Cost Comparison of Alternative Methods of Chrome Removal

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180

Mackenzie L. Davis,

MaryAnn Steinert, Linda Sturgess Michigan State University, East Lansing, MI

The rising cost of S idge disposal and the potential liability for ground water contamination is a major stimulus for platers to reevaluate their alternatives for treating chrome plating wastewater. The six processes selected for evaluation (hydroxide precipitation, sulfide precipitation, electrochemical precipitation, evaporation, ion exchange and liquid ion exchange) are described. Capital and operating costs for a typical countercurrent-rinse wastewater (30 gpm and 50 mg/L hexavalent chrome) are compared. Equipment suppliers and a local plater supplied the cost data. Without using a credit for the recovered chrome, the most expensive process is sulfide precipitation and the cheapest is the reciprocating flow ion exchange system. It is concluded that all of the chroae recovery processes are less expensive than the precipitation processes. A major factor leading to this conclusion is the reduction in utility costs because of the abilitv to recycle the water and, hence, avoid the cost of purchasing raw ~~

water and paying sewer discharge fees.

181

COST COMPARISON OF ALTERNATIVE METHODS OF CHROME REMOVAL FROM METAL PIATING WASTEWATER

Introduction

Chrome plating is a commonly practiced electrochemical process used to add luster to the final finish of metal parts in the automobile and small appliance industries. The plating process is followed by a series of rinse baths to remove the chrome solution (Figure 1). Historically these rinse waters have been considered too dilute to recycle and they have been discharged as a waste. Under regulations implementing the Clean Water Act of 1977, metal finishing facilities are required to treat this spent process wastewater to remove pollutants before the wastewaters are discharged to natural water bodies or to publicly-owned treatment works (POTU). The US EPA has classified metal finishing wastes as hazardous materials to be regulated under the Resource Conservation and Recovery Act (RCRA) of 1976 (11.

The most commonly selected method for control has been reduction of hexavalent chrome to trivalent chrome followed by alkaline precipitation. Recently, several factors have caused platers to reconsider their treatment alternatives.

The rising cost of sludge disposal has been a major stimulus for Michigan platers to reevaluate their alternatives. The cost has risen from $22.50 per cubic yard (cu yd) in 1982 to $120/cu yd in 1986. A related and equally important factor is the potential liability for groundwater contamination from sludges disposed of in landfills.

A third factor of increasing concern is the U.S. reliance on chromium imports. The world's major producers of chromium are the USSR (33- 35%), South Africa (24-26%), Turkey (10-12%) and the Phillipines (8- 9%) [2]. Virtually no chrome has been mined in the U.S. for 20 years because of the low grade of the ore. Most of the chromium used in the U.S. is imported from South Africa. This situation has profound implications for corporate strategic planning.

As a result of these factors, a local plater asked our assistance in evaluating alternative methods for removing chrome from his waste stream. The plater selected six processes for comparison: (1) hydroxide precipitation; (2) sulfide precipitation; (3) electrochemical treatment; (4) evaporation; ( 5 ) ion exchange; and (6) liquid ion exchange. The hydroxide precipitation method was selected as the base for comparison of the other methods. The sulfide precipitation method was selected to see if the elimination of the reduction step would offset the increased costs of sludge disposal. The electrochemical process was selected because of anticipated lower capital and operating costs. Neither the sulfide precipitation process nor the electrochemical process offer advantages in eliminating the hazardous waste sludge. The remaining processes were selected because they effectively eliminated the production of sludge.

Process Descriptions

The existing plating line is shown in Figure 1. Each of the alternate treatment schemes was designed for the hot and cold rinse water from this line. The average wastewater flow from these two tanks is 30 gallons per minute (gpm). The average effluent total chromium concentration from the combined flows from these two tanks is 50 mg/L. The rinse water is segregated from the other plating line waste streams and flows to an equalization basin where the flows are leveled prior to treatment.

