of 1.6 where almost 100% of copper and more than 70% of Nickel were dissolved, whereas the iron dissolution does not exceed 30% over that period.The separation may take place from initial pH of 0.5 over a period of 25 minutes and leveling up at 1 where 70-80% of copper and 60-70% of nickel were dissolved, however iron is dissolved only within 20-50%.
The separation may also occur by leaching from initial pH of 1.5, but over a longer period of about 240 minutes where pH levels up at 2. Under these conditions 100% of copper and 95% of nickel were dissolved while iron does not exceed 19%.
Based on the previous results we suggest the following procedure for copper and nickel dissolution from hydroxide sludge as seen in figure 18.
initial consumable concentration of metal.( C0=0) and the leaching reaction is:

Fig. 2 :
Fig. 2: The variation of the hydroxides solubility with pH

(
temperature, pH, specific conductivity), The bottle was shaken continuously, and the pH and specific conductivity were measured as function of time.At different times, small volumes (1 mL) of solution were taken and filtered immediately.The metal concentrations in the filtrate were measured by atomic absorption spectroscopy.Thermodynamic Aspect different amounts of Ni(OH)2, Cu(OH)2 and electroplating sludges were put into a 50 mL screw cap glass bottle to which 25 mL of a solution ranging from 1 to 0.01 mol/L of acidic solution was subsequently added.The pH of the bottled solutions was adjusted by adding a predetermined amount of NaOH, H2SO4 and H2O, in such a way that the volume in the bottle was not substantially changed.The bottles were shaken for 8 h at 25 0 C, after which the pH was measured and the solution was filtered immediately.The metal concentrations in the filtrate were measured by atomic absorption spectroscopy.Dissolution Kinetics A-Effect of Temperature on the Dissolution Rates: Figure 3 illustrates the difference between the leaching process when the initial pH is 0.5, 1 and 1.5, Figs. 4. 5. 6. show the temperature effect with respect to the starting pH value.

Fig. 4 :
Fig.4: The temperature effect on leaching of hydroxide sludge when the initial pH is 0.5

Fig 9 :
Fig 9: The variation of pH and the concentration of dissolved nickel ion from nickel hydroxide (Ni(OH)2)

Fig 10 :Fig 11 :Fig 12 :
Fig 10: The variation of pH and the concentration of dissolved copper ion from copper hydroxide (Cu(OH)2)

Table 1 :
Compositions of four electroplating sludges, g kg-1 A mass m (in g) of Ni(OH)2 , Cu(OH)2 and electroplating sludges respectively, was put into a 50 mL using a screw-cap glass bottle with known initial conditions

Table 2 :
the dissolution kinetics of Iron (III) hydroxide at t=0;