US3904491A

US3904491A - Controlling electrolyte concentration in electrolytic and rinsing treatment of chrome plated steel strip

Info

Publication number: US3904491A
Application number: US405893A
Authority: US
Inventors: Robert B Smith
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1973-10-12
Filing date: 1973-10-12
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09

Abstract

In treating steel strip subsequent to chrome plating, concentration in a bath in a zone functioning both as a rinsing zone and an electrochemical treatment zone is controlled by controlling rinse water input in response to line speed and bath conductivity.

Description

United States Patent Smith Sept. 9, 197 5 54] CONTROLLING ELECTROLYTE 3,526,486 9/1970 Smith et al 204/28 CONCENTRATION IN ELECTROLYTIC 23 882 13;? i 393 uc anan AND RINSING TREATMENT OF CHROME 3,574,069 4/1971 RobeIts 61 al. 204/29 PLATED STEEL STRIP 3,755,091 8/1973 Austin 204 35 R [75] Inventor: Robert B. Smith, Crown Point, Ind.

[73] Assignee: National Steel Corporation, Primary Examiner-Herbert T. Carter Pittsburgh, Pa. Assistant ExaminerWayne A. Langel [22] Filed: Oct. 12, 1973 Attorney, Agent, or FzrmShanley, O Neil and Baker [21] Appl. No.: 405,893

ABSTRACT [52] US. Cl 204/28; 204/29; 204/35 R;

204/41; 204/42 In treating steel strip subsequent to chrome plating, [5 Int. CLZ. oncentration in a bath in a one functioning both as Fleld of Search 204/28, 35 R, 41, 42 a rinsing zone and an electrochemical treatment zone is controlled by controlling rinse water input in re- [56] Referen Clted sponse to line speed and bath conductivity.

UNITED STATES PATENTS 3,518,169 6/1970 Oyama et a1 204/35 R 11 Claims, 2 Drawing Figures PORTION OF SYSTEM SPRAY g ig 5 CHROME PLAYING DEFINED m S WASHING T0 01mm;

. FIGURE 2 m '9 5 2 5 \m DEMINERALIZED 660 a 46 WATER STORAGE I I 26 J20 PLATER SOLUTION G 56 gn X E STORAGE TANK I EXCHANGE APPARATUS C cmzomc A010 CONTROLLING ELECTROLYTE CONCENTRATION IN ELECTROLYTIC AND RINSING TREATMENT OF CHRONIE PLATED STEEL STRIP BACKGROUND OF THE INVENTION This invention relates to treating steel strip subsequent to chrome plating. More particularly, this invention relates to a process involving rinsing to remove residual plating electrolyte and electrochemical treatment to impart a hexavalent chrome oxide coating. Rinsing and electrochemical treatment are disclosed in Smith et a1. U.S. Pat. No. 3,526,486 and in a paper entitled Chromium Coated Steel for Container Application presented by E. J. Smith on May 25, 1967 at the 75th General Meeting of American Iron and Steel Institute.

A preferred rinsing system is a multistage counterflow system. This type of system includes a plurality of tanks each containing a bath of rinse solution. Strip moves successively through the tanks with the first tank the strip enters being denoted the tank at the strip entrance end and the last tank being denoted the tank at the strip exit end. Water for rinsing is introduced into the tank at the strip exit end, and rinse solution moves from tank to tank in a direction opposite to the direction of strip movement so that the least concentrated rinse solution is in the tank at the strip exit end with progressively more concentrated rinse solution in each successive tank with the most concentrated rinse solution in the tank at the strip entrance end.

Utilizing one of the aforementioned tanks not only for rinsing but additionally for electrochemical treatment provides the advantage of eliminating the need for one tank in obtaining any specified amount of rinsing compared to where electrochemical treatment is carried out in a tank which is not a rinsing tank. And in a system in being with a fixed number of tanks or where there is room for only a particular number of tanks, utilizing one tank not only for rinsing but also for electrochemical treatment maximizes the amount of residual electrolyte removed from the strip in rinsing compared to where electrochemical treatement is carried out separate from rinsing, thereby minimizing the load on a subsequent different type of washer and thereby minimizing the load on any subsequent ion exchange system or waste disposal system.

Utilizing a tank functioning both as a rinsing zone and an electrochemical treatment zone to obtain the aforesaid advantages presents a problem with respect to controlling the concentration in the bath in that tank so as to get more or less uniform electrochemical treatment results and good quality control.

U.S. Pat. No. 3,574,069 discloses a counterflow rinsing system for chrome plated steel strip with an electrolytic zone in fluid communication with rinsing zones. This disclosure does not recognize the above discussed problem nor does it disclose or teach a means or method for the solution of that problem.

It is an object of this invention to provide a solution to the above problem, that is to provide a method of controlling the concentration of the bath in a zone in a counterflow rinsing system functioning both as a rinsing zone and an electrochemical treatment zone and in particular to a method of controlling the concentration of hexavalent chromium in the form of anions in that zone.

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

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION With continuing reference to FIG. 1 of the drawings, steel strip having been preliminarily treated, for example in electrolytic cleaning and pickling steps, follows a travel path 10 and is subjected to a chrome plating step 12 which includes a final treatment where objectionable oxide coating is removed. In this step the strip passes through a series of tanks and containing a bath of electrolyte. There is electrolytic action in each of the tanks except in the final tank. Such electrolytic action is imparted by rendering the strip cathodic, for example utilizing contact rolls, and passing it adjacent anodic grids. In the tanks where electrolytic action occurs, chrome plating is applied. The final tank where no electrolytic action is applied functions to remove objectionable oxide coating. The electrolyte in each of the tanks is the same, namely an aqueous solution of chromic acid. The chemistry of chromic acid is quite complicated and what exactly is present is not known. However, at the pH of less 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 a range of to 250.

