US3885018A

US3885018A - Regenerating anion exchange zone containing hexavalent chromium

Info

Publication number: US3885018A
Application number: US366544A
Authority: US
Inventors: Robert B Smith
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1973-06-04
Filing date: 1973-06-04
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20

Abstract

In recovering hexavalent chromium from chrome plated steel strip which is wetted with plating solution as a result of dragout from the plating process, an aqueous solution containing hexavalent chromium in relatively low concentration is concentrated in a process comprising treating in a first cation exchange zone, treating in an anion exchange zone, regenerating the anion exchange zone and treating the effluent of regeneration in a cation exchange zone different from the first cation exchange zone. The concentrated solution is passed to a storage zone and recirculation is carried out between the storage zone and the plating zone and solution is passed from the storage zone to an evaporation zone and more concentrated solution is passed from the evaporation zone to the storage zone. In regenerating the anion exchange zone, only a portion of the hexavalent chromium is removed.

Description

United States Patent 1191 Smith 1 51 May 20, 1975 [75] Inventor: Robert B. Smith, Crown Point, Ind.

[73] Assignee: National Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 4, 1973 [21] Appl. No.: 366,544

[52] US. Cl. 423/54; 423/6585; 210/30; 210/31; 210/32; 210/37; 210/38; 204/51; 75/101 BE [51] Int. Cl ..C01g 37/14; B01d 15/06 [58] Field of Search 423/54, 6585; 204/51; 210/37, 38, 3032; 75/101 BE [56] References Cited UNITED STATES PATENTS 2,733,204 1/1956 Costa 423/54 X 3,223,620 12/1965 Obuhofer 210/37 X 3,658,470 4/1972 Zievers et a1. 210/37 X 3,681,210 8/1972 Zievers et a1. 204/51 X OTHER PUBLICATIONS Paulson et al., Plating, Sept. 1953, pp. 1005-1009. Culotta et al., Plating, Mar. 1970, pp. 251-255.

Primary Examinerl-lerbert T. Carter Attorney, Agent, or Firm-Shanley, ONeil and Baker [571 ABSTRACT In recovering hexavalent chromium from chrome plated steel strip which is wetted with plating solution as a result of dragout from the plating process, an aqueous solution containing hexavalent chromium in relatively low concentration is concentrated in a process comprising treating in a first cation exchange Zone, treating in an anion exchange zone, regenerating the anion exchange zone and treating the effluent of regeneration in a cation exchange zone different from the first cation exchange zone. The concentrated solution is passed to a storage zone and recirculation is carried out between the storage zone and the plating Zone and solution is passed from the storage zone to an evaporation zone and more concentrated solution is passed from the evaporation zone to the storage zone. In regenerating the anion exchange zone, only a portion of the hexavalent chromium is removed. I

8 Claims, 2 Drawing Figures REGENERATING ANION EXCHANGE ZONE CONTAINING HEXAVALENT CHROMIUM BACKGROUND OF THE INVENTION This invention relates to regenerating an anion exchange zone incident to recovering hexavalent chromium and increasing the concentration of the same in aqueous solution. Such allows practical use for plating of chromium values which would otherwise be lost.

Methods have been disclosed for recovery of hexavalent chromium involving washing plated articles wetted with chromium-containing solution, recovering an aqueous solution containing hexavalent chromium in relatively high concentration, recovering an aqueous solution containing hexavalent chromium in relatively low concentration, concentrating low concentration solution by ion exhange and regeneration of an anion exchanger, and concentrating by evaporation, In this regard, see Paulson et al, Plating, pages 1000l009, September, I953; Culotta et al, Plating, pages 251-255, March, 1970; Zievers et al, US. Pat. No. 3,658,470.

It is an object of this invention to provide a novel method of regenerating an anion exchange zone in which resin in the hydroxide form has been used to remove hexavalent chromium in the form of anions from a solution, whereby to maximize the concentration of hexavalent chromium in the effluent provided by regeneration,

This object and others will be evident from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B constitute a flowsheet depicting a process for chrome plating steel strip and for recovering hexavalent chromium from the solution wetting the plated strip for reuse and illustrate the inventive concepts herein.

DETAILED DESCRIPTION With continuing reference to FIGS. 1A and 1B of the drawings, steel strip having been preliminarily treated, for example in electrolytic cleaning and pickling steps, follows a travel path and passes successively through

chrome plating tanks

12, 14, 16 and 18 which define a plating zone. In each of the plating

tanks

12, 14 and 16, the strip follows the travel path so as to enter a plating tank in a substantially horizontal plane, then passes over a contact roll and advances downwardly, then passes under a sink roll and advances upwardly, then passes over another contact roll and leaves the tank in a substantially horizontal plane. In the plating tank 18, the strip follows the travel path so as to enter the tank in a substantially horizontal plane, then passes over a contact roll and advances downwardly, then passes under a sink roll and advances upwardly, then passes over a deflector roll and leaves the tank in a substantially horizontal plane. In FIG. 1A the contact rolls at the strip entrance sides of

tanks

12, 14, 16 and 18 are respectively denoted 12a, 14a, 16a, and 1841'; the sink rolls are respectively denoted 12b, 14b, 16b, and 18b; the contact rolls at the exit sides of the

tanks

12, 14 and 16 are respectively denoted 12c, 14c, and 16c; and the deflector roll at the exit side of tank 18 is denoted 180. The

contact rolls

12a, 14a, 16a, 18a, 12c, 14c, and 160 perform a deflecting function and also render the strip cathodic. At the entrance end of each of the tanks 12,

l4, l6 and 18, snubber rolls respectively denoted 12d, 14d, 16d and 18d force the strip against adjacent contact rolls to provide good contact.

In the first three tanks, that is

tanks

12, 14 and 16, there are two sets of anodic grids which are vertically oriented with one of the sets being between the contact roll at the entrance end of a tank and a sink roll and the other set being between a sink roll and the contact roll at the exit end of a tank so that the strip passes through one set of grids on its downpass through a tank and through the other set on its up pass. The grids for the downpass are respectively denoted 122, Me and 16e, and the grids for the up pass are respectively denoted 12f, 14f and 16f. In the fourth tank, tank 18, there is only one set of vertically oriented grids, and this set is between the contact roll at the strip entrance end of the tank and the sink roll 50 that the strip passes through this set on the downpass and does not pass between any grids on the up pass; this set of grids is denoted 186.

