THEORET I CAL AND APPLIED ASPECTS OF US I NG ELECTROFLOTAT I ON METHOD FOR WASTEWATER TREATMENT OF SURFACE FINISHING INDUSTRY

Dr. V.A. Kolesnikov, Dr. V.N. Kudryavtsev Mendeleyev University of Chemical Technology of Russia, Moscow

Abstract Process of electrof lotation removal of various ions of

heavynon-ferrous metals like Cu, Ni, Zn, Cd, Sn, Pb, Fe, Al, Cr from liquid wastes of platin productions has been studied. It has been shown that electroflota f ion removal is a complex colloidal-chemical process. Effectiveness of the process depends on pH and com osition of solution, current density, size and char e of bub % les and dispersed particles, nature of flocculants, hydroxides, phosphates, carbonates based on Cu, Ni, Zn, Cd, Fe, A and other ions have been determined. Technologies and electroflotation unit with a capacity of 10 m3/hr for treatin waste waters of electro lating shops which provide for residua concentrations of 0.05- 8 .01 mg/l for Cu, Ni, Zn, Cr, A l , Fe ions have been developed and implemented. Electroflotation method can be successfully used for removing greases, oils, surfactants, petroleum products.

process

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solu f ion flow rate etc. Optimum conditions for

I n t r cduc t i on The electroflotation method is based on the principle of the

electrochemical processes of oxygen and hydrogen evolution during electrolysis of waste waters. Finely dispersed gas bubbles rise tc the surface trapping along with them dispersed particles present in solution. The foam layer obtained (flotosludge) is removed fron the surface of the solution by mechanical or other means.

Electroflotation is a multi-stage process. For its successful realization, it is necessary to have substances being removed from solution to be in their insoluble form. There are many methods for converting metal ions, organic and polymer molecules into dispersed form. The simplest of them is conversion soluble metal compounds into their insoluble hydroxides. Usually, alkali is added to acidic solution or acid is added to alkaline solution adjusting pH to the pH of hydroxide formation. The hydroxide dispersed phase can also be formed during the electrolysis of acidic (alkaline) solution in the cathode (anode) chamber of a double-chamber electrolyzer. There, the solution is alkalified

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ACIDIC

E I- E CTRO

ELECTRQLYTE

( p H - 3 )

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Fig.1. Electroflotator with pH electrocorrector (Dimension - 2000 x 1200 x 1115)

1 - anode chamber; 2 - cathode chamber; 3 - electroflotation chamber; 4 - foam collector; 5 - cathode; 6 - foam receiver; 7 - membrane; 8 - anode; 9 - cathode.

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(acidified) to the appropriate pH due to electrolytic hydrogen) (oxygen) evolution.

By another method, precipitator-ions should be added to the solution. As a result, precipitator-ions react with metal ions to form dispersed insoluble compounds.

Operation principle of elctroflotator unit The scheme of a flow-type electroflotator unit with pH

3lectrocorrector, used for the treatment of acidic solutions is presented in Fig.1, as an example.

Acidic rinse water containing Cu, Ni, Zn, Cr(III> or other :ations together or separately is pumped to the cathode chamber 2 Df a diaphragm type cell (it is called pH electrocorrector). During the electrolysis of rinse water in this type of a cell, hydrogen is generated at the cathode 5. So, pH of the solution increses up to pH of hydroxide formation. This process is sccompanied with the formation of insoluble metal hydroxide particles. The hydroxide particles adhere to hydrogen bubbles. The flotocomplexes formed have density lower than that of the solution. Therefore, they are able to rise to the surface of the solution and form a stable foam layer. The efficiency of waste water purification done by primary flotation is about 90-95%. Simultaneously, with the flotation process, anions migrate through the membrane 7 and get into the anode chamber 1, where the rinse water is partially demineralized.

The foam layer is removed from the surface of the solutior with the help of a foam collector 6 (or a water-jet pump) to the tank called foam receiver 4. The solution with remaining particle: passes into chamber electroflotator 3 for further electroflotatior tertiary treatment. The chamber has insoluble anodes 8 anc cathodes 9.