182

Lnromium rrom rne n o c aiiu SUIU w a L S L , A L 6 V L u &, _.. _.._ hexavalent (Cr VI) form. Cr VI cannot be precipitated as a hydroxide. It first must be reduced to the trivalent form (Cr 111). Reduction is usually done with sulfur dioxide (SO,) or bisulfite (see upper left of Figure 2). Because this reaction proceeds more rapidly at a lower pH, an acid is added to lower the pH to about 2. Oxidation-reduction potential (ORP) is used to monitor and control the conversion from Cr VI to Cr 111. The neutralization step raises the pH to a range that will precipitate the chrome. The finely divided precipitate is mixed with a polymer to flocculate the suspension and improve its setling characteristics. The settled sludge is thickened in a gravity thickener. The thickened sludge is dewatered with a plate filter press.

The Permutit Company [3] has patented a insoluble sulfide precipitation (ISP) process with the trade name SulfexTM. This process is shown schematically in Figure 3 . The Cr VI waste enters at the lower left of the schematic. The waste is then pumped to the precipitator-settler where iron sulfide and polymer are added. The polymer serves the same functipn here that it does in the hydroxide precipitation process. The settled sludge is centrifuged to dewater it. The filter provides a polishing step to remove any chrome sulfide precipitate not removed in the precipitator-settler. Freshly prepared ferrous sulfide is used as the source of the sulfide ions. The process operates on the principle that FeS will dissociate into ferrous ions and sulfide ions as predicted by its solubility product. As sulfide ions are consumed in reducing the Cr VI to Cr 111, additional FeS will dissociate to maintain the equilibrium concentration of sulfide ions. In alkaline solutions the iron will precipitate. Because most heavy metals have sulfides less soluble than ferrous sulfide, they will precipitate as metal sulfides. An advantage of the ISP process over the soluble sulfide precipitation (SSP) processes is that the hydrogen sulfide off-gases that result from the SSP process as a by-product of the reaction are effectively eliminated. In addition the ISP process is less susceptible to inter- ference from complexing agents than hydroxide precipitation [4]. The major disadvantages of the process include considerably higher than stoichio- metric reagent consumption and significantly higher sludge generation rates than either hydroxide or soluble sulfide precipitation [ 5 ] .

The pH is adjusted in two stages for better control.

The electrochemical process selected for evaluation is a proprietary process marketed by Andco Environmental Processes, Inc. (61. Electrochemi- cal processes use consumable iron electrodes and an electric current to reduce Cr VI to Cr I11 in the following reaction [7]:

3Fe2 + CrO: + 4 H,O = 3 FeS + CrS + 80 H

If the pH of the wastewater flowing into the process is between 6 and 9, no pH adjustment is necessary. The hydroxyl ions generated in the reaction are sufficient to precipitate the Cr 111. Chelating agents do not interfere with the reaction. As shown in Figure 4, the wastewater first enters the electrochemical cell. The effluent from the cell contains metal hydroxides as suspended solids. As in the previous processes, polymer is added to enhance flocculation. The suspension is then carried to the clarifier to remove the solids. The solids are dewatered with a filter

+ press.

Evaporation achieves recovery of chromium from the waste stream by distilling off the water until there is a sufficient concentration of chromium to allow reuse in the plating operation. Because of the thermal energy cost of operation, evaporators usually are not considered for recovery of plating chemicals from dilute wastewaters. The most effective use of the evaporator is when it is applied to the concentrated solution at