The electrolyte in each of the

tanks

12, 14, 16 and 18 is the same, namely an aqueous solution of chromic acid, and is depicted as a body ofliquid in each tank extending to a level above the anodic grids. The chemistry of chromic acid is quite complicated and what exactly is present is not known. However, at the pH ofless I than 1 which is characteristic of the electrolyte utilized,

the chromic acid is mostly in the form of dichromic acid (H Cr O The concentration in the electrolyte of the chromic acid expressed as grams of CrO per liter is in the range of to 250. Small amounts of silicofluorides (magnesium, sodium or potassium) and sulfate in the form of sulfuric acid are typical additives.

Prewet sprays 19 are present at the entrance end of tank 12 upstream of contact roll 12a to spray plater solution, that is electrolyte, at each side of strip before it reaches the contact roll, The reason for this is to put a good conducting medium on the strip so that it will make good contact with the contact roll thereby negating arcing between the contact roll and the strip.

The strip having been chrome plated exits from tank 18 and follows travel path 10 through a tank 20. The strip enters the tank 20 in a horizontal plane, passes over a deflector roll 20a, then turns downwardly traveling vertically, then passes under a sink roll 20b, then advances vertically upwardly and passes over a deflector roll 20c and then advances horizontally out of the tank. The tank 20 contains the same solution as each of the

plating tanks

12, 14, 16 and 18 but no electrolytic action is imparted during the passage of the strip through this solution. Passage of the strip through the tank 20 has the effect of removing objectionable oxide coating from the strip.

At the exit side of tank 20 downstream of deflector roll 200 is a pair of wringer rolls 22 between which the strip passes. These rolls 22 wipe plater solution from the strip to minimize dragout of electrolyte from the plating process.

The

tanks

12, 14, 16, 18 and 20 are continuously supplied with electrolyte, that is plater solution, respectively through valved

lines

12g, 14g, 16g, 18g and 20g. These lines are supplied with plating solution by a header 24. Header 24 also communicates with valved lines 26a and 2612 which in turn communicate with sprays 19. The header 24 communicates with and is fed by line 28 which in turn communicates with and is fed by valved line 30 which in turn communicates with plater solution storage tank 32 which defines a plater solu- I from solution wetting the strip.

tion storage zone. A pump 34 is provided in line 28 to continuously move the plater solution from tank 32 via lines 30 and 28 and into header 24 and from there via

lines

26a and 26b and 12g, 14g, 16g, 18g and 20g into tanks l2, l4, 16, 18 and 20. A valved line 36 communicates with line 30 so that plater solution can be drained from the system. Overflow outlets are provided in each of the

tanks

12, 14, 16, 18 and 20, and plater solution continuously overflows through these outlets through the

respective lines

12h, 14h, 16h, 18h and 2011. These overflow lines communicate at their downstream ends into a main exit line 38 which communicates with another line 40 which in turn communicates with the plater storage tank 32. Thus, there is provided between tank 32 and between

tanks

12, 14, 16, 18 and 20 a recirculation loop whereby recirculation is continuously carried out between the storage zone defined by tank 32 and the plating zone defined by

tanks

12, 14, 16 and 18.

The plater storage tank 32 is also in a recirculation loop with a heat exchanger 42 defining a heat exchange zone with .a valved line 44 communicating between tank 32 and heat exchanger 42 and line 46 leading back from heat exchanger 42 into storage tank 32. Line 44 contains a pump 48 whereby plater solution is recirculated through this loop. Water is passed through heat exchanger 42 countercurrent to the plater solution as indicated by lines 50a and 50b to remove heat from the plater solution (that is, from the plating solution storage zone defined by tank 32). This is done because there is a heat buildup in the plating portion of the system due to the grid current. In other words, the heat gained because of the current supplied by the grids exceeds the heat lost through the tanks and pipes of the system. Inasmuch as the best cathode efficiency is achieved utilizing a plater solution having a temperature of 105 to 125F., control of heat buildup is desirable.

The chrome plated strip having had undesirable oxides removed in tank 20 and having passed through wringer rolls 22 still is wetted with a significant amount of plating solution comprising aqueous chromic acid solution. So as to recover the hexavalent chromium from such solution, the strip is advanced along travel path through a first rinse tank 52 and a second rinse tank 54. The strip is then passed through a chemical treatment tank 56. The strip is then passed through a third rinse tank 58 to recover hexavalent chromium The tank 52 contains a strip entrance end deflector roll 52a, a sink roll 52b and a strip exit end deflector roll 52c and the strip following travel path 10 turns downwardly over roll 52a and moves vertically downwardly turning under roll 52b and then advances vertically upwardly over roll 52c whereupon it leaves tank 52.

The tank 54 contains a strip entrance end deflector roll 54a, a sink roll 54b and a strip exit end deflector roll 54c. The strip entering tank 54 after passage out of tank 52 turns downwardly over roll 54a and moves vertically downwardly, then turns under roll 54b, and then advances vertically upwardly and turns over roll 54c and moves out of tank 54.

The tank 56 contains a strip entrance end contact roll 56a, a sink roll 56b and a strip exit end contact roll 56c. The strip exiting from tank 54 passes over roll 56a and then passes vertically downwardly, then passes under roll 56b, then advances vertically upwardly and passes over roll 56c exiting from tank 56 horizontally.

The tank 58 contains a strip entrance end deflector roll 58a, a sink roll 58b and a strip exit end deflector roll 58c. The strip exiting from tank 56 passes into tank 58 and over roll 58a, then advances vertically downwardly, then passes under roll 58b, then passes vertically upwardly, then passes over roll 58c and exits horizontally from tank 58.

In

tanks

52 and 54, the strip is subjected to rinsing to wash off plater solution on the strip by dragout from the

plater tanks

12, 14, 16 and 18 and from tank 20. In the chemical treatment tank 56, the strip is subjected to electrolytic treatment to provide a hexavalent chrome oxide coating. In tank 58, washing is provided to remove solution on the strip by dragout from the chem treat tank 56.

In the chemical treatment tank, the electrolyte is preferably aqueous chromic acid solution containing 35 grams per liter of chromic acid expressed as CrO Electrolytic action is supplied in this tank, and it oper ates and is constructed generally as one of the

plater tanks

12, 14, 16 or 18 with vertically oriented

anodic grids

56e and 56f on either side of the strip respectively on a downpass and on an up pass and contact rolls 56a and 56c rendering the strip cathodic and snubber roll 56d forcing the strip against contact roll 56a at the inlet to the tank. As with the plater tanks, a dc current is supplied. The electrolyte is initially supplied into the tank 56 from a chemical treatment storage tank, not depicted, via valved line 60. Solution is recirculated between the chemical treatment tank and the chemical treatment storage tank during the progress of the process with electrolyte flowing from the storage tank into tank 56 via line 60 and returning to the chemical treatment tank by overflow gravity feed via line 62. The recirculation loop includes a heat exchanger, not depicted, where recirculating solution passes in indirect heat exchange with hot water to heat the recirculating liquid and control the temperature of solution in the storage tank, for example to 105. A pair of wringer rolls 64 on either side of the strip at the exit end of tank 56 wipe electrolyte from the strip and minimize the amount of the electrolyte leaving the tank 56 by dragout on the strip.