The strip emerges from the aforedescribed treatment wetted with residual plating electrolyte as a result of dragout from the baths in the previously described tanks. This residual plating electrolyte contains hexavalent chromium in the form of anions. It is important to recover the hexavalent chromium in this electrolyte so as to minimize wastage and to minimize or eliminate disposal problems. Such recovery is obtained by rinsing treatment. In addition, it is important to initially protect the chrome plating against corrosion until it is provided with an organic protective coating (e.g paint, varnish, lacquer, enamel), by the application of a controlled hexavalent chrome oxide coating. Such application is carried out in an electrochemical treatment step. The present invention has particular application and is involved with the rinsing and electrochemical treatment portions of the chrome plating line. These portions of the line including the present invention are particularly depicted in FIG. 2 and will be discussed in detail later. The portion of the system depicted in FIG. 2 is generally denoted on FIG. 1 by the reference numeral 14.

The strip having been treated in the portion of the system denoted by reference numeral 14 is still wetted with some aqueous hexavalent chromium containing solution, for example about 2% of the amount removed in the prior rinsing. It continues along travel path -10 through a spray washing step 16 wherein the strip is washed to recover a solution comprising cations (for example, iron and trivalent chromium ions) and a rela tively low concentration'of hexavalent chromium.

The strip exiting from the spray washing treatment 16 contains only trace or no amounts of chromic acid wetting it. It follows travel path and isdried and coiled.

The solution recovered from the rinsing treatment, that is, the portion of the system 14, is an aqueous solution of chromic acid containing for example from about 85 to about 100 grams per liter of chromic acid expressed as CrO It flows via

conduits

18 and 20 into a plater solution (that is, plating electrolyte) storage tank The solution recovered from the spray washing treatment 16 comprises an aqueous solution of chromic acid containing for example about parts per million of chromic acid expressed as CrO It leaves the spray washing treatment 16 via a conduit 24 through which it flows to a chrome recovery system 26.

In the system 26 solution from line 24 passes into a cation exchange zone which takes out cations including trivalent chromium and iron ions and replaces them with hydrogen ions. Effluent from the cation exchange zone flows into an anion exchange zone which taks out hexavalent chromium in the form of anions and other anions and replaces these with hydroxyl ions. Effluent from the anion exchange zone is demineralized water, and it flows via a line 28 into a demineralized water storage tank denoted by reference numeral 30 which supplies demineralized water through a valved line 32 containing a pump 34 to spray washing treatment 16 via a valved line 35 and to the portion of the system denoted by reference numeral 14 via a line 36. Hexava lent chromium is recovered from the anion exchange zone by regenerating that zone utilizing sodium hydroxide 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 entering the system 26 via line 24. This effluent is passed through a cation exchange zone to replace cations with hydrogen ions, and the effluent from the cation exchange zone comprises an aqueous solution of chromic acid containing for example 10 to 50 grams per liter of chromic acid expressed as CrO This chromic acid leaves the chrome recovery system 26 via a valved line 38 containing a pump 40 whereby it is pumped to plater solution storage tank 22.

The plater solution storage tank 22 is in a recirculation loop with chrome plating zone 12, and this loop includes line 20 for continuous flow between

zone

12 and 22 and a valved line 42 containing a pump 44 for continuous flow from tank 22 into zone 12. The tank 22 is also in a recirculation loop with an evaporator 46 where solution is heated against steam passing in indirect heat exchange relation as indicated by lines 48a and 48b. The evaporator has a concentrated liquid exit line 50 which communicates with tank 22 and a vapor exit line 52. Also communicating with the evaporator is a valved liquid feed line 54 containing a pump 56 and communicating at its upstream end with the bottom of plater solution storage tank 22. Recirculation through the evaporator functions to controlliqu id volume in the platin system and therefore the concentration of the plating electrolyte (plater solution). The combination of providing circulation between tank 22 and zone 12, passing solution from lines18 and 38 into tank 22 and passing solution from tank 22 to the evaporation zone defined by evaporator 46 to reduce liquid volume and provide concentrating effectmaximizes the concentration of the material being treated by evaporation and permits treatment of a relatively large volume of material during evaporation so as to minimize concentration control difficulties which would be inherent in treating the small volume from

lines

18 and 38 and further minimizes temperature control difficulites during evaporation in keeping the temperature of the liquid being treated below its flash point.

The tank 22 is also in a recirculation loop with a heat exchanger 58 defining a heat exchange zone with a valved line 60 communicating between tank 22 and,

heat exchanger 58 and a line 62 leading back from heat exchanger 58 into storage tank 22. Line 60 contains a pump 64 whereby solution is recirculated through this loop. Water is passed through exchanger 58 countercurrent to the plater solution as indicated by lines 66a and 66b. This is done because there is a heat buildup in the plating portion of the system due to the grid current. This recirculation through the heat exchanger 58 controls the temperature to that where the best cathode efficiency is achieved in the plating zone 12, namely to a temperature of F. to F.

The plating system of zone 12, the spray washing system denoted by reference numeral 16, the chrome recovery system denoted by reference numeral 26 and the various recirculation loops involving tank 22 are described in detail in application Ser. No. 366,966 of Robert B. Smith and Richard K. Dickey, filed June 4, 1973 and in application Ser. No. 366,544 of Robert B. Smith, filed on that same date. The system described in those applications differs from'that of the present invention, however, in the portion of the system involving rinsing prior to spray washing and in the portion of the system involving electrochemical treatment inasmuch as in the applications of Ser. Nos. 366,966 and 366,544 there is no fluid communication between the electrochemical treatment system and the rinsing system wherein in the present invention as particularly described hereinafter there is such fluid commuication.