183

the end of a counter current rinse system [ E ] . A major assumption we used in preparing this design and cost estimate that was not required in any of the other designs was that the plating process line could be modified to a closed loop, countercurrent rinse system (Figure 5). This change increased the concentration of chromium flowing to the evaporator and reduced both the water and sewer service charges and eliminated the production of sludge. Of the four types of evaporators (rising film, flash, submerged tube and atmospheric), the rising film type was selected for comparison. Rising film evaporators are built so that the evaporative heating surface is covered by a wastewater film . The complete unit consists of a reboiler, separator, and condenser. The reboiler is a shell-and-tube heat exchanger. The heat is supplied by low pressure steam. Because plating chemicals are susceptible to degradation at high temperatures, evaporation is accomplished at pressures reduced below atmospheric. The wastewater leaves the reboiler as a vapor/droplet mixture and enters the separator. The separator separates the water vapor from the droplets. The vapor is condensed in a shell-and-tube heat exchanger and the distillate is returned to the rinse tanks or to other plant uses. In double effect evaporators, the vapor from the separator enters the second-effect reboiler and con- denses to provide the thermal energy to reach the final concentration of the plating solution. Although the capital costs are increased approximately 30 percent for double effect evaporators, the steam and cooling water rates are approximately 50 percent of those for a single effect unit [9]. The purification unit shown in Figure 5 is an ion exchange system to provide makeup water free of hardness to reduce scale buildup.

The reciprocating flow ion exchanger (RFIE), shown schematically in Figure 6 , was especially developed for treating rinse water overflow [lo]. This unit operates on the principle that for the short period the unit goes off stream for regeneration, the buildup of contaminants in the rinse stream in negligible. The anion bed exchanges hydoxyl ions for the chromate ions. When this bed is regenerated with sodium hydroxide, the chromate is eluted as sodium chromate. The sodium chromate is passed through a strong acid cation resin to convert it to chromic acid. This can be used for chemical make-up in the plating process. The product concentration is approximately 10 percent chromic acid [ll]. The unit is not intended for high concentrations of chrome. The purified rinse water is returned to the rinse tank so that the loading of cations and anions passing to the RFIE after the initial pass is limited to the chrome and the ions in the makeup water. While the unit is on stream the "spare" cation bed is regenerated. Ideally, the unit is on line for 4 hours and off line for regeneration for about 20 minutes. Approximately EO gallons of water is purged to the POTW each cycle [12]. Before the purge water is discharged to the sewer it must be neutralized.

Liquid ion exhange (LIX) is, in concept, very similar to distillation. In distillation, a mixture of two substances is separated by creation of two phases (one liquid and one vapor). In LIX two liquid phases are formed by the addition of an immiscible liquid. The LIX material is an organic solvent that behaves like an ion exchange resin. The solvent extracts the chrome into an organic matrix that is immiscible in water. This matrix, once separated from the waste stream, is stripped to recover both the solvent and the chrome. The LIX process, as shown in Figure 7 , consists of two major parts: extraction and stripping. In the extraction process, the pH of the wastewater is lowered to about 2 to improve the extraction efficiency. A gravity separator is used to allow the two immiscible layers to separate. The raffinate is the bottom layer from the separator. Because the raffinate pH is low, it must be neutralized prior to discharge to the POTW sewer. The supernatant from the separator contains the LIX reagent and chrome. The chrome laden solvent is then stripped of chrome by the addition of sodium hydroxide. The chrome and hydroxide phase is separated from the LIX in another gravity separator. The LIX is recycled to the treatment process while the chrome

It is then mixed with the LIX reagent.

is reconstituted and reused in the plating operation, Although the process has yet to find commercial application in the plating industry in the U.S., it has found application in Europe and it is widely used in the metallurgical industry for ore processing.

Method of Analysis

Equipment costs were obtained from manufacturers. The cost of installation was estimated by the plater based on his experience using costs of labor and materials that he would expect to pay. No investment tax credit was taken. The equipment was depreciated over 5 years using the post 1985 ACRS schedule. A tax rate of 34 percent was used to compute after tax cash flow. The after tax cash flow was annualized over a 5 year life at an interest rate of 10 percent.

