FR2520007A1 - PROCESS FOR THE CONTINUOUS REGENERATION OF FLUXING BATHS IN STEEL GALVANIZATION OF STEEL PARTS - Google Patents

Abstract

1. A process for regenerating the content of fluxing tanks, which is formed by chlorides of zinc and ammonium in aqueous solution in which steel components are dipped prior to galvanisation, the process comprising, in order to remove the harmful dissolved iron, taking off in a closed circuit the content of the tank which is maintained at a pH-value of between 3 and 6 and adding oxygenated water to the material taken off so as to cause all the iron present to go to the trivalent state and form insoluble sludge of ferric hydroxide oxide which is removed prior to returning the solution to the tank, characterised by taking off a flow at a practically constant flow rate, simultaneously adding to said flow flows of oxygenated water and ammonia which are respectively regulated in such a way that the oxygenated water is substantially in stoichiometric equilibrium with the iron to be oxidised and the pH-value is maintained at between 3,9 and 4,1, allowing the formation of flakes of ferric hydroxide to be concluded in the flow in a practically non-turbulent flow condition, and transferring the flow into a settlement zone where it circulates upwardly in such a way that the hydroxide sludge accumulates at the bottom from which it is discharged, and collecting at the surface the solution of chloride of zinc and ammonium from which iron has been removed, said solution being heated in the settlement zone by the application of heat at the surface at a temperature which is at least equal to that of the solution in the tank.

Description

The invention relates to a method for regenerating the

  held by fluxing tanks, consisting of zinc and ammonium chlorides in aqueous solution, where parts are immersed

  in steel before hot dip galvanizing.

  During the dip galvanizing process, that is to say by immersion of the parts in a bath of molten zinc, the parts undergo preparative surface treatments including degreasing in an alkaline bath, pickling in an acid bath, and a so-called fluxing operation which consists of covering

  parts of a thin layer of zinc chlorides and ammonium

  nium (flux), by immersing them in a tank containing a so-

  aqueous lution of these salts, then optionally by evaporating

in the solution water oven.

  To reduce the transport of degreasing bath in the pickling bath, and of this pickling bath in the fluxing bath, intermediate rinses are carried out nevertheless

  the bath contained in the fluxing tank is gradually enriched

  iron, essentially in the divalent state, which comes from the transport of pickling bath, diluted by rinsing, and

  iron attack by the fluxing solution.

  The entrainment of iron salts in the galvanizing tank

  tion causes a reaction of the molten zinc with these salts of

  iron, with formation of mattes, zinc / iron compounds, which precipitates

  try in molten zinc and tend to collect at the bottom of the tank These mattes can cause defects in the

  zinc recovery, and cause zinc loss.

  When the flux bath contains more than traces

  of dissolved iron, part of this iron in the divalent state accompanied

  gne the chlorides of zinc and ammonium in the flow, while another part oxidizes in the trivalent state under the action of atmospheric oxygen, and precipitates in the state of ferric hydroxide; ferric hydroxide is partially entrained by the fluxing solution To the mattes which form by attack

  direct iron by zinc in the presence of flux during gal-

  vanization are added, undesirably, the mat-

  tes from the presence of dissolved iron in the fluxing bath.

  To remedy this difficulty, it is conventional to ef-

  periodically regenerate the flux baths.

  To do this, add oxygenated water to the contents of the tank.

  born, until practically all the iron contained passes to the trivalent state, and precipitates in the form of ferric hydroxide, which is in the form of flakes. It is allowed to form at the bottom of the tank hydroxide sludge, which it is then removed by pumping The clear bath which remains is completed in level

and in content.

  It is easy to understand that this regeneration operation, which blocks the operation of the galvanizing line for a significant period, is only carried out when the content of the

  fluxing bath in dissolved iron reaches prohibiti-

  resulting in a significant drop in the quality of galva-

  In addition, the removal of sludge from the bottom of the tank causes a significant flux bath to entrain the sludge is bulky, and relatively rich in zinc and chloride. It is expensive to treat this sludge to avoid

  pollution Current practice still has a problem

  comes from: oxidation of iron in a trivalent state by oxy-

  gene should not be driven with agitation, under penalty of

  see ferric hydroxide form in small-sized flakes

  pick which are not capable of settling in one time

  reasonable It follows that hydrogen peroxide cannot be

  part quickly in the mass of the bath to oxidize the iron.

  During slow diffusion, a significant amount of hydrogen peroxide spontaneously decomposes, and the consumption of this

reactive is excessive.