Turning now with particularity to the protion of the system denoted 14 on FIG. 1 and with continuing reference to FIG. 2, the strip having been treated in plating and oxide removal zones (denoted by reference numeral 12 on FIG. 1) continues along travel path 10 and is advanced through a tank 70 which forms structure for a first rinsing stage (zone), then through a tank 72 which forms structure for both a rinsing zone and also an electrochemical treatment zone and is denoted a second stage, then through a tank 74 whichforms structure for a third rinsing stage (zone) and then through a tank 76 which forms structure for a fourth rinsing stage (zone).

The tank 70 contains a strip entrance end deflector roll 70a, a sink roll 70b and a strip exit end deflector roll 70c and contains a rinse bath 70d. The strip following travel path 10 turns downwardly over roll 70a and moves vertically downwardly into and through rinse bath 70d turning under roll 70b and then advances vertically upwardly out of bath 70d and over 700 whereupon it leaves tank 70. Snubber roll 70c presses the strip against roll 70a and snubber roll 70f presses the strip against roll 70c thereby peforming a wringer function to remove liquid form the moving strip.

The tank 72 contains a strip entrance end contact roll 72a, a sink roll 72b, and a strip exit end Contact roll 720. It contains a bath of liquid 72d. The strip exiting from tank 70 being advanced along travel path 10 passes over roll 72a and turns vertically downwardly entering bath 72d, then passes under roll 72b then advances vertically upwardly passing out of bath 72d, then passes over roll 72c exiting from tank 72 horizontally. Located within bath 72d on either side of travel path in its downpass between rolls 72a and 72b is one set of vertically oriented anodic grids 72g. A second set of anodic grids 72h is located on either side of travel path 10 within bath 72d in its up pass between roll 72b and roll 72c. Snubber roll 72e forces the strip into contact with roll 72a and snubber roll 72f forces the strip into contact with roll 72c. A pair of wringer rolls 78 within tank 72 on either side of the strip in its travel downstream of roll 72c wipes liquid from the strip thereby minimizing the amount of liquid leaving tank 72 by dragout on the strip. The tank 72 is in a recirculation loop with a heat exchanger 80 where recirculating solution passes in indirect heat exchange with hot water to heat the recirculating liquid and control the temperature of solution in tank 72 to that preferred for electrochemical treatment, for example, 120F. The passage of the hot water in indirect heat exchange is countercurrent to the direction of the recirculating solution and is indicated by arrows 82a and 82b. The recirculation loop includes a line 84 containing a pump 86 whereby solution is pumped from tank 72 through exchanger 80 and a line 88 through which heated solution travels from heat exchanger 80 back into tank 72.

The tank 74 contains a strip entrance end deflector roll 74a, a sink roll 74b and a strip exit end deflector roll 740. It contains a bath of rinse solution 74d. The strip exiting from tank 72 passes into tank 74 and following travel path 10 is advanced over roll 74a, then passes vertically downwardly into bath 74d, then passes under roll 74b, then passes vertically upwardly out of bath 74d, then passes over roll 74c and exits horizontally from tank 74. Snubber roll 74e forces the strip against roll 74a and snubber roll 74f forces the strip against roll 74c thereby performing a wringer function to wring liquid from the strip.

The tank 76 contains a strip entrance end deflector roll 76a, a sink roll 76b and a strip exit end deflector roll 760. It contains a bath of rinse solution 76d. The strip exiting from tank 74 follows travel path 10 and is advanced over roll 760, then is moved vertically downwardly into bath 76d and passes under roll 76b, then passes vertically upwardly and leaves bath 76d, then passes over roll 76c and is advanced horizontally out of tank 76, then follows travel path 10 to the spray washing system (denoted by reference numeral 16 on FIG. 1 Snubber roll 76e forces the strip against roll 76a and snubber roll 76f forces the strip against roll 76c thereby performing a wringer function to wring liquid from the strip.

The

tanks

70, 72, 74 and 76 make up an interconnected washing system including a rinsing system with washing, that is rinising, solution passing counterflow to moving strip wherein the strip is washed to remove hexavalent chromium from solution wetting the strip and recover an aqueous solution comprising a relatively high concentration of hexavalent chromium in the form of anions. Each of the

tanks

70, 72, 74 and 76 constitutes structure for a rinsing zone (that is stage), and the baths in each zone (respectively 70d, 72d, 74d and 76d) constitute rinse baths each comprising an aqueous solution of hexavalent chromium in the form of anions, In its passage through the tanks, the strip passes through a succession of rinsing stages, that is it passes in succession through the rinse baths 700?, 72d, 74d and 76d. Water for rinsing is introduced into the last of these rinsing stages that the strip encounters; in

other words, it is introduced into tank 76. Rinse solution moves from stage to stage in a direction opposite to strip movement; in other words, solution moves from tank 76 to tank 74, from tank 74 to tank 72, and from tank 72 to tank 70. The concentration of hexavalent chromium in the form of anions progressively increases from stage to stage in a direction opposite strip movement with rinse bath 76d having the lowst concentration and bath 74d having a higher concentration, bath 72d having a still higher concentration and bath d having the highest concentration.

The tank 72 which forms structure for an intermediate rinsing stage (it is intermediate the stages of tanks 70 and 74) also forms structure for an electrochemical treatment zone. In this elecrochemical treatment, a dc current is supplied, contact rolls 72a and 72c render the strip cathodic, grids 72g and 72h function as anodes, and bath 72d in tank 72 besides constituting a rinse bath also constitutes the electrolyte. This electrochemical treatment functions to apply a hexavalent chrome oxide coating to the chrome plated strip to protect it from corrosion.