The

tanks

52, 54 and 58 make up an interconnected washing system with washing solution passing counterflow to the strip wherein the strip is washed in a first washing step to recover an aqueous solution comprising a relatively high concentration of hexavalent chromium in the form of anions.

On startup, rinse solution, preferably demineralized water is introduced into tank 52 via valved line 66. Also, rinse solution is initially introduced into tank 54 through valved line 68 and into tank 58 through a valved line 70.

Lines

68 and 70 are supplied through header 72 which is supplied from a rinse water storage tank, not depicted.

During the process, demineralized water is added continuously into tank 58 through spray nozzles 74 which are supplied by line 76 containing valve 78. The spray nozzles 74 are on either side of the strip in tank 58 on its up pass just prior to its passage over roll 58c. Flow through the spray nozzles 74 is perferably controlled to maintain a CrO concentration in tank 54 equal to the CrO concentration in tank 56 by manually setting the flow through the nozzles and controlling valve 78 in response to periodic chemical tests carried out on tank 54 solution. These sprays aid in washing off residual electrolyte, that is aqueous hexavalent chromium containing solution, on the strip by dragout from tank 56. Washing action is also provided by a bath 71 in the bottom of tank 58 through which the strip passes in its downpass and in its up pass through tank 58. The demineralized water introduced through the-sprays 74 and solution washed off of the strip by these sprays falls by gravity into the lower portion of tank 58 and becomes part of the bath 71. Solution continuously overflows through an overflow outlet in tank 58 providing a constant level in that tank.

The liquid leaving tank 58 through its overflow outlet enters aline 80 and flows therethrough by gravity into a collection tank 82. The level of liquid in, tank 82 is sensed by lever sensor 84. A level controller 86 operates in response to the level sensed, and a pump 88 operating in response to a set point on the controller 86 operates to pump solution from tank 82 via a line 90 into tank 54.

The tank 54 has an overflow outlet, and this acting in concert with addition of solution through line 90 provides a substantially constant level bath in tank 54.

The strip passing through tank 54 is washed by the bath.

on its downpass and up pass through tank 54 whereby aqueous hexavalent chromium containing solution on the strip by dragout from the plating tanks and tank 20 is rinsed therefrom.

Solution passes out of the overflow outlet in tank 54 and passes via a line 92 into tank 52. A substantially constant level of rinse solution is maintained in the lower portion of tank 52 as a result of an overflow outlet in that tank. The bath of rinse solution in tank 52 provides washing of the strip on its downpass from roll 52a to roll 52b and on its up pass from roll 52!; to roll 52c whereby aqueous hexavalent chromium containing solution on the strip by dragout from the plating tanks and tank 20 is washed therefrom.

Solution overflows through the outlet in tank 52, flows by gravity through a line 94 and then into main line 38 and through line 40 into plater storage tank 32.

The concentration of chromic acid expressed as CrO in the baths in each of the

tanks

52, 54 and 58 is as follows: in the bath in tank 52, 85 to 100 grams per liter; in the bath in tank 54, 30 to 40 grams Per liter, perferably 35; in the bath 71 in tank 58, 5 to gram per liter, preferably 10. These concentrations are controlled by control of valve 78.

Following its exit from tank 58, the chrome plated strip still wetted with some aqueous hexavalent chromium containing solution, for example one-tenth of the amount removed in the prior washing in

tanks

52, 54 and 58 continues along travel path 10 through a spray washer 96 depicted in FIG. 1B wherein the strip is washed in a second washing step to recover an aqueous solution comprising cations (for example, iron and trivalent chromium ions) and a relatively low concentrationof hexavalent chromium in the form of anions.

The spray washer comprises four open

top compartments

98, 100, 102 and 104 in series along its length with compartment 98 at its strip entrance end, followed by compartment 100, followed by compartment 102 and finally followed by compartment 104 at its strip exit end. A wall 106 separates compartments 98 and 100. A wall 108 separates

compartments

102 and 100. A wall 110 separates

compartments

104 and 102. The

path and is horizontally oriented. There are five sets of wringer rolls that guide the strip over the compartments in the horizontal path. These sets of wringer rolls are respectively denoted 98a, 100a, 102a, 104a and l04b with set 98a being at the strip entrance end of the washer over compartment 98, set 100a being over compartment 100 near wall 106, set 102a being over compartment 102 near wall 108, set 104a being over compartment 104 near wall 110 and l04b being near the strip exit end of the washer 96.

Over each of the

compartments

98, 100, 102, 104 there are two spray nozzles, one below the strip and one above the strip so that the strip is sprayed from above and below. The spray nozzles above tank 98 are denoted 98c; those above compartment 100 are denoted l00c; those above compartment 102 are denoted 102C and those above compartment 104 are denoted 104C. Spray nozzles 1040 are supplied with demineralized water by valved line 112. The water from the spray nozzles 104C washes residual solution containing hexaof compartment 102 through a valved line 114 by a pump 116 to the spray nozzles 1020. The solution from the nozzles 102 washes residual solution containing hexavalent chromium from the strip and resultant solution falls into compartment 102. The solution in compartment 102 accumulates to the level of the top of wall 108 and then continuously cascades over this wall into compartment 100. Solution from the bottom of compartment flows through a valved line 118 and is pumped by a pump 120 to spray nozzles 100;. Solution from the spray nozzles 100s washes residual solution containing hexavalent chromium from the strip and the resultant solution falls into compartment 100. Solution accumulates in compartment 100 to fill that compartment and thereafter cascades over wall 106 into compartment 98. Solution flows from the bottom of compartment 98 through a valved line 122 and is pumped by a pump 124 to spray nozzles 98c. Solution from the spray nozzles 98c washes residual solution containing hexavalent chromium from the strip and the resultant liquid falls into compartment 98. Solution overflows from compartment 98 into line 126.

. The spray washer 96 constitutes a counterflow spray washing system whereby solution on the strip containing hexavalent chromium is washed therefrom and a solution comprising cations (for example, iron and trivalent chromium ions) and a relatively low concentration of hexavalent chromium, that is, for example, from 0.1 to 0.3 grams per liter of chromic acid expressed as CrO overflows into line 126. When the strip leaves washer 96, it contains only trace or no amounts of chromic acid wetting it.