In the cathode chamber 2 of the pH electrocorrector, thc liquid (containing dispersed particles) and hydrogen bubbles flov directions coincide. But, it is otherwise in the flotator - thf hydroxide-containing liquid flow is counter directed towards thf

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;as bubbles stream. Liquid with the remaining particles of lispersed phase moves from up downwards, while oxygen and hydrogen Iubbles are rising up. The counter-movement of dispersed particles ind gas bubbles creates a "filtration" effect of dispersed ,articles through "sieve" or "filter" of bubbles. This results in i sharp increase i n the efficiency of dispersed particles removal "om the solution. The rising gas bubbles work as a compact 'ilter. This filter is so compact that only liquid can pass ;hrough it, while hydroxide particles together with bubbles rise ~p top the surface. The foam layer (hydroxide) is removed from loth cells by a mechanical device 6.

The purified water with metal ions content of 0.5 rngA is 2cidified to pH 6.5-8 using acidic solution generated in the anode 2hamber 1 of pH electrocorrector. Then, it can be used repeatedly 2t the rinsing stage. To prevent salinization of the reclaimed Hater, a small part of the solution , not more than 5%, is removed from the system and replenished with fresh water.

Using the double-chamber electroflotation system describec sbove we can make efficiency of the dispersed phase removal tc get close to 100%.

Electroflotation treatment of waste water is limited by thf naximum concentration of soluble hazardous substances in waste waters - 200 mg/l. The optimum concentration is 10-100 mg/l.

There are some basic characteristics that determint effectiveness of electroflotation process. These include: curren' density, I,; electroflotation time, pH of solution, ions conten' in solution, solution flow rate through the flotator (foi F 1 e.. C W r r h ,111; + ) 1 l U & - b J y C U l l l b /

Influence of current density and electrof lotation time on process effectiveness

Regulation of current density in the unit plays an

The higher the current density the more gas bubbles

important role in the controlling of electroflotation process.

be formed per time unit, i.e. the higher the solution saturatior

will

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degree with bubbles. With a change in current density the bubble's radius changes, also. For example, in acidic solution, where current density is increased from 50 to 300 A/m the average diameter of gas bubbles increases from 10 to 25-35 mm.

It has been found that the dependence of particles removal efficiency current density on for all compounds studied goes through a maximum. At low current density, the degree of gas saturation of any liquid is low. Therefore, the process is ineffective. A t high current density, a significant turbulence accompanied by foam layer destruction occurs in the solution. As a result of this, the removal efficiency drops.

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Thus, three cases can be observed: a) I, < I s , o p t . - process proceeds at low rate; b) I, = Is o p t . - process proceeds with maximum effectiveness; c ) I, > I s , o p t . - intensive flotocomplexes formation takes place. Large portion of them is destroyed not even reaching the foam layer.

Experimental results revealed that optimum current density de- pends on the composition and concentration of floating system. Let us consider electroflotation of manganese hydroxide, as an example

The dependence of Mn(OH>Z removal efficiency on current density for two g i v e n concentration, - 200 (curves 1 and 2 ) and 250 mg/l (curve 3 and 4) - at various electroflotation times is shown in Fig.2. Maximum can be observed on t h e curves at I, = 18C mA/cm . At the beginning of electrolysis, current density has more inf hence on manganese hydroxide removal efficiency (curves 1,3) . From these data it can be seen that at, t.he s m s current density the longer the electrolysis time the higher the removal efficiency (compare curves 1,3 with 2,4).

flotoconcentrate, particles' charge and size. This is shown ir Fig.3. For copper hydroxide optimum current density is 150 A/m , for phosphate - 100 A/m2 and for sulfide - 50 A/m2+ All these particles have different properties and that is why optimun

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Optimum current density depends on nature of

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60 120 180 240 300 360 'ig.2.Dependence of Mn(OHI2 removal efficiency on current lensity for solutions with initial Mn2+ concentration (mg/l): 200 (1,2) and 250 (3,4)Process time (mink 1,3

01, %

Fig.3. copper

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- 8 ; 2,4 - 12.