On startup, rinse solution, preferably demineralized water is introudced into tank 70 via a valved line 90. Liquid to form the bath 72d in tank 72 is initially introduced into that tank through a valved line 92 from an electrochemical treatment storage tank not depicted. Rinse solution is initially introduced into tank 74 via a valved line 94 and into tank 76 via a valved line 96;

lines

94 and 96 are supplied through a header 98 which in turn is supplied from a rinse water storage tank (not depicted) via a line 100.

During operation, that is as strip is moved through the line, demineralized water is added continuously into tank 76 through spray nozzles 102 which are supplied by line 36 containing a valve 106. The valve 106 is such as to allow flow out of nozzles 102 at a rate for example of 0 gallons per minute when fully closed to 5 gallons per minute when at full open. The spray nozzles 102 are on either side of the travel path 10 that the strip follows through tank 76 on its up pass just prior to its passage over roll 760. These sprays aid in washing off aqueous hexavalent chromium containing solution residually on the strip; washing action is also provided by bath 76d through which the strip passes in its downpass and in its up pass through tank 76. The demineralized water introduced through the sprays 102 and solution washed off the strip by these sprays falls by gravity into the lower portion of tank 76 and becomes part of the bath 76d. Solution continuously overflows through an overflow outlet in tank 76 providing a constant level in that tank.

The liquid leaves tank 76 through its overflow outlet, enters a line 108 and flows therethrough by gravity into a collection tank 110. The level of liquid in tank 110 is sensed by level sensor 1 12. A level controller 114 operates in response to the level sensed, and a pump 1 16 operating in response to a set point on the controller 114 operates to periodically pump solution from tank 1 10 via a line 118 into tank 74. The use of the tank 110 instead of overflowing directly from tank 76 to tank 74 allows a low liquid level in tank 76 whereby dragout from tank 76 is minimized.

The tank 74 has an overflow outlet, and this acting in concert with addition of solution through line 118 provides a substantially constant level bath in tank 74. The strip passing through tank 74 is washed by the bath on its downpass and its up pass through that tank whereby aqueous hexavalent chromium containing solution on the strip is rinsed therefrom.

Solution passes out of the overflow outlet in tank 74 and passes via a line 120 into tank 72. A substantially constant level of liquid is maintained in tank 72 as a result of an overflow outlet in that tank. This liquid which forms forms bath 72d provides washing of the strip on its downpass from roll 72a to roll 72b and its up pass from roll 71b to roll 72c whereby aqueous hexavalent chromium containing solution residually on the strip is washed therefrom thereby reducing the amount of hexavalent chromium in solution wetting the strip. The bath 72d also functions as an electrolyte for the electrochemical treatment which is carried out in tank 72 simultaneously with washing.

Solution passes out of the overflow outlet in tank 72 and passes via a line 122 into tank 70. A substantially constant level of solution is maintained in tank 70 as a result of an overflow outlet in that tank acting in concert with the addition of liquid through line 122. The bath of solution in tank 70 provides washing of the strip as it follows its travel path through that hath whereby aqueous hexavalent chromium containing solution residually on the strip is washed therefrom.

Solution overflows through the outlet in tank 70, then flows by gravity through line 18, then flows into line 20 (FIG. 1) and into plater solution storage tank 22 (FIG. 1).

Each of the

baths

70d, 72d, 74d, and 76d comprise aqueous solutions of chromic acid, that is aqueous hexavalent chromium containing solutions. The concentration of chromic acid expressed as CrO in each of these baths is as follows: in bath 70d, 85 to 100 grams per liter; in

bath

72d, 20 to 60 grams per liter, preferably 35 to 45 grams per liter, most preferably 40 grams per liter (controlled as described later); in bath 74d, to grams per liter; in bath 76d, l to 3 grams per liter. Thus, each successive tank in the direction of strip movement contains a bath of progressively lower concentration with respect to hexavalent chromium, and the strip is wetted with solution containing progressively less hexavalent chromium as it follows the travel path 10 through the

tanks

70, 72, 74 and 76 (in other words, the amount of hexavalent chromium in solution wetting, the strip is reduced as the strip passes through each one of the tanks).

To achieve a uniform hexavalent chrome oxide coating application in electrochemical treatment in tank 72, it is desirable that some particular concentration be selected within the aforedescribed range and deviation from such selected level be no more than plus or minus 3 grams, preferably no more than plus or minus 2 grams, per liter of chromic acid (that is hexavalent chromium in the form of anions) expressed as CrO Because in the aforedescribed system, tank 72 is in fluid communication with

tanks

74 and 70 so that it functions as part of the rinsing system, control of the chromic acid concentration in bath 72 to within the aforedescribed limits is a problem, and it has been discovered herein that a particular method and system of automatic control as described hereinafter minimizes variation in hexavalent chrome oxide coating applied in water through sprays 102 with -moving of vavle 106 toward open position providing an increased input of demineralized water into the system through sprays,

102 thereby lowering the concentration of chormic acid in tank 72 and the moving of valve 106 toward closed position providing a decreased input of demineralized water into the system through sprays 102 thereby increasing the concentration of chromic acid in tank 72. The controller 124 operates valve 106 by sending an electronic output signal via conductor 126 to a transducer 128 which converts current to a pneumatic signal which is transmitted via a line 130 to operatethe valve 106.

The ratio controller 124 operates in response to an electronic signal via line 132 from a summation transmitter 134 and to another electronic signal (a feedback signal) via a line 136 from a differential pressure transmitter 138.

The summation transmitter 134 receives an electronic signal via a line 140 from a line speed transmitter 142 and another electronic signal via a line 144 from a bias transmitter 146. The summation transmitter 134 adds or subtracts from each other the signals received via

lines

140 and 144 to provide a summation signal, and this summation signal is the signal sent to controller 124 via line 132 and, when thissummation signal exceeds the set point on controler 124, that controller signals the valve 106 to open more, and when the summation signal is less than the set point on controller 124, that controller signals valve 106 to close more. The feedback signal from transmitter 138 indicates to controller 124 when the flow condition called for by the signal through 126 has been achieved.