Operating and maintenance costs were based on local union labor rates (including fringe benefits), local utility costs and current prices for chemicals found in the Chemical Marketing Reporter [13]. These were computed on an annual basis for 24 hour work schedule, 7 days a week for a 350 day year. In the first analysis no credit was taken for the recovered chrome. In the second analysis a credit was taken for the chrome that might be recycled. The unit costs are shown in Table 1.

Results

The detailed tabulations are given in the appendicies. The summary comparison is shown in Table 2.

The sulfide precipitation process was found to be the most expensive for four reasons: (1) it had a very high capital cost; (2) the utility costs for water and sewer were added expenses not seen in the recycle processes; (3) labor costs were high; ( 4 ) sludge disposal was a significant part of the operating cost. As expected the hydroxide precipitation process was quite expensive for similar reasons: (1) high capital costs; (2) high labor and utility costs for water and sewer; (3) substantial sludge disposal costs. The lower capital and operating cost for the electrochemical process resulted in an annual cost that was $25,000 less than the hydroxide precipitation process. The major savings was in the lower labor cost. The more favorable cost for the liquid ion exchange process cost was due to the lower capital cost. The labor and utility costs were comparable to the processes that generate a sludge but the savings in sludge disposal costs were offset by the high cost of chemicals. The favorable position for the evaporator resulted from low labor and utility costs. Ion exchange (RFIE) was the cheapest. The capital cost was $93,400 less than the hydroxide precipitation. The operating and maintenance costs were 75 percent of that of the hydroxide precipitation. The reduction in operating cost for both the evaporation and RFIE processes were the result of three factors: (1) reduced labor costs (2) no charges for water and sewer service because the water is recycled and (3) the absence of sludge disposal charges.

When the value of the recovered metal was considered (Table 2 ) , the economic picture for liquid ion exchange improved over that for the electrochemical process. However, ion exchange still remained by far the Cheapest.

Conclusions

Michigan chrome platers have the four major concerns in treating their chrome wastwater: (1) meeting increasingly stringent discharge limits; (2) rising sludge disposal costs; ( 3 ) liability for sludges dispose on the land; and ( 4 ) long term availability of cheap chrome. These concerns are easily resolved by selection of one of the three recovery processes (liquid

184 185

1.

2.

3.

4 .

5.

6.

7.

8 .

9.

10.

11.

12.

13.

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ion exchange, evaporation or RFIE. The decision is made easy by the goo to excellent economic position of the recycling alternatives relative t the sludge generating alternatives. A major factor leading to the favor able economic position of the chrome recycling processes is the reductio in utility costs because of the ability to recycle the water and, hence avoid the purchase cost of raw water and sewer discharge fees.

References

National Materials Advisory Board, Utilization, National Academy of Scien

The Permutit Company, Princeton, N.J., 08540.

R.M. Schlauch and A.C. Epstein, Sulfide Precipitation, US EPA Pub.

ANDCO Environmental Processes, Inc., 595 Commerce Drive, Amherst, NY 14150.

Environmental Pollution Control Alternatives. Ibid, p. 19.

Evauorators. Ibid, pp. 19-20 and 26.

Environmental Pollution Control Alternatives. Ibid, pp. 56-57.

Ion Exchanee. Ibid, p. 40.

Chemical MarketinP Reporter, Schnell Publishing Co., Septemb 1986.

TABLE I. UNIT COSTS (1986 Basis)

LABOR Salary Hourly

MAINTENANCE 6% of installed cost

UTILITIES Electricity $ O.O4021/kwh Sewer $ 1.40/1000 gal Steam $ 6.14/1000 lb Water $ 1 .40 /1000 gal

CHEMICALS Calcium hydroxide Chromium oxide Ferrous sulfate LIX solvent Polymer Sodium bisulfite Sodium hydroxide Sodium sufhydrate Sulfuric acid

$ 0.0195/lb $ 1.90/lb $ 0.075/lb $25,00/lb $ 1.10/lb $ 0.28/lb $ 0.375/1b $ 0.25/lb $ 0.04/lb

