EffEct of iron on EnErGY conSUMPtion AnD cUrrEnt ... - ABM

One method <strong>of</strong> production <strong>of</strong> electrolytic zinc is electrowinningusing sulfate solutions. The deposition <strong>of</strong> electrolytic zincfrom alkaline zincate solution was investigated recently. (2,3) Currentis applied through insoluble electrodes and causes zinc depositionon cathode. Several parameters affect zinc electrolysis and itscontrol. (4) The presence <strong>of</strong> impurities in the electrolyte is a majorproblem for the zinc electrowinning industry. (5) The synergisticinteractions among impurities determine the quality <strong>of</strong> zinc depositfrom the solution. (6)Literature about the effect <strong>of</strong> impurities on zinc electrowinningis still scarce and restrict to specific impurities such asantimony, (7) nickel, (8) cadmium, <strong>iron</strong> and copper. (5)A promising development in this area is the application <strong>of</strong>cyclic voltammetry and <strong>of</strong> electrochemical impedance spectroscopyto the investigation <strong>of</strong> the electrodeposition process. (5,9)The aim <strong>of</strong> this work is to study the effect <strong>of</strong> <strong>iron</strong> in the zincelectrowinning process using electrochemical techniques such asgalvanostatic deposition and cyclic voltammetry. Two electrolyteswere studied: a zinc sulfate electrolyte, pH 5.05, and an acid industrialelectrolyte <strong>of</strong> zinc sulfate with addition <strong>of</strong> 180 g.L -1 <strong>of</strong> H 2SO 4.2 MATERIALS AND METHODSTwo electrolytes <strong>of</strong> zinc sulfate were used. A solution <strong>of</strong>60 g.L -1 <strong>of</strong> zinc was prepared dissolving 262.8 g <strong>of</strong> ZnSO 4.7H 2O in1000 mL <strong>of</strong> deionized water. Solutions containing 60 g.L -1 <strong>of</strong> zincand 5, 10, and 15 mg.L -1 <strong>of</strong> <strong>iron</strong> were prepared using ZnSO 4.7H 2O,FeSO4.7H 2O, and deionized water.Iron was added in a second electrolyte, used in an industrialplant <strong>of</strong> zinc electrowinning. Chemical characterization <strong>of</strong> thisindustrial acid electrolyte is shown in Table 1. Chemical analysiswas performed using the technique <strong>of</strong> atomic absorption spectrophotometry.Contents <strong>of</strong> 5 mg.L -1 , 10 mg.L -1 , and 15 mg.L -1 wereadded in the industrial electrolyte.Table 1. Chemical composition <strong>of</strong> the industrial electrolyteElement Content (g.L -1 )Zn 51.59Fe 8.06.10 -4Mn 1.36Mg 7.27Cd 0.06Electrochemical tests <strong>of</strong> galvanostatic deposition using apotentiostat/galvanostat AUTOLAB PGSTAT 30 were performed.Electrochemical cell was <strong>of</strong> three electrodes, an anode <strong>of</strong> Pb-Ag(1% wt), a cathode <strong>of</strong> aluminum and the reference electrode wasAg/AgCl. Zinc was deposited using galvanostatic deposition, applyinga current density <strong>of</strong> 500 A.m -2 , which is used in the industrial plant <strong>of</strong>zinc electrolysis. A deposition time <strong>of</strong> 3 hours was used. Superficialarea <strong>of</strong> cathode and anode was 1.0 cm 2 . Before testing, cathodeswere polished with 600-mesh sandpaper, washed in distilled waterand dried. The zinc was removed from the cathode surface with astainless steel spatula. Zinc deposits were weighedin a precision balance for measurements <strong>of</strong>0.0001 g. The aluminum cathode was also weighedbefore and after deposition (with the zinc deposits).Each experiment was performed five times.Anderson-Darling normality analysis wasperformed to verify if the obtained data <strong>of</strong> depositedzinc mass have a normal distribution. F test<strong>of</strong> variance analysis (Minitab s<strong>of</strong>tware, version 15)was used to verify the equality <strong>of</strong> media. ApplyingFaraday law, the theoretical mass <strong>of</strong> zinc wascalculated using the equation: (10)dnidQI = nF =(1)dt dtwhere I is the electric current, in amperes, n isthe moles <strong>of</strong> electrons, F is the Faraday constant,dni/dt represents the rate <strong>of</strong> reaction in moles persecond, Q is the quantity <strong>of</strong> electricity or chargein coulombs (C), and t is the time. The theoreticalzinc mass calculated was 184.71 mg. The currentefficiency can be expressed by the ratio <strong>of</strong> depositedzinc mass and theoretical zinc mass.Energy consumption (kWh/ton) was calculatedusing the expression:5Vm.8.4.10(2)10.CEwhere Vm is the average potential (V), and CE thecurrent efficiency (%).Cyclic voltammetry tests were performedusing a potentiostat/galvanostat AUTOLABPGSTAT 30. Electrochemical cell was <strong>of</strong> threeelectrodes: an anode <strong>of</strong> aluminum, a counter electrode<strong>of</strong> platinum and the reference electrodewas Ag/AgCl. Superficial area <strong>of</strong> cathode andanode was 1 cm 2 . The scan rate was 10 mV.s -1 ,and the potential range was from 0 to –1600 mV.Characterization <strong>of</strong> the electrodepositedzinc was performed using scanning electronmicroscopy (SEM), and energy dispersive spectroscopy(EDS). Image analysis using Quantikov (11)s<strong>of</strong>tware was carried out to evaluate porosity <strong>of</strong>the deposit.3 RESULTS AND DISCUSSION3.1 Current Efficiency and EnergyConsumption3.1.1 Electrolyte <strong>of</strong> acid solutionTable 2 shows current efficiency andenergy consumption obtained in acid electrolyteswithout Fe and with additions <strong>of</strong> 5 mg.L -1 ,10 mg.L -1 , and 15 mg.L -1 <strong>of</strong> Fe.62 Tecnol. Metal. Mater. Miner., São Paulo, v. 7, n. 1-2, p. 61-66, jul.-dez. 2010