The strip exiting from the spray washer is dried and coiled.

Merging with line 126 is a line 128 containing a metering pump 130. The upstream end of line 128 communicates with a chrome waste storage tank, not shown in the drawings. Various of the tanks can be emptied into the chrome waste storage tank, for example on shut down, to capture chrome laden water which can be metered by pump 130 into line 126 for chrome recovery.

Solution from line 126 enters a collection tank 132 having an exit line 134.

Valved line 133 is provided communicating with tank 132 for the passage thereto of chrome laden water from sources other than compartment 98 and line 128; for example, chrome laden water from washing strip exiting from a chemical treatment step in an electrotinplating line can be passed into tank 132 through line 133.

The

valved lines

128 and 133 constitute means for admixing aqueous solution recovered from the second washing step with hexavalent chromium containing solution from other sources to provide in tank 132 an admixture which is an aqueous solution comprising cations and a relatively low concentration of hexavalent chromium in the form ofanions.

Solution is pumped from tank 132 by a pump 136 in line 134 in response to a level control 138 operating a valve 140 in line 134.,The pump 136 runs continuously and the level control operates to close the valve more if the level in the tank drops. This setup operates to provide continuous flow through line 134.

The solution from line 134 flows through heat exchanger 142 where it is cooled against the countercurrent flow of cold water as denoted by arrows 142a and 142b. Cooling is to a temperature suitable for treatment of the solution in the ion exchange system which is described below.

Solution leaves the heat exchanger 142 via a line 144 and passes through a filter 146 which removes particles of solid material. The filtered solution flows through line 148 into an ion exchange system which is described below.

The ion exchange system comprises three sets of ion exchangers, each set consisting first of a cation exchanger defining a first cation exchange zone and then of an anion exchanger defining an anion exchange zone. The cation exchangers are denoted 150a, 1501: and 1500. The anion exchanger associated with cation exchanger 150a is denoted 152a; the anion exchanger associated with cation exchanger 15012 is denoted 152b," and the anion exchanger associated with cation exchanger 1500 is denoted 1520.

Valved lines

154a, 154b and 1540 respectively communicate between line 148 and

cation exchangers

150a, 150b, and 1500. Line 156a communicates between cation exchanger 150a and anion exchanger 152a; line 1561) communicates between cation exchanger 15011 and anion exchanger 15211; and line 1560 communicates between cation exchanger 1500 and anion exchanger 1520. The

lines

1560, 156b and 1560 each contain valves. Three sets of the ion exchangers are provided so that two sets can be on line while the exchangers of the other set are being regenerated.

Resins for use in the

cation exchangers

150a, 150b and 1500 are of the strong acid type and are in the hydrogen form. They contain sulfonic acid functional groups and are prepared, for example, by the nuclear sulfonation of styrenedivinylbenzene. These resins are not as highly oxidative resistant as the resins utilized in the cation exchanger for treating regeneration effluent which is described later. Suitable cation exchange resins are, for example, Amberlite IR-l20 or Amberlite IR-l20 Plus manufactured by Rohm and Haas Company.

The ion exchange resin for use in the anion exchangers is a strong base anion exchange resin of the quaternary ammonium type. It is utilized in the hydroxide form. A suitable resin is Amberlite IRA-900 which is supplied in the chloride form and is converted to the hydroxide form for use; it is available from Rohm and Haas Company.

Solution flows from line 148 into whichever two of the cation exchangers a, 15% and 1500 are on stream. The cation exchangers take out cations, including trivalent chromium and iron ions and replace them with hydrogen ions. Effluent from a cation exchanger is aqueous solution containing hexavalent chromium in the form of anions, other anions and hydrogen ions and flows into an associated

anion exchanger

152a, 152b or 1520. The anion exchangers take out hexavalent chromium in the form of anions and other anions and replace these with hydroxyl ions. Effluent from the anion exchangers is demineralized water. Valved exit lines for flow of effluent from anion exchangers 152a, 152b and 1520 are respectively denoted 158a, 158k and 1580.

The

lines

158a, 158b and 1580 each communicate with a main line 160 whereby demineralized water effluent fiows into a demineralized water storage tank 162. Included in the line 160 is a device 164 for deaerating the demineralized water. The storage tank 162 has a valved exit line 166 for flow of water to lines 112 (see FIG. 1B) and 76 (see FIG. 1A). A pump 168 is provided in line 166 to pump the water through line 166.

When a set of ion exchange units is to be regenerated, it is isolated from flow from line 148 and from line 160 by closing the appropriate valves, and another set of ion exchange units is brought into communication with line 148 and line 160.

Regeneration of the cation exchange unit in a set of units is carried out, for example as suggested by the manufacturer of the particular resin utilized, and the particular method of regeneration forms no part of the present invention. Regeneration of a cation exchange unit can be carried out for example, by backwashing, introducting 10% sulfuric acid as a regenerating agent and rinsing.

Regenerating of an anion exchange unit involves a regeneration cycle comprising backwashing, removal of residual backwash water, introduction of regenerating agent, removal of resulting solution, introduction of rinse water and removal of resulting solution, introduction of water, and recirculation. For simplification purposes some valves associated with these steps are not depicted. For further simplification, the state of each valve in the system at each point in time during the cycle will not be described; the conduits described as functioning at a particular point in time are open or have valves positioned as otherwise described and the other conduits in the regeneration system at that point in time are closed unless otherwise stated. The regeneration is carried out to produce an aqueous effluent comprising cations (from the regenerating agent) and a concentration of hexavalent chromium in the form of anions substantially higher than the relatively low concentration leaving compartment 98 or in tank 132. Re-

generation of an anion exchange unit is described in detail below.

Backwashing of the anion exchange column is carried out to fluff up the resin and remove extraneous solids and resin fines from the resin bed to permit good contact between the resin and regenerating agent so that regeneration is carried out efficiently. Backwash water having cations removed therefrom, for example by passage through the cation exchanger associated with the anion exchanger being regenerated, is introduced into the bottom of the anion exchange column (the flows other than backwash flows through the various ion exchange columns herein are from the top as is conventional; however, the direction of flow constitutes no part of the present invention). Piping from the introduction of backwash water is represented by valved line 169a, and piping for outlet of backwash water is represented by valved line 169b, both depicted as associated with column 1526. Corresponding piping is provided for columns 152a and 15212 but such is not depicted. When backwashing has been completed, the valve in the appropriate backwash water introduction line (for example 169a) is closed. Residual backwash water is removed from the anion exchanger utilizing a pressurized air purge. An air supply line 170 supplies pressurized air for this purpose with communication between line 170 and

exchangers

152a, 152b and 1526 being provided respectively by valved lines 172a, l72b and 1720. Solution leaves the column during backwashing and the subsequent purging via backwash outlet line (for example, line 16%) and is routed to waste disposal.