The line speed transmitter 142 operates in response to sensing of the speed of the strip being advanced along travel path 10 as indicated by electronic signal line 148 and transmits a signal indicating line speed via electronic signal line 140 to the summation transmitter 134.

The bias transmitter 146 receives an electronic signal via line 150 from a conductivity cell 152 in bath 72d in tank 72 indicating the conductivity of the liquid making up the bath 72a. Inasmuch as conductivity is related to concentration in the present system, the signal transmitted via line 150 is a signal indicating concentration of hexavalent chromium actually present in the bath 72d.

The bias transmitter 146 ratios the effect of the conductivity signal in relation to the line speed signal so that controller 124 is influenced much more by the Signal from line speed transmitter 142 than it is affected I by the signal from conductivity cell 152. Particularly,

the bias transmitter 146 is set so that the signal it transmits is accorded 5 to 20% of the weight of the signal transmitted by line 140. In other words, the bias transmitter 146 operates so that the signal related to conductivity (concentration) is accorded 5 to 20% of the weight of the signal related to line speed. The bias transmitter 146 preferably is adjustable so the weight accorded the signal related to conductivity can be adjusted with respect to the weight accorded the signal related to line speed. Thus, the bias transmitter 146 functions so that the water introduction rate is controlled in response to strip speed as a primary control element and bath (72d) conductivity as a secondary control element. The reason for the bias transmitter 146 is to provide a slow correction if there is a temporary deviation in concentration in the system due to reasons other than line speed (these reasons will be discussed later), and the biasing is set such to provide a slow correction in such cases so that a large slug of water which could not be handled by the system and would result in improper coating application in the electrochemical treatment step will not be introduced.

Differential pressure transmitter 138 provides a signal indicative of the flow rate in line 36, that is of the water introduction rate through sprays 102. This signal which is transmitted via line 136 is received as a feedback signal by controller 124. Communication between differential pressure transmitter 138 and demineralized water line 36 is provided by

valved line

154 and 156, line 154 communicating downstream of an orifice plate 158 in line 36 and line 156 communicating upstream of orifice plate 158. The

lines

154 and 156 meet within differential pressure transmitter 138 at a diaphragm which gives an indication of the differential pressure and therefore of the flow rate through line 36. The valves in

line

154 and 156 can be utilized to take the differential pressure transmitter out of the system, for example for maintenance.

It is helpful in understanding the control system to denote the signal from the line speed transmitter 142 as A, the signal from the bias transmitter 146 as B, the signal from the ratio controller 124 as C. and the feedback signal from differential pressure transmitter 138 as D. The signal from summation transmitter 134 would be A plus or minus B. The signal C would be null when the output signal from summation transmitter is equal to signal D.

The above described system functions to provide a four tank counterfiow rinsing system whereby a significantly increased percentage of the hexavalent chromium residually wettiing the strip is removed therefrom and the amount of hexavalent chromium that need be removed by the spray washing system 16 (FIG. 1) (and therefore the amount of ion exchange resin utilized in the chrome recovery system 26) is substantially reduced compared to a four tank system where three of the tanks constitute a counterflow rinsing system and a fourth tank is utilized only for electrochemical treatment (that is for electrochemical treatement and not for washing). In other words, the most rinsing is accomplished with the fewest number of tanks being used for rinsing and electrochemical treatment because tank 72 is utilized to function both as a rinsing zone and also as an electrochemical treatment zone. 5

The present invention assures high quality control in said electrochemical treatment step and in particular the automatic control system utilized assures this high quality control. In summary, this automatic control system operates to control the concentration of the bath 72d by controllingthe water introduction rate through the sprays 102 into tank 76 in response to line speed as a primary control element and in response to the conductivity of the bath 72d as a secondary control element and more particularly by controlling the water introduction rate in response to a signal which is the sum or difference of signals indicating deviation of line speed and conductivity from predetermined values with the signal related to conductivity being accorded 5 to of the weight of the signal related to line speed. The automatic control is carried out with the ob- 5 ject of obtaining in bath 72d a concentration of hexavalent chromium in the form of anions at a particular level within the range of 20 to grams per liter expressed as CrO preferably at a particular level within the range of 35 to 45 grams per liter expressed as CrO most preferably of 40 grams per liter expressed as CrO with the deviation from that particular level being no more than plus or minus 3 grams per liter, preferably no more than plus or minus 2 grams per liter at conditions and changes normally encountered. The operation of the control system will now be described in more detial.

Changes in line speed will affect the concentration in tank 72. At any particualr water introduction rate, increased line speed results in more hexavalent chromium being rinsed from the strip per unit time thereby increasing concentration of bath 72d. At any particular water introduction rate, decreased line speed results in less hexavalent chromium being rinsed from the strip per unit time thereby decreasing concentration of bath 72d.

Variations in line speed can be caused for example due to a variety of line problems. For example, a problem in the welder necessitating a slow-up in the looper until the problem is corrected (the welder and looper are not depicted on the drawings; they are upstream of the plating step 12 denoted on FIG. 1) may slow line speed temporarily thereby initially resulting in decreased line speed and thereafter return to normal line speed when the problem is corrected. Similarly, there might be a problem in plating, and the line would be slowed up unitl the problem was corrected.

The system of the present invention operates to maintain concentration in bath 72d as hereinbefore described regardless of line speed changes because controller 124 operates in response to sensing of line speed as a primary control element thereby controlling the amount of water that enters the system so that: if line speed increases, controller 124 signals valve 106 to move toward open position thereby introducing more water into the system to provide the desired concentration in bath 72d; and if line speed decreases, controller 124 signals valve 106 to move toward full closed thereby introducing less water into the system to provide the desired concentration in bath 72d.