  The subject of the invention is a process free from the disadvantages

  aforementioned, and proposes for this purpose a method of regeneration

  ration of the content of fluxing tanks, consisting of zinc and ammonium chlorides in aqueous solution, where steel parts are immersed before dip galvanizing, method of removing harmful dissolved iron, according to which one adds

  to the contents of the tank of hydrogen peroxide so as not to-

  be virtually all of the dissolved iron in the trivalent state and

  to form insoluble ferric hydroxide sludge, and

  mine this insoluble sludge, characterized in that one draws from the tank a current at a practically constant flow rate of its content, to this current is added flow rates of hydrogen peroxide and ammonia regulated respectively so that the oxygenated water either substantially in stoichiometric equilibrium with the iron to be oxidized and that the p H is maintained substantially at 4, the formation of ferric hydroxide is allowed to end in a calm environment, and the hydroxide sludge is removed by decantation by returning

  in the tank the solution of zinc chlorides and ammonium de-

iron barred.

  The process, carried out on a bath where the concentration

  dissolved iron remains low, since elimination is perma-

  nente, makes it possible to add the reagents, hydrogen peroxide of oxidation and ammonia for adjusting p H, to the bath current so that the redox power and the p H are optimally adjusted in the bath in reaction; oxidation of

  iron (II) in iron (III) is done with the consumption of oxy-

  gene practically reduced to a minimum In addition, it takes advantage of the fact that the speed of oxidation of iron is notably higher than that of precipitation of ferric hydroxide; thus the current, relatively faster in the injection phase of reagents, corresponding to the oxidation step, is slowed down to allow the flakes to mature, at favorable p H

  ferric hydroxide The size of the flakes promotes the reduction

  clarity and efficiency of decantation.

  Various arrangements are made to improve the process, so after introduction of the reactants, the bath stream is led to the bottom of a reactor by a pipe, so that the oxidation time is close to the time taken by the bath to run in the pipe, while in the reactor where the current is almost imperceptible, the hydroxide can come to maturity After that the bath, where the hydroxide has developed

  for the most part, is sent to a decanter where the hy-

  drooxide will gather at the bottom It will be wise

  to heat the decanter on the surface, to avoid turbulence

  convection which could bring the surface of the hydro-

  xyde if the surface layers in the decanter

  cold, became denser than the deep layers.

  Finally, the ferric hydroxide sludge discharged by periodic purging of the settling tank, in reduced volumes, can

  easily be subjected to filtration, to recover the solu-

  tion of flow entrainée by the purges.

  The characteristics and advantages of the invention stand out

  will also derive from the description which follows

  example, with reference to the appended drawing which diagrammatically represents the arrangement of the material used for setting up

process work.

  According to the embodiment chosen and shown, the bath

  flux 10, aqueous solution of zinc chlorides and ammonium

  nium is contained in a tank 1, with a capacity of approximately 40 cubic meters, which is conventionally part of a line of

  dip galvanizing This line includes tanks for

  greasing, first rinsing, acid pickling, second rinsing, the fluxing tank 1, followed by a drying oven and the galvanizing tank, which contains a bath of molten zinc. To remove the iron dissolved in the fluxing bath 10,

  iron from the pickling tank drives

  of, and of the surface attack of the parts by the bath of

  fluxing, the content of the tank 1 is withdrawn in a regulated current

  bind by a positive displacement pump 12, through a suction pipe 11 which opens towards the bottom of the tank 10 The flow rate of the pump 12 is approximately 100 liters per hour, ie

roughly 0.03 liters per second.

  The pump 12 delivers the solution taken from a reactor 2, of 80 liters capacity, through a reaction tube 21 which enters the reactor as far as the bottom A

  upper part of the reaction tube 21 open two nozzles

  stages 22 b and 23 b, connected to two reservoirs 22 and 23 with reagents of 100 liters capacity, the reservoir 22 containing water

  oxygenated and the ammonia reservoir 23, at concentra-

  two standard metering pumps 22 a and 23 a collect the

  reagents in tanks 22 and 23 to send them to the

  stages 22 b and 23 b These metering pumps have flow rates controlled by sensors 22 c and 23 c, respectively for the redox potential and the p H, so that, in the reactor 2, the p H is maintained at 4 (+ 0.1) and that towards the surface of the

  reactor, the redox potential corresponds to a con-

  iron (II) concentration below a given value (0.5 g / l),

in the presence of iron (III).

  An overflow pipe 24 pitted on the reactor 2, and which defines the surface level, opens out in the center and towards the bottom of a cylindrical decanter 23, with a diameter of 1 meter and a height of 1, 300 meters ( volume about 1 cubic meter) The reactor is fitted with a peripheral gutter

  31 into which the decanted liquid overflows, which

  turns to fluxing tank 1 through line 32, outlet

  edge near the surface of the bath 10 From the bottom of the floor

  tor 3 leaves a drain line 33, to evacuate the

  solid particles which have collected at the bottom of the decanter 3.

  A battery of 4 immersion heaters 34 of unit power

  500 watts allows surface heating of the contents of the decan

author 3.