After backwash water has been so purged, pressurized air flow is stopped by closing the valve in the appropriate line 172a, 172b or 172C and the valve in the appropriate backwash outlet line, for example line 169b, is closed. The anion exchanger being regenerated is then vented to the atmosphere by means of a valve not depicted. Then, regenerating agent is flowed into the anion exchanger. For this purpose, a regenerating agent supply line 174 is provided, and it communicates respectively with

anion exchangers

152a, 152b and 1526 by means of

valved lines

176a, 176b and 176C. The regenerating agent comprises alkali metal hydroxide, for example, potassium or sodium hydroxide or mixtures thereof. Preferably, the regenerating agent comprises sodium hydroxide utilized in aqueous solution at a concentration of by volume. The alkali metal hydroxide reacts with the resin to provide resin in the hydroxide form and release anions containing hexavalent chromium and produce alkali metal salts containing hexavalent chromium in a negative radical, that is alkali metal chromates and dichromates. The amount of alkali metal hydroxide introduced is based on calculations of how much chromium entered and therefore is in a particular column with a sufficient amount being added so that when solution is removed from the column as described hereafter as the effluent of regeneration as defined hereafter, the effluent of regeneration will contain from about 90 to about 95% of the hexavalent chromium in the column just prior to initiation of regeneration. When the appropriate amount of regenerating agent has been introduced, flow of such into the column is stopped, and liquid is forced from the column utilizing a pressurized air purge. The pressurized air for the air purge is supplied through line 170. When the air purge has forced the liquid from the column that readily is removed thereby, air introduction is continued and rinse water is simultaneously routed into the column for a predetermined time and is forced therefrom by the pressurized air. Rinse water flow is provided through piping described below. The streams leaving the column both as a result of purging of liquid in the column from addition of regenerating agent and as a result of purging liquid from the rinse water introduction make up the effluent of regeneration. These streams leave the columns 1520, 152b and 1520 respectively through

exit lines

178a, 178b and 1786. Thus, alkali metal hydroxide is introduced to provide an aqueous effluent from the column containing cations and from aboout 90 to about 95% of the hexavalent chromium present in a column just prior to initiation of regeneration and to leave in the anion exchange column the remainder of the cations and hexavalent chromium. Preferably, the amount of hexavalent chromiun removed from the column in the effluent is about 95% of that present just prior to initiation of regeneration.

The

exit lines

178a, 178b and 178C communicate respectively with

three'way valves

180a, 18% and 1800. During forcing of effluent of regeneration from a column, the appropirate three-way valve is positioned so 4 as to route liquid into a respective communicating

line

182a, 182b or 1826. The lines 182a, 18211 and 1820 communicate with a main line 184.

After the effluent of regeneration has left the column being regenerated and has been routed to line 184, the pressurized air supply is stopped by closing of the valve in the appropriate line 172a, l72b or 172e, but water introduction is continued to fill the column and simultaneously with the shutting off of the air supply the appropriate three-way valve a, 1801) or 1800 is repositioned so as to connect the respective line 178a, l78b or 1786' into a recirculation loop described below. Piping for this water introduction as well as the rinse water introduction to provide part of the effluent of regeneration comprises a main water line 186 which in turn communicates with valved branch lines 188a, 188!) and 1886 which respectively communicate with cation exchanger entrance lines a, 19% and 190C with 188a and 190a being associated with exchanger 150a, 188b and 19% being associated with exchanger 15012 and 188C and 1906 being associated with exchanger 1500. The piping for the water addition system also includes a pipe 192a connecting exchangers 150a and 152a, a pipe 192b connecting exchangers 150i) and 152b, a pipe 192C connecting exchangers 150C and 1526, the pipes 192a, 19212 and 1926' each contain valves which are not depicted. Thus, in introducing water into anion exchanger 152a, thewater from main line 186 enters valved line 188a and then line 190a, then passes through cation exchanger 150a, then through line 192a and into anion exchanger 152a to fill the same. The exchangers 15211 and 1520 are filled with water in corresponding fashion when they are in a regeneration cycle. The passage of the water through the cation exchanger before entry into the anion exchanger provides the advantage of removing cations therefrom including divalent cations such as calcium and magnesium ions so that service water can be utilized without danger of hardness in the water clogging the exchanger system. In other words, the water can be drawn from a source of water containing divalent cations (for example, calcium and magnesium ions) since these cations are removed prior to entry of the water into the anion exchanger.

Once the anion exchange column has been filled with water, water addition is stopped by closing the valve in the

appropriate line

188a, 188b, 1880, the appropriate vent valve is closed, and recirculation between the anion exchange column and its associated cation exchange column is started. The recirculation loop including

ion exchange columns

150a and 152a consists of line 178a, a line 194a containing a pump 196a, valve 180a joining

lines

178a, 182a and 194a and positioned to provide flow between

lines

178a and 194a, line 190a communicating at its upstream end with the downstream end of line 194a, and line 192a. The recirculation loop including ion exchange columns 150b and l52b consists of line 178b, a line 194b containing a pump 196b, valve

180b joining lines

178b, 182k and 194b and positioned to provide flow between

lines

178b and 194b, line 190b communicating at its upstream end with the downstream end of line 194b, and line 192b. The recirculation loop including ion exchange columns 150c and 1520 consists of line 1780, a line 194c containing a pump 196e, valve 1800 joining

lines

178c, 182c and 194C and positioned to provide flow between lines 1780 and 194e, line 1900 communicating at its upstream end with the downstream end of line 1940, and line 1920. To carry out recirculation, the

appropriate pump

196a, 196b or 1966 is operated. Recirculation is carried out between the anion exchanger and the cation exchanger in a loop until anions including the hexavalent chromiun containing anions in the anion exchanger being treated react with the anion exchanger resin in that exchanger and cations react with the resin in the cation exchanger.

Conductivity sensors

198a, 198b and 198c are provided respectively sensing in

lines

178a, 178b and 178C to indicate when reaction has been completed.

After reaction has been completed, recirculation is stopped by stopping the

appropriate pump

196a, 196b or 1960.

Preferably, the regeneration cycle including backwashing, air purging, introduction of regenerating agent, removal of resulting solution, introduction of rinse water and removal of resulting solution, introduction of water and recirculation is carried out automatically so that the various valves including the valves in

lines

188a, 188b, 188C, 158a, 158b and 1586, 169a, 16% and the

valves

180a, 18% and 1806 are operated automatically in accordance with a predetermined schedule.