More particularly, if line speed is increased, the line speed transmitter 142 sends a higher positive signal (the signal is proportional to the line speed) to the summation transmitter 134. The summation transmitter 134 sends a positive signal to the controller 124 which signal exceeds the set point on the controller, and the controller 124 signals the valve 106 to move toward more open position. If line speed is decreased, the line speed transmitter 142 sends a lower positive signal (proportional to line speed) to the summation transmitter 134, and the summation transmitter 134 sends a signal to the controller 124 which is less than the set point on the controller whereupon the controller signals the valve 106 to move toward closed position.

There are a number of reasons for change in concentration in bath 72d besides increased or decreased line speed, and these changes are corrected for by controlling the waterintroduction rate through sprays 102 in plater solution concentration is high. In such case, the

conductivity in bath 72d as measured by cell 152 is above the set point on the bias transmitter 146, and the bias transmitter 146 sends a positive signal to the summation transmitter 134. The summation transmitter 134 adds the signal to the signal from the line speed transmitter 142. A signal from the summation transmitter goes to the controller 124. If line speed is constant or does not vary to a degree to overwhelm the signal from bias transmitter 146, the signal from 134 exceeds the set point on controller 124, and controller 124 signals the valve 106 to openmore.

The concentration of hexavalent chromium in bath 72d in tank 72 can be low, for example, if the system contains very new efficient snubber rolls, if the plater solution concentration were low or if strip width decreases. In such case, the conductivity as measured by cell 152 would be lower and below the set point on bias transmitter 146. The bias transmitter 146 would send a negative signal to the summation transmitter 134 which subtracts the signal from the signal received from line speed transmitter 142. The summation transmitter then sends the summation signal to the controller 124 and the signal sent is below the set point on the controller. Thereupon the controller signals valve 106 to move toward closed position thereby correcting for the descreased concentration (as indicated by the low conductivity) in bath 72d.

The inventive concepts herein are illustrated in the following specific example.

EXAMPLE The method and system of FIGS. 1 and 2 is utilizied. The method and system is generally operated as described above. The details of operation are presented below with respect to the portions of the system pertinent to concepts of the present invention.

Steel Strip (36 inch width) is treated.

The line speed is 1550 feet per minute.

The strip having been preliminarily treated in elec' trolytic cleaning and pickling steps is subjected to a step 12 comprising passage in series through three tanks where chrome plating is applied and then passage through a fourth tank where objectionable oxide coating is removed. Each of the tanks contains 1700 gallons of plater solution, that is, electrolyte. The plater solution is an aqueous solution comprising 150 grams per liter of chromic acid expressed as CrO in other words, the concentration of hexavalent chromium in the form of anions in the plating electrolyte is 150 grams per liter expressed as CrO The recirculation system associated with step 12 functions so that 100 gallons per minute of solution enters each tank and 100 gallons per minute of solution each tank. The temperature of the plater solution is 1 15F.

The tank 70 contains 800 gallons of aqueous solution which comprises on the average about 90 grams per liter of hexavalent chroimum in the form of anions expressed as CrO Tank 72 contains 1250 gallons of aqueous solution. The concentration of hexavalent chromium in the form of anions expressed as CrO in bath 72d is automatically controlled as hereinbefore generally described to 40 grams per liter with a deviation from that level of no more than plus or minus 2 grams per liter. Circulation is carried out through heat exchanger to maintain the temperature of the solution in tank 72 at F. In tank 72 the strip surface is treated cathodically for 0.23 seconds at 200 amperes per square foot to thereby provide a surface treatment of 46 coulombs per square foot.

The tank 74 contains 1700 gallons of aqueous solution ccrnprising hexavalent chromium in the form of anions at an average concentration of about 10 grams per liter expressed as CrO The tank 76 contains 800 gallons of aqueous solution. This solution has a average concentration of hexavalent chromium in the form of anions expressed as CrO of about 2 grams per liter.

The flow through sprays 102 is automatically controlled by the control system generally described hereinbefore with the object of obtaining in tank 72, that is in bath 72d the aforedescribed concentration of 40 grams per liter of hexavalent chromium in the form of anions expressed as CrO With the wringer or snubber rolls operating efficiently the water introduction rate through sprays 102 is controlled to about 1 gallon per minute. With bad wringer or snubber rolls the control system maintains the concentration in bath 72d by causing the water introduction rate to increase to about 1550 feei per minute, the automatic control system' causes the water introduction rate through sprays 102 to increase proportionally per minute, whereas if strip width is decreased to, for example, 28 inches and line speed is maintained the water introduction rate is automatically controlled to decrease proportionally.

The flow out of tank '76 is continuous and is the same as the flow into tank 76. The average rate of flow into tank 74 corresponds to the rate of flow through sprays 102 inasmucn as pump 116 periodically empties tank 1 10 thereby introducing into tank 74 that solution that has flowed out of tank 76. Flow out of tank 74, flow into tank 72 through line 120, flow out of tank 72 through line 122, flow into tank 70 and flow out of tank 70 through line 18 is at the same rate as the flow into tank 74 through line 118.

The bias transmitter 146 is adjusted so it accords to the signal related to conductivity 10% of the weight of the signal related to line speed.

The control system operates to maintain a concentration of hexavalent chromium in the form of anions in tank 72 of 40 grams per liter expressed as CrO with a deviation from that level of no more than plus or minus 2 grams per liter despite the changes in line speed, changes in strip width or changes in efficiency of wringer or snubber rolls normally encountered thereby resulting in the application of a substantially uniform hexavalent chrome oxide coating in electrochemical treatment and good quality control. Without the control system operating, the concentration in bath 72d of hexavalent chromium in the form of anions will fluctuate under changes normally encountered often deviating substantially from the 40 grams per liter desired resulting in application of a coating in electrochemical treatment which is not uniform and poor quality control.