TABLE 11. COMPARISON OF ALTERNATIVES

ROCESS

ulfide precipitation lydroxide precipitation .lectrochemical .iquid ion exchange ivaporation on exchange

ANNUAL COST WITHOUT CREDIT

$148,300 $132,200 $107,300 $116,700 $ $ 86,400 86,600

:redit' = market value of recovered chrome

187

ANNUAL COST WITH CREDIT

$148,300 $132,200 $107,300 $96,600 $72,500 $59,300

PARTS TO BE]-t

PLATED

Z 0 F 4 I- a a w CK a

-

SPRAY CATHODIC -HZHTHTb RINSE RINSE

I r

COLD HOT WATER - WATER

RINSE RINSE RINSE RINSE CrO 3 RECOVERY RECOVERY

PLATE - -

FIGURE 1. EXISTING PLATING LINE

ACID - 1

I

NEUTRALIZATION

!&GEM2 SULFONATOR

1 .Y

SLUDCf

IC] CHLORINATOR PRESS

FIGURE 2. CHROME REDUCTION/ H YD R 0 XI DE P R FCI PI TAT1 ON

188

I

1-j

w

FIGURE 4. ELECTROCHEMICAL TREATMENT

PRODUCT FLOW DRAG IN AND DRAG-OUT _---

> EVAPORATOR

UNIT PACKAGE

--

.. .

STEAM OR HOT - WATER - COOLING WATER

.

PLATING I BATH

I

PURIFICATION I UNIT

DISTILLATE

1 - CONCENTRATED PLATING

CEMICALS

FIGURE 5. COUNTERCURRENT RINSE WITH EVAPORATOR RECOVERY

190

STORAGE

I I

PURIFIED RINSE

1

SPENT RINSE WATER

CATION El I I I I I I I I I I I I I

PACKAGE I _ _ _ _ _ _ _ _ _ _ _ _ _ UNIT COMPONENTS J I

UTILITIES 7 *WATER - AIR

FIGURE 6. RECIPROCATING FLOW ION EXCHANGE [RFIE)

191

RECOVERED SOLVENT1

SEPARATION

EASE EASE

RECOVERED SOLVENT

SEPARATION

RECOVERED Cr

1 CHROME LADEN SOLVENT

I STRIPPING SOLVENT MAKE-UP I

FROM PLATING RINSE

TO POTW t U

SOLVENT EXTRACTION

FIGURE 7. LIQUID ION EXCHANGE

APPENDIX A COST ESTIMATE FOR HYDROXIDE PRECIPITATION

(1986 Basis)

EQUIPMENT Chromium reduction system Neutralizer Flocculation/clarification Storage tanks Filter press Installation

Total equipment

OPERATION AND MAINTENANCE Operator (2,100 h) Supervisor (100 h) Maintenance Electricity (125,300 kwh) Water Sewer Chemicals

Sodium bisulfite ( 3 lb/lb Cr) Sodium hydroxide (2 .3 lb/lb Cr) Sulfuric acid (2 lb/lb Cr)

Sludge disposal (37 cu yd/y)

Total O&M

$38,000 $35,300 $31,500 $ 8,000 $25,000 $65 ,600

$203,400 - - - - - - -

$39,650 $ 2 ,150

$ 5,000 $12,200

$21,200 $21,200

$ 5,300 $ 5 ,440 $ 500 $ 5 , 5 5 0

$118,190 - - - - - - -

APPENDIX B COST ESTIMATES FOR SULFIDE PRECIPITATION

(1986 Basis)

EQUIPMENT Precipitator, clear well, centrifuge,etc. Filter and transfer pump Neutralization system Storage tanks Installation

Total equipment

ANNUAL OPERATION AND MAINTENANCE Operator (2.100 h) Supervisor (100 h) Maintenance Electricity (106,743 kwh) Water Sewer Chemicals