The liquid passing through line 184 comprises an aqueous solution containing cations (alkali metal ions from the regenerating agent for the anion exchange zone) and a concentration of hexavalent chromium in the form of anions substantially higher than the relatively low concentration leaving compartment 98 or in tank 132. It flows into a holding tank 200. Intermittently liquid is pumped from tank 200 by pump 202 through a valved line 204 through a cation exchange column 206 referred to hereafter as cation exchanger 206, exchanger 206 and column 206 (see FIG. 1A). Effluent from the cation exchanger passes into plater solution storage tank 32 via a line 208. When the liquid passes through the cation exchanger 206, cations in the liquid react with a resin in the exchanger and are exchanged for hydrogen ions, and the effluent from the cation exchanger is an aqueous solution of chromic acid containing, for example to 50 grams per liter of chromic acid expressed as CrO that is an acidic aqueous solution comprising a concentration of hexavalent chromium in the form of anions substantially higher than the concentration of such in the stream leaving compartment 98 through line 126 or in tank 132. Me ters 210a and 210b (pH meters) are provided on either side of exchanger 206 for the easy determination of whether the resin bed in exchanger 206 is depleted, that is whether the resin in the exchanger is spent and needs to be regenerated.

The ion exchange resin utilized in cation exchanger 206 is of strong acid type and is used in the hydrogen form. It contains sulfonic acid functional groups in a polymer matrix and is prepared, for example by the nuclear sulfonation of styrene-divinylbenzene. The resin utilized is highly resistant to oxidation because the concentration of chromic acid to which the column is exposed is such as to be highly oxidative. The resin contains a relatively high level of cross-linking to provide such oxidation resistance. A suitable resin is sold under the tradename Amberlite 200 by Rohm and Haas; this resin is sold in the sodium form and is converted to the hydrogen form for use.

The solution leaving compartment 98 through line 126 contains, for example, 0.1 to 0.3 grams per liter of chromic acid expressed as CrO and the solution leaving the exchange system through line 208 contains for example 10 to 50 grams per liter of chromic acid expressed as CrO Thus, the ion exchanger system serves to concentrate the chromic acid solution so that is can be treated as described hereafter so as to be suitable for reuse in the plater tanks.

An evaporation system is provided to control the volume of liquid in the system and to remove water from the system inasmuch as liquid from line 94 comprises chromic acid at a concentration of, for example, to grams per liter of chromic acid expressed as CrO and the liquid from line 208 has a concentration of chromic acid expressed as CrO of, for example, 10 to 50 grams per liter. In other words, the water added via line 76 and the excess water added into the system through line 174 has to be removed to concentrate solution to plating strength. The evaporation system comprises an evaporator 211 defining an evaporation zone where solution is heated against steam passing in indirect heat exchange relation as indicated by

lines

212a, and 212b. The evaporator has a concentrated liquid exit line 214 anda vapor exit line 216. Also communicating with thee vaporator is a liquid feed line 218 containing a pump 220 and a valve 222 and communicating at its upstream end with the bottom of plating tank 32. In utilizing the evaporator, solution from plater storage tank 32 is passed from the plater storage tank to the evaporator and concentrated solution is passed from the evaporator to the plater storage tank. More particularly, the liquid is pumped by pump 220 from the lower portion of plater storage tank 32 through line 218 into the evaporator where is is heated indirectly against steam passing as indicated by lines 212a and 21217 and subjected to vacuum whereby water is flashed off leaving through line 216 (such water is condensed and disposed to waste) and concentrated liquid leaves the evaporator via line 214 and is fed back into plater storage tank 32.

An inventive concept herein concerns the addition of regenerating agent to provide an effluentcontaining from about 90 to about 95% of the hexavalent chromium present in the anion exchange resin just prior to initiation of regeneration and then recirculating the remaining to hexavalent chromium and reacting it with the anion exchange resin to establish a continuing level of hexavalent chromium in the resin. This procedure maximizes the concentration of hexavalent chromium in the solution flowing into tank 200 from line 184. Once the 5 to 10% level of hexavalent chromium is established in the resin, additional chromium need not be utilized to establish this level during the life of the resin; therefore, the establishment of this level is provided with minimum usage of'hexavalent chro EXAMPLE The system of FIGS. 1A and 1B is utilized. The system is generally operated as described above. The specities are presented below.

Steel strip (36 inch width) having been preliminarly treated in electrolytic cleaning and pickling steps is passed through the system. Chrome plating is applied in tanks 12, l4, l6 and 18. Objectionable oxide coating is removed in tank 20. The plated strip is washed in

tanks

52 and 54. In tank 56, hexavalent chrome oxide coating is applied. In tank 58 the strip is washed. Finally the strip is further washed in spray washer 96. The strip exiting from washer 96 is ready for drying. The line speed is 1,550 feet per minute.

Each of the tanks l2, l4, 16, 18 and 20 contains 1,700 gallons of plater solution. The plater solution is an aqueous solution comprising 150 grams per liter of chromic acid expressed as CrO -1 gram per liter of sulfuric acid expressed as S0 and 3 grams per liter of metal silicofluorides expressed as SiF 100 gallons per minute of plater solution enters each tank and 100 gallons per minute of plater solution leaves each tank. Of the 100 gallons per minute entering tank 12, 2. gallons per minute enters through prewet sprays 19. The temperature of the plater solution is 1 Tank 52 contains 800 gallons of aqueous solution containing about 90 grams per liter of chromic acid expressed as CrO Solution enters and leaves tank 52 at the average rate of about 1.5 gallons per minute.

Tank 54 contains 1,700 gallons of aqueous solution containing about 35 grams per liter of chromic acid expressed as CrO Solution enters and leaves tank 54 at the average rate of about 1.5 gallons per minute.

Tank 56 contains 1,700 gallons of aqueous solution containing chromic acid at a concentration of about 35 grams per liter expressed as CrO 0.15 grams per liter of sulfuric acid expressed as $0., and 0.6 grams per liter of metal silicofluorides expressed as SiF Solution passes in and out of tank 56 at a rate of approximately 150 gallons per minute. The temperature of solution in tank 56 is 120.

Tank 58 contains 800 gallons of aqueous solution. This solution has a concentration of chromic acid expressed as CrO of 10 grams per liter. 1.5 gallons per minute of solution continuously pass in and out of this tank.