The rinsing system of tanks170, 72, 74 and 76 functions to remove more than 98 percent of the hexavalent chromium in the form of anions in solution wetting the strip entering tank 70 thus leaving only the difference to be removed in the spray washing stepl6 and to require recovery'inion exchange system 26.

In comparison, in a system utlizing four tanks for rinsing and electrochemical treatment where the electrochemical treatment tank is not in fluid communication with a rinsing tank as described in the example in Smith et al. application Ser. No. 366,966 filed June 4, 1973 where the same line speed is utilized and the same strip width is utilized and the same plater solution concentration is utilized, only about 90 percent of the hexavalent chromium in the form of anions on the strip entering the rinsing system is removed by the rinsing system leaving the remainder to be removed by the spray washing system and requiring ion exchange for recovery in useful form.

In the above described runs, the method of the present invention results in a savings of over 80% of ion exchange resin and ion exchange resin regenerating agent for step 26 compared to the aforedescribed four tank system where the electrochemical treatment tank is not in fluid communication with rinse tanks.

Thus, the system and method of the present invention for a particular number of tanks utilized for rinsing and electrochemical treatment maximizes the amount of residual electrolyte removed from the strip in rinsing compared to where electrochemical treatment is carried out separate from rinsing utilizing the same number of tanks for rinsing and electrochemical treatment thereby minimizing the load on spray washer l6 and minimizing the load on the ion exchange system 26 resulting in a substantial reduction of usage of ion ex change resin and regenerating agent. At the same time, the control system of this invention functions to maintain the concentration in the electrochemical treatment tank so that a uniform hexavalent chrome oxide coating is applied in electrochemical treatment thereby assuring good quality control.

In the drawings the dashed lines represent electronic signal lines.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.

For example, pump 1 16 can be of a type which operates continuously and the system can be designed so there is continuous flow into tank 74.

Moreover, a different total number of tanks for rinsing and electrochemical treatment than is hereinbefore described can be utilized. It is desirable to pass the strip through a rinse prior to electrochemical treatment, and it is desirable to add the water for rinsing in a rinse tank the strip passes through subsequent to electrochemical treatment. Thus, it is desirable to use at least a total of three tanks for rinsing and electrochemical treatment. While utilizing a total of three tanks can result in less efficient rinsing compared to a four tank system, such system is still vastly superior to a three tank system where the electrochemical treatment tank is not utilized for rinsing. Utilizing more than four tanks provides better rinsing than utilizing a total of four tanks but the cost of an extra tank might not justify the benefit derived from the extra rinsing. In any case regardless of the number of-tanks utilized the method and system of this invention will still provide an advantage with respect to amount of hexavalent chromimum removed from thest'rip compared to a system where the chemical treatment tank is independent of the rinsing system and the method of concentration control described herein makes possible the obtainment of this advantage without sacrificing good quality control in electrochemical treatment. I

Therefore, inview of the variations that are readily understood to come with the limits of the invention, such limits are defined by the scope of the appended claims.

I claim:

1. In a method for treating chrome plated steel strip wetted with residual plating electroylte containing hexavalent chromium in the form of anions where moving strip is subjected to counterflow rinsing to remove hexavalent chromium from solution wetting the strip by passage through a succession of rinsing stages each containing a bath of rinse solution comprising an aqueous solution of hexavalent chromium in the form of anions, with water for said rinsing being introduced in the last of said stages the strip encounters and rinse solution moving from stage to stage in a direction opposite to strip movement, the improvement comprising subjecting said moving strip in an intermediate rinsing stage both to rinsing to remove hexavalent chromium from soultion wetting the strip and to electrochemical treatment to apply a hexavalent chrome oxide coating to the strip with the rinse solution for this intermediate rinsing stage also forming the electrolyte for said electrochemical treatment and controlling the concentration of hexavalent chromium in said rinse solution forming said electrolyte by controlling the water introduction rate into said last stage in response to the speed of strip movement and also in response to conductivity of said rinse solution forming said electrolyte, said controlling of water introduction rate being accomplished by automatic mechanism having input signals varying with measured strip speed and electrolyte conductivity.

2. Method as recited in claim 1 in which strip speed is utilized as the primary control element.

3. Method as recited in claim 2 in which water introduction rate is controlled in response to an electronic signal which is the sum or difference of electronic signals indicating deviations of line speed and conductivity from predetermined values with the electronic signal related to conductivity being accorded 5 to 20% of the weight of the electronic signal related to line speed.

4. Method as recited in claim 3 in which control is carried out to obtain in the electrolyte for electrochemical treatment a concentration of hexavalent chromium in the form of anions at a particular level within the range of 20 to 60 grams per liter expressed as CrO with deviation from that particular level being no more than plus or minus 3 grams per liter.

5. Method as recited in claim 4 in which said concentration is at a particular level within the range of 35 to 45 grams per liter expressed as CrO with the deviation from that particular level being no more than plus or minus 2 grams per liter.

6. Method as recited in claim 5 in which said concentration is 40 grams per liter expressed as CrO 7. Method as recited in claim 1 in which the concentration of hexavalent chromium in the form of anions in the plating electrolyte ranges from 140 to 250 grams per liter expressed as CrO counterflow rinsing comprises four rinsing stages with each stage containing a bath of solution with the stage initially rinsing the strip denoted the first stage, the stage next treating the strip being the stage involving electrochemical treatment as well as rinsing and denoted the second stage, the stage next treating the strip being denoted the third stage and the stage next treating the strip being denoted the fourth stage; and water for rinsing is introduced into the fourth stage.