  The purge sludge extracted from the settling tank 3 can be, via a three-way valve 41, either

  sent directly to the workshop evacuation to an ins-

  waste treatment plant, either led by a pump

  eg 42 on a filter 4, to recover the zinc chlorides and

ammonium from the filtrate.

  If we pay attention to the purification cycle of

  fluxing solution, we note that, with an extrac-

  tion of 100 liters per hour, the purification period of the con-

  held 10 of the tank 1, is 400 hours for a volume of 40 cubic meters Furthermore, the duration of mixing of the reactants with the flow of solution in the tube 21 is of the order of ten seconds, and the residence time in reactor 2

  is approximately 50 minutes With a reactor height 2 of

  about 0.80 meters, the rate of rise of the solution

  in this reactor is about 0.3 mm per second, which

  responds to an almost turbulence-free flow, where the redox reactions between iron (II) and hydrogen peroxide occur

  occur with the minimum of side reactions and de-

  spontaneous composition of hydrogen peroxide, while the nucleus

  ation of ferric hydroxide occurs in the mass of so-

  lution It should also be noted that, due to the small importance

  relative tance of the fluxing solution sample in the tank 1 vis-à-vis the total volume of solution, the composition of the sample flow corresponds to the composition of the contents of the tank, which varies slowly, while the returns through the

  line 32 are not likely to alter sensitive-

  ment the composition of the fluxing bath over a short period. Also the contributions of reagents, hydrogen peroxide and ammonia, by the metering pumps 22 a and 23 a, also vary very slowly, so that the measurements of the sensors 22 c and 23 e are carried out on quantities, redox potential

  and p H, in a substantially permanent regime.

  In the next step, in decanter 3, the drying time

  solution day is 10 hours, which corresponds to a

  upward speed of about 0.04 mm / sec, for di-

  reactor dimensions specified above Not only does the maturation of the ferric hydroxide flakes have time to be complete, but also these flakes have an apparent density

  such that their falling speed can greatly exceed the

  ascending tesse of the liquid It will be noted that, in the reaction

  tor 2, where the flakes have barely exceeded the nucleation stage, while the ascending speed of the liquid

  is almost eight times larger, the drive of the hydro-

  ferric oxide is substantially complete.

  Furthermore, it is common practice to carry out the

  fluxing at a temperature above ambient, in order to operate

  rer with baths more concentrated in chlorides, brought to an acceptable viscosity The solution which stays 10 hours in

  the decanter 3 would cool, both through the walls of the decanter

  on the surface The temperature inequalities in the de-

  cantor 3 would generate convection movements in the liquid, from the bottom in the central part to the surface in the peripheral part, consequently causing the ferric hydroxide towards the tank 1 This is the reason why the surface layers of the decanter are heated 3, so as to form a warm layer on the surface, stable and capable of

  protect the underlying layers against cooling.

  Of course, the invention is not limited to the example

  describes, but embraces all variants, no-

  especially by adapting to the volume of flux baths to be purified.

  In addition, when the work rates are regular, we

  can save on controlled feedback flow rates

  tifs, by periodically checking the conditions of p H and

redox.

2520007 '

Claims (4)

R E V E N D I C A T I 0 N S   1 Process for regenerating the content of fluxing tanks, consisting of zinc and ammonium chlorides in solution   water, where steel parts are immersed before galvanizing-   dipping, harmful dissolving iron removal process   according to which we add oxygenated water to the contents of the tank   born so as to pass practically all of the dissolved iron to the trivalent state and form insoluble ferric hydroxide sludge, and this insoluble sludge is eliminated, characterized in that a current is drawn from the tank at practically constant flow of its content, there are added to this flow rates of oxygenated water and ammonia, regulated respectively so   that the hydrogen peroxide is substantially in stoichiometric equilibrium   stick with the iron to be oxidized and that the p H is kept sensitive   at 4, the formation of ferric hydroxide is allowed to end.   risk in a calm environment, and the hydroxide sludge is removed by decantation, returning the chlorides solution to the tank   zinc and ammonium free of iron.   2 Method according to claim 1, characterized in that   keep the current of chlorine solution in circulation   rures after addition of hydrogen peroxide and ammonia on a par-   long enough for the oxidation of iron to be felt   if completely complete, before leaving to finish in a calm environment the formation of hydroxide.   3 Process according to claim 1 or claim 2, characterized in that the chloride solution is transferred, after the major part of the ferric hydroxide has been formed, in a settling zone.   4 Method according to claim 3, characterized in that the chlorine solution is heated on the surface in the decantation zone.   Method according to any of claims 1 to   4, characterized in that the sludge discharged by decantation is subsequently subjected to a filtering operation in   sort of recovering from the zinc chloride solution and an- moniu carried away by this mud.