In washer 96 20 gallons per minute of demineralized water enters through sprays 104c; 20 gallons per minute overflows from compartment 104 to compartment 102; 20 gallons per minute is sprayed by sprays 1020; 20 gallons overflows from compartment 102 to compartment 100; 20 gallons per minute is sprayed through sprays 100C, 20 gallons per minute overflows from compartment 100 to compartment 98; 20 gallons per minute is sprayed through sprays 98c; and 20 gallons per minute leaves through line 126. the water entering through sprays 104C is at a temperature of 180. The use of water at this temperature aids in strip washing and drying.

The solution leaving through line 126 has a concentration of chromic acid expressed as CrO of 0.2 grams per liter. The concentration of chromic acid expressed as CrO in compartment 104 is in low parts per million. The strip leaving washer 96 contains trace or no amounts of chromic acid.

. Tank 132 is a 6,000 gallon tank and controller 138 operates valve 140 to maintain a continuous stream of liquid through pipe 134. This stream of liquid is an aqueous solution having a concentration of hexavalent chromium in the form of anions expressed as CrO of 0.2 grams per liter.

The heat exchanger 142 operates to cool the solution from pipe 134 to 100.

The solution having been so cooled is routed to two of the three

cation exchangers

150a, 150b and 1506. These exchangers and each of the other ion exchangers in the system are 10 feet high and 54 inches in diameter and of the conventional type where the ion exchange resin is maintained upon a screen which is positioned in the bottom of the exchanger, the exchanger is vertically oriented and the solution to be treated enters the top and leaves the bottom. The ion exchange resin utilized in the exchangers 150a, l50b and 1506 is Amberlite IR- Plus, and it is utilized in the hydrogen form. The effluent from the two cation exchangers (of a, l50b and 1500) that are on stream is passed to the two anion exchangers which communicate with such cation exchangers. The

anion exchangers

152a, 152b and 152C contain as an ion exchange resin Amberlite IRA-900. This resin is supplied in the chloride form and is converted to the hydroxide form for use. The effluent from the anion exchangers is demineralized water which is routed to storage tank 162.

. Every eight hours one set of cation and anion exchangers is taken out of the line and a fresh set of cation and anion exchangers is put in the line. For example, if

cation exchangers

150a and 150b and

anion exchangers

152a and 152b are being utilized and the resin in

exchangers

150a and 152a is the most depleted, exchangers 150a and 152a are taken out of the line and exchangers 150C and 152c are put in the line.

The cation exchanger taken out of the line is regenerated prior to regenerating the associated anion exchanger. The regeneration comprises backwashing, then treating with a regenerating agent consisting of 10% aqueous sulfuric acid and then rinsing. Flow rates and times are in accordance with the resin manufacturers recommendations. i

The anion exchanger which is taken out of the line is regenerated first by backwashing for 10 minutes utilizing service water having cations removed therefrom introduced through the appropriate backwash water introduction line. When the ten-minute time period has ended, the valve in the backwash water line is closed automatically; then the appropriate valve in the air purge line (that is, the valve in the appropriate branch line 172a, l72b or 172a) is automatically opened and the residual backwash water is forced from the anion exchanger through the appropriate backwash water outlet line. After the valve for air purging has been open for 10 minutes, it automatically closes and the anion exchanger is automatically vented to the atmosphere.

Then the valve in the appropriate regenerating agent introduction line 176a, 1761: or 1760 automatically opens. The regenerating agent utilized is aqueous sodium hydroxide solution containing 10% sodium hydroxide by volume. Four hundred gallons of regenerating agent is introduced over a 40 minute time period. At this point, the valve in the regenerating agent introduction line automatically closes, and the valve in the appropriate line 172a, 172b or 1726 automatically opens and the appropriate three-way valve 180a, 18017 or 1801' is automatically positioned to route liquid to tank 200. After 10 minutes, the air has forced substantially all of the liquid from the column being regenerated. At this point the valves in the

appropriate lines

188a, 188b, 1880, 1920, 192b, 1926 automatically open whereby water passes through a cation exchanger and then into the anion exchanger which is being regenerated. The water is introduced at the rate of 60 gallons per minute. After 1 minute, the appropriate valve is closed to shut off the air. The 60 gallons of water introduced before the air is shut off serves to rinse out residual solution heavily laden with chrome, and carry it to tank 200. The 460 gallons routed to tank 200 contain 95% of the hexavalent chromium which was in the resin in the column just prior to initiation of regeneration.

When the air is shut off, the appropriate three-way valve 180a, 1801; or 1800 is automatically repositioned to recirculation position (that is, to provide communication, for example between

lines

178a and 194a) and water introduction is continued to fill the column. Then water introduction is automatically stopped by means of a conductivity switch, the vent valve closes and the appropriate pump 196a, 19612 or 1966 is automatically started and recirculation between the anion exchanger being regenerated and its associated cation exchanger is carried out. This recirculation is continued until the

conductivity cell

198a, 198b or 1980 registers 25 micromhos thereby indicating reaction of the sodium chromates and dichromates with the resins to establish a .level chromate and dichromate in the anion exchange resm.

Except during regeneration of column 206, solution is pumped by pump 202 from tank 200 through exchanger 206. The exchange resin in this exchanger is Amerlite 200 which has been converted to the hydrogen form; it is much more highly oxidative resistant than the cation exchange resin Amberlite IR-l Plus utilized in the cation exchangers 150a, 1501; and 150C. Passage of the solution through the cation exchange resin removes cations and the effluent from the cation exchanger 206 is aqueous chromic acid solution containing grams of chromic acid per liter expressed as CrO The differential pH as indicated by meters 210a and 210b indicates resin bed depletion, and a predetermined higher pH being registered by meter 210a indicates that the resin bed is ready to be regenerated. Re-

generation is carried out utilizing backwashing, introduction of regenerating agent and rinsing in accordance with the manufacturers directions for the particular resin being utilized. The regenerating agent utilized is 10% by volume aqueous sulfuric acid.

Tank 32 contains about 6,000 gallons when the line tanks are full.

Recirculation is carried out between tank 32 and evaporator 211 with gallons per minute being pumped by pump 200 whereby the concentration of solution in tank 32 is maintained at 150 grams per liter of chromic acid expressed as CrO Steam is introduced through line 212a to supply heat. A vacuum of 26 inches of Hg is utilized in evaporator 21 1; this vacuum is provided by an eductor on line 216.

The recirculation through

lines

44 and 46 between heat exchanger 42 and tank 32 is at the rate of 500 gallons per minute and cooling water is passed through exchanger 42 via lines 50a and 50b countercurrent to the flow of plater solution to remove heat from the system and maintain the temperature of the plater solution at 1 15F.