8. Method as recited in claim 7 in which control is carried out to obtain in the bath in the second stage a concentration of hexavalent chromium in the form of anions of 40 grams per liter expressed as CrO with the deviation from that level being no more than plusor minus 2 grams per liter.

9. Method as recited in claim 8 in which the concen-' tration of hexavalent chromium in the form of anions expressed as grams per liter of CrO in the first stage ranges from to 100, in the third stage ranges from 5 to 15 and in the fourth stage ranges from 1 m3.

10. Method as recited in claim 9 in which strip speed is utilized as the primary control element.

11. Method as recited in claim 10 in which Water introduction rate is controlled in response to an elec-,

line speed.

PATENT NO.

DATED INVENTOR(S) I nornrzn STATES PATENT oFFrcE CERMHCATE OF CQRRECTION September 9 1975 Robert B. Smith It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column Column Column Column Column Column Column line 7, line 14, change "7lb to -72b.

line 59 before "each", leaves-- is omitted.

DUOUJUJ line line line line line line electrochemical.

Column Column Column Column Column Column Column Column Column Column Column Column Column Column Column Column Column Column line line line line line line line line line line line line line line line line line line "introuced" -occurring-. inasmuch. -chromium-. -solution-. should be inserted.

Signed and Sula] this second Day of December1975 Attest:

RUTH C. MASON Arresting Offirer C. MARSIHALL DAMN ('onmlissimwr oflarems and Trademarks

Claims

1. IN A METHOD FOR TREATING CHROME PLATED STEEL STRIP WETTED WITH RESIDUAL PLATING ELECTROYLTE CONTAINING HEXAVALENT CHROMIUM IN THE FORM OF ANIONS WHERE MOVING STRIP IS SUBJECTED TO COUNTERFLOW RINSING TO REMOVE HEXAVALENT CHROMIUM FROM SOLUTION WETTING THE STRIP BY PASSAGE THROUGH A SUCCESSION OF RINSING STAGES EACH CONTAINING A BATH OF RINSE SOLUTION COMPRISING AN AQUEOUS SOLUTION OF HEXAVALENT CHROMIUM IN THE FORM OF ANIONS, WITH WATER FOR SAID RINSING BEING INTRODUCED IN THE LAST OF SAID STAGES THE STRIP ENCOUNTERS AND RINSE SOLUTION MOVING FROM STAGE TO STAGE IN A DIRECTION OPPOSITE TO STRIP MOVEMENT, THE IMPROVEMENT COMPRISING SUBJECTING SAID MOVING STRIP IN AN INTERMEDIATE RINSING STAGE BOTH TO RINSING TO REMOVE HEXAVALENT CHROMIUM FROM SOULTION WETTING THE STRIP AND TO ELECTROCHEMICAL TREATMENT TO APPLY A HEXAVALENT CHROME OXIDE COATING TO THE STRIP WITH THE RINSE SOLUTION FOR THIS INTERMEDIATE RINSING TAGE ALSO FORMING THE ELECTROLYTE FOR SAID ELECTROCHEMICAL TREATMENT AND CONTROLLING THE CONCENTRATION OF HEXAVALENT CHROMIUM IN SAID RINSE SOLUTION FORMING SAID ELECTROLYTE BY CONTROLLING THE WATER INTRODUCTION RATE INTO SAID LAST STAGE IN RESPONSE TO THE SPEED OF STRIP MOVEMENT AND ALSO IN RESPONSE TO CONDUCTIVITY OF SAID RINSE SOLUTION FORMING SAID ELECTROLYTE, SAID CONTROLLING OF WATER INTRODUCTION RATE BEING ACCOMPLISHED BY AUTOMATIC MECHANISM HAVING INPUT SIGNALS VARYING WITH MEASURED STRIP SPEED AND ELECTROLYTE CONDUCTIVELY.

4. Method as recited in claim 3 in which control is carried out to obtain in the electrolyte for electrochemical treatment a concentration of hexavalent chromium in the form of anions at a particular level within the range of 20 to 60 grams per liter expressed as CrO3 with deviation from that particular level being no more than plus or minus 3 grams per liter.

5. Method as recited in claim 4 in which said concentration is at a particular level within the range of 35 to 45 grams per liter expressed as CrO3 with the deviation from that particular level being no more than plus or minus 2 grams per liter.

6. Method as recited in claim 5 in which said concentration is 40 grams per liter expressed as CrO3 .

7. Method as recited in claim 1 in which the concentration of hexavalent chromium in the form of anions in the plating electrolyte ranges from 140 to 250 grams per liter expressed as CrO3; counterflow rinsing comprises four rinsing stages with each stage containing a bath of solution with the stage initially rinsing the strip denoted the first stage, the stage next treating the strip being the stage involving electrochemical treatment as well as rinsing and denoted the second stage, the stage next treating the strip being denoted the third stage and the stage next treating the strip being denoted the fourth stage; and water for rinsing is introduced into the fourth stage.

8. Method as recited in claim 7 in which control is carried out to obtain in the bath in the second stage a concentration of hexavalent chromium in the form of anions of 40 grams per liter expressed as CrO3 with the deviation from that level being no more than plus or minus 2 grams per liter.

9. Method as reciTed in claim 8 in which the concentration of hexavalent chromium in the form of anions expressed as grams per liter of CrO3, in the first stage ranges from 85 to 100, in the third stage ranges from 5 to 15 and in the fourth stage ranges from 1 to 3.

11. Method as recited in claim 10 in which water introduction rate is controlled in response to an electronic signal which is the sum or difference of electronic signals indicating deviation of line speed and conductivity from predetermined values with the electronic signal related to conductivity being accorded 5 to 20% of the weight of the electronic signal related to line speed.