Calcium hydroxide ( 1 . 3 lb/1000 gal) Ferrous sulfate (2.07 lb/1000 gal) Sodium sulfhydrate ( . 25 lb/ 1000 gal) Polymer ( . 04 lb/1000 gal)

Sludge disposal (207.5 cu yd)

Total O M

$61,400 $ 6,800 $76,900 $16,000 $59,200

$220,300 _ - - - - - -

$39,650 $ 2,150 $13,220 $ 4 , 3 0 0 $21,200 $21,200

$ 410 $ 2 ,000 $ 820 $ 580 $31,130

$136,660 - - - - - - -

192 193

APPENDIX C COST ESTIMATES FOR ELECTROCHEMICAL PROCESS

(1986 Basis)

EQUIPMENT Electrochemical process Storage tanks Installation

Total equipment

ANNUAL OPERATION AND MAINTENANCE Operator (1050 h) Supervisor (100 h) Maintenance Electricity Water Sewer Chemicals

Iron (electrode replacement) Polymer

Sludge disposal (68 cu yd/y)

Total O&M

$118,000 $ 16,000 $ 20,000 - - - - - - - $154,000

$19,820 $ 2 ,150 $ 9,230 $12,300 $21,200 $21,200

$ 4 , 2 0 0 $ 1 , 3 0 0 $10,200 - - - - - -

$101,600

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX D COST ESTIMATES FOR EVAPORATORS

(1986 Basis)

EQUIPMENT Evaporators Heat exchanger Ion exchange system Tanks Installation

Total equipment

ANNUAL OPERATION AND MAINTENANCE Operator (1050 h) Supervisor (100 h) Maintenance Electricity (47,600 kwh) Steam (1 ,780 ,800 lb) Water Sewer Chemicals

Sulfuric acid

Total O&M

194

$105,000 $ 5,000 $ 10.000 $ 12,000 $ 30,500

$162,500 - - - - - - - -

$19.830 $ 2 :150 $ 9 ,750 $ 1 ,910 $10,930 $ 8,470 $ 8,470

$ 950

$62,460 - - - - - - -

APPENDIX E COST ESTIMATES FOR RECIPROCATING FLOW 1ON EXCHANGE

(1986 Basis)

EQUIPMENT RFIE (cartridge filter and 3 ion exchange beds) $57,000 pH adjustment of effluent rinse $16,000

Installation $25,000 Storage tanks $12,000

Total Equipment $110,000 - - - - - - -

ANNUAL OPERATION AND MAINTENANCE Operator (1050 h) Supervisor (100 h) Maintenance Electricity Compressed air Water Sewer Chemicals

Sodium hydroxide Sulfuric acid Resin replacement

Total O&M

$19,820 $ 2 ,150 $ 6 ,600 $ 6 ,300 $ 400 $ 2 ,820 $ 2 ,820

$34,970 $12,300 $ 1,000 - - - - - - -

$89,180

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX F COST ESTIMATES FOR LIQUID ION EXCHANGE

(1986 Basis)

EQUIPMENT Mixer (pH adjust) Mixer (solvent) Separator Neutralizer (effluent) Mixer (stripping) Settler (stripping) Pumps, piping, etc. Installation

Total equipment

ANNUAL OPERATION AND MAINTENANCE Operator (2100 h) Supervision (100 h) Maintenance Electricity Water Sewer Chemicals

Calcium Hydroxide Kerosene LIX Sodium hydroxide

Sulfuric acid

Total O&M

195

$ 800 $ 300 $ 5,200 $16,000 $ 800 $ 800 $32,000 $14,000

$69,900 - - - - - - -

$39,650 $ 2,150 $ 4,200 $ 5,000 $21,200 $21,200

$ 100 $ 1,640

$ 5,100 $ 6,000 $ 4,450

$110,690 - - - - - - -

Documents

Cost Comparison of Alternative Methods of Chrome Removal