The use of different cation exchangers for the cation exchange upstream of the anion exchange and for the cation exchange downstream of the anion exchange allows the use of different cation exchange resins so that a highly oxidative resistant resin can be utilized in exchanger 206 where such resin is desirable because of the relatively high concentration of hexavalent chromium in the form of anions while a resin which is not highly oxidative resistant but which is less expensive can be utilized in exchangers 150a, l50b and 1500 where the concentration of hexavalent chromium in the form of anions is lower and less oxidation resistance is needed. Thus, the use of different cation exchangers and different resins significantly contributes to the economy of the system. The removal of only of the hexavalent chromium from the anion exchangers 152a, 1521; and 1526 during regeneration maximizes the concentration of chromic acid in the effluent of regeneration thereby significantly contributing further to the economy of the system by reducing the load on the evaporation system. Finally, the recirculation between the plater tanks and tank 32 in combination with adding the streams from

lines

94 and 208 into tank 32 and treating solution from tank 32 in the evaporator provides significantly more accurate plater solution volume and concentration control than if

streams

94 and 208 were passed directly to an evaporator and has the further advantage of allowing continuous evaporator operation on a relatively large volume of liquid compared to the volume of thestreams from 94 and 208 thereby permitting continuous evaporator operation without danger of exceeding the flash point of the solution being treated.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Therefore, in view of the variations that are readily understood to come within the limits of the invention, such limits are defined by the scope of the appended claims.

What is claimed is:

1. Method both forrecovering chromium from strong base anion exchange resin which has been used in the hydroxide form to remove hexavalent chromium in the form of anions from a solution and for regenerating the resin for reuse for removing hexavalent chromium in the form of anions from a solution; comprising the steps of a. introducing water into a column containing the resin thereby backwashing the resin to remove extraneous solids and resin fines,

b. removing residual backwash water,

0. passing aqueous alkali metal hydroxide into the column containing the resin and providing effluent from the column containing from about 90 to about 95% of the hexavalent chromium present in the column while leaving in the column alkali metal cations and the remainder of the hexavalent chromium in the form of anions to thereby optimize the concentration of hexavalent chromium in the effluent,

d. introducing water into the column and recirculating resulting liquid between that column and a column containing cation exchange resin of the strong acid type in the hydrogen form to react hexavalent chromium in the form of anions with the anion exchange resin and to react the alkali metal cations with the cation exchange resin, to establish a continuing to level of hexavalent chromium in the anion exchange resin thereby conserving chromium.

2. Method as recited in claim 1, in which the alkali metal hydroxide is sodium hydroxide.

3. Method as recited in claim 1, in which in step (b) residual backwash water is removed utilizing pressurized air 4. Method as recited in claim 3 in which liquid resulting in the column containing the anion exchange resin from passing aqueous alkali metal hydroxide thereinto is removed from the column utilizing pressurized air.

5. Method as recited in claim 4 in which subsequent to the removal of liquid resulting in the column containing the anion exchange resin from passing aqueous alkali metal hydroxide thereinto and prior to step (d), rinse water is introduced into that column and resulting solution is removed utilizing pressurized air to provide a portion of the effluent of step (c).

6. Method as recited in claim 5 in which the rinse water and pressurized air are introduced into the column containing the anion exchange resin simultaneously.

7. Method as recited in claim 1 in which the'water introduced in step (d) is passed through a column containing cation exchange resin of the strong acid type in the hydrogen form prior to such introduction 8. Method as recited in claim 1 wherein in step (c) the effluent contains about of the hexavalent chromium.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 885 Dated 20 19 75 lnventor( ert B. Smlth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 19 "1000" should read 1005 Column 2 line 17 "50" should read so Column 16 line 9 "200" should read 220 Signed and Scaled this twenty-third a) or September 1975 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN :"H X lfi (nmnlissium'r uflarcnrs and Trurlcmurkx USCOMM-DC 60376-5 69 U S GQVKRNME'JY PRXNYING OFFICE a 9 9 FORM PO-105OUO-69)

Claims

1. METHOD BOTH FOR RECOVERING CHROMIUM FROM STRONG BASE ANION EXCHANGE RESIN WHICH HAS BEEN USED IN THE HYDROXIDE FORM TO REMOVE THE HEXAVALENT CHROMIUM IN THE FORM OF ANIONS FROM A SOLUTION AND FOR REGENERATING THE RESING FOR REUSE FOR REMOVING HEXAVALENT CHROMIUM IN THE FORM OF ANIONS FROM A SOLUTION; COMPRISING THE STEPS OF A. INTRODUCING WATER INTO A COLUMN CONTAINING THE RESIN THEREBY BACKWASHING THE RESIN TO REMOVE EXTRANEOUS SOLIDS AND RESIN FINES, B. REMOVING RESIDUAL BACKWASH WATER, C. PASSING AQUEOUS ALKALI METAL HYDROXIDE INTO COLUMN CONTAINING THE RESIN AND PROVIDING EFFLUENT FROM THE COLUMN CONTAINING FROM ABOUT 90 TO ABOUT 95% OF THE HEXAVALENT CHROMIUM PRESENT IN THE COLUMN WHILE LEAVING IN THE COLUMN ALKALI METAL CATIONS AND THE REMAINDER OF THE HEXAVALENT CHROMIUM IN THE FORM OF ANIONS TO THEREBY OPTIMIZE THE CONCENTRATION OF HEXAVALENT CHROMIUM IN THE EFFLUENT, D. INTRODUCING WATER INTO THE COLUMN AND RECIRCULATING RESULTING LIQUID BETWEEN THAT COLUMN AND A COLUMN CONTAINING CATION EXCHANGE RESIN OF THE STRONG ACID TYPE IN THE HYDROGEN FORM TO REACT HEXAVALENT CHROMIUM IN THE FORM OF ANIONS WITH THE ANION EXCHANGE RESIN AND TO REACT THE ALKALI METAL CATIONS WITH THE CATION EXCHANGE RESIN, TO ESTABLISH A CONTINUING 5 TO 10% LEVEL OF HEXAVALENT CHROMIUM IN THE ANION EXCHANGE RESIN THEREBY CONSERVING CHROMIUM.

3. Method as recited in claim 1, in which in step (b) residual backwash water is removed utilizing pressurized air.

4. Method as recited in claim 3 in which liquid resulting in the column containing the anion exchange resin from passing aqueous alkali metal hydroxide thereinto is removed from the column utilizing pressurized air.

7. Method as recited in claim 1 in which the water introduced in step (d) is passed through a column containing cation exchange resin of the strong acid type in the hydrogen form prior to such introduction

8. Method as recited in claim 1 wherein in step (c) the effluent contains about 95% of the hexavalent chromium.