I,, t-nA/cm2 I I I I I

50 100 150 200 250 Dependence of removal efficiency on current density for hydroxide (1) , copper phosphate (2>, copper sulfide ( 3 ) ;

pH=9.5; time = 10 mini Cc,2+=20 mg/l. 6

a , % 100

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5 3 T , min

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4 8 12 16 Fig.4. Dependence of nickel ( 1 1 ) ions removal efficiency on electrolysis time for I,: 1 - 50; 2 - 100; 3 - 150; 5 - 250 Ah2; CNi2+ = 300 mg/l

4 - 200;

xrrent densities for their removal are very different. The flotation rate and obviously, the flotation time depend

3n current density as well as on the initial concentration of substances in the solution being treated. Under these conditions zertain optimum current density exists for each concentration. For sxample, the dependence of the removal efficiency on electrolysis time at various current densities are presented in Fig.4. As it is shown, at low current densities 50-200 A/m retardation curve zones in eiectroflotation rate are notable. Existence of the lag effect at high metal concentrations and low current densities is connected with insufficient gas saturation required for metal hydroxide flotation. At current densities of 250 and higher- when gas saturation is high, the retardation curve zone is not Dbserved.

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Inf hence of Flocculants Flocculants play important role in electrof lotation process.

Flocculants can increase particles’ size, increase their sedimentation rate and improve their flotability. Mechanism of flocculants’ action is complex. However, it is known that the most effective flocculants are those that neutralize surface charge of dispersed particles, or recharge the surface. In our processes, almost in all cases flocculants were used resulting in increase in flotation rate.

Using radioisotope method, it was confirmed that the flocculant adsorption occured on all types of the hydroxides studied. The diameter of hydroxides in the presence of floccu- lants increases up to 70-75 mm. Increase in the size of floating particles makes possible to intensify the process of electro- flotation removal of hydroxides due to shorter process time.

Influence of acidity of the medium Acidity of the medium has a significant influence on complete

conversion of ions into insoluble hydroxides, the charge of solid particles formed and the flotation activity of aggregates (dispersed particles) formed. The flotation acitivity is defined as efficiency of interaction between the dispersed particles and gas bubbles.

Three cases of pH influence on the electrof lotation efficiency can be observed: 1) PH < PHopt. - process of electrof lotation removal takes place at low rate, and its achievable removal efficiency is not high; 2 ) PH = pHGpt. - t h e process proceeds at an aiierage rate, wniie the removal efficiency reaches its maximum value; 3 ) pH ) pHopt. - process rate is highest, but there is drop in t h e removal efficiency.

For the analysis of pH influence, it is convenient to compare removal efficiency at various differences between the current pl

0 - this means that the process proceeds at; pH higher than the and its optimum value, i.e. defined as ApH = pH - pHopt. . I f ApH >

a

value of optimum pH. When ApH<O the process proceeds at lower pH. Dependences of the removal efficiency on ApH value for

various hydroxides of divalent metals - Cu, Ni , Zn, Cd, Mn as well as for trivalent - Fe and Cr are presented i n Fig.5. In acidic medium, Le. when ApH < 0, a drop i n hydroxides removal efficiency is observed compared to the optimum. The lowest influence is observed for C U ( O H ) ~ and Fe(OH)31 the highest for Cr(OH13- I n the zone where ApH > 0, the most intensive pH influence is observed for Ni(OH);! and Cr(OH>3, the lowest is for

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APH I I I I 1 I I I I

- 2 - 1 0 + 1 + 2 Fig.5. Change in efficiency of removing some hydroxides as 2

jependence of value of pH difference from the optimum: i - Cu(OHI2; 2 - Cd(OHI2, M I I ( O H ) ~ ; 3 - Ni(OH)2; 4 - ZII(OH)~; 5 - 3 - ( O H ) 3 ; 6 - Fe(OHI3.

From practical point of view, it is more technological tc 2arry out the process when a is less dependent on ApH, for instance, in case of Cu(0H) , Fe(OH> . It is more difficult tc remove Ni and Cr hydroxides. Drop in removal efficiency at ApH < C

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is connected with hydroxide dissolution and ions formation. In alkaline medium at ApH > 0, dispersed phase is stable. However, in the presence of excess OH- ions surface of the hydroxides is recharged from positive charge to negative. As it has been shown in the studies, all prototype systems (carbonates, phosphates etc.), which are negatively charged, float much worse than the flotoconcentrates that are either positively charged or neutral.

For example, the dependence of nickel hydroxide removal ef f i- ciency on the pH of solution for various concentration of disper- sed phase within the range 50-500 m g / l is presented in Fig.6.

a , % 100

80

60

40

PH

8 9 10 11 IFig.6. Dependence of nickel ( 1 1 ) ions removal efficiency on pH of solution for concentration (mg/l): 1 - 50; 2 - 100; 3 - 200; 4 - 500: flotation time - 3 min; current d e n s i t y - 100 mA/cm

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The data given in Fig.6 show that for the all nickel concentrations studied, the maximum removal efficiency is observed at pH = 9.5-10. In the same pH interval nickel hydroxide has the lowest solubility. At pH lower than 9.0 a big portion of nickel are present in the solution in form of ions. At pH higher thar 10.5 the surface of the hydroxide is recharged resulting in 101

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nickel removal as foam product. With increase in nickel concentration in solution removal efficiency decreases significantly. Extremial form of the dependence of a = f(pH) is peculiar to all hydroxides studied.

Influence of the particles’ surface charge on the electrof lotation process

Interaction between evolving gas bubbles and pollutants particles is an important stage of the electroflotation process. In this case, the surface charge of the particles is a determining factor.

It is known that electrolytic hygrogen and oxygen bubbles being evolved are charged. Thus, during hydrogen evolution, at the moment of its detachment from the cathode, the hydrogen bubble is negatively charged, while oxygen is positevely charged.

When electrical charges of bubbles and particles are equal, the barrier arising due to electrostatic forces of repletion can hinder particles moving towards each other, resulting in adherence. When bubbles and particles are counter charged, the flotosludge formation proceeds more effectively.

However, the probability of charged bubble formation will greatly depend on pH of the solution. Heavy metal ions (Cu, Ni, Zn, etc.) removal takes place mostly at pH 8. Therefore, the most probable in the first stage of flotocomplex formation is a heterogeneous process with involvement of the dispersed phase (positively charged) and gaseous phase of hydrogen bubbles (negatively charged). The flotocomplex formed will be charged neutral. And due to the particles aggregation, its size will increase 5-10 times.

Ions present in the solution can cause recharging of the surface and thereby influence the entire electroflotation proces: either by intensifying or hindering it.

If the initial particles’ surface is charged positively, i t means that anions with low adsorption capability should be addec for the process intensification. Obviouslv. when the initial

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surface of the particle is negatively charged, it means that =ations with low adsorption capability or a very low quantity of sations with high adsorption capability should be added in order to intensify the process.

If the dispersed phase is charged negatively then floto- 2omplex formation is difficult and the flotation effectiveness nust fall. This was proved by low f lotoactivi ty of phosphates, sulfides, carbonates, tartrates, pyrophosphates and other disper- sed particles. That is why it is necessary to use special floc- xlants, which can increase the flotability of these particles.

Additional removal of metal ions from waste water can be xhieved by adding Na3PO4* Na2HP04, S- ions etc. In this case zompounds whose solubility is in 12-17 folds lower than that of their respective hydroxides, are formed. Strong oxidising agents 2an also be added to waste water containing Ni ions for 3xidation to form compounds of higher oxidation number, for sxample, NiOOH. Residual metal ions concentration after treatment is 0.01-0.05 mg/l.

2 +

Conc 1 us i on Electroflotation method using insoluble electrodes for water

treatment has been successfully applied in Russia and abroad for the removal of dispersed contaminants, effluents clarification? water demineralization, as well as for the removal of petroleur products, surfactants, oils, greases from waste water and dr: alkaline photoresist polymer film from waste water of PCl production. This method is also can be applied for the removal o! aluminium from aluminium etching solutions. High effectiveness easy operation, possible aiitotiiation of i n s t a l l a t ion , low energ; consumption, low reagents consumption, small production area, arc some of the numerous advantages of this method.

Various designs of electroflotator and electroflotator wit1 pH electrocorrector units for the realization of the processe: described above have been developed and being fabricated. Thl units’ capacity ranges from 1 to 10 m3/h.

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