Wastewater Characterization

Home Based Recycling Business

Make Money in the Recycling Business

Get Instant Access

Water is used in the chrome pigment industry mainly to cool most of the equipment such as heat exchangers, generate steam in the boilers, make slurry of raw materials, scrub the reactor vent gases, and wash the precipitated products. Wastewater generated as a result of water use varies in quantity of pollutant, which is directly related to the raw materials use. Generally, these wastewaters contain dissolved chromium and pigment particles. Wastewaters emanating from the chrome yellow and chrome orange facilities often contain additional sodium and lead salts. A chrome oxide plant uses more volume of water because of the additional scrubber; the process water contains sodium sulfate and sometimes sodium borate and boric acid, particularly, when boric acid is one of the essential raw materials in the preparation of hydrated chromic oxide. Sodium salts and lead salts similar to those present in the chrome oxide plant wastewater are found in wastewater emanating from the molybdenum orange facility. In addition, the wastewater includes chromium hydroxide and silica. Wastewater from the zinc yellow plant contains soluble zinc salts, hydrochloric acid, sodium chloride, and potassium chloride. In the chrome green process plant, the wastewater generated contains sodium nitrate, sodium chloride, ammonium sulfate, ferrous sulfate, sulfuric acid, and iron blue pigment particulates, when iron blue is one of the essential raw materials.

Results of raw waste load found in verification sampling of a chrome pigment plant are given in Table 22.10.

22.3.3.3 Wastewater Treatment Process

In addition to the heavy metals stated in Table 22.10, ferro- and ferricyanide are also part of the pollutants in the wastewater generated in a chrome pigment plant. These wastes are generally combined and treated through reduction, precipitation, equalization, and neutralization to be followed by clarification and filtration processes. Most of the heavy metals are precipitated using lime or caustic soda at specific pH. Chromium is reduced by SO2 to a trivalent form, wherein it is precipitated as chromium hydroxide at specific pH. Sodium bisulfide is also employed to precipitate some of the metals at a low pH. The treated water is recycled for plant use while the sludge is sent to landfills (Figure 22.7).

Some other types of treatment processes can be employed in the chrome pigment industry in order to achieve safer industrial practices in terms of pollution. Processes such as ion exchange, biological oxidation, and use of glass for filtration before settling have potential application in this industry.

TABLE 22.10

Summary of Raw Waste Loadings Found in Verification Sampling of Chrome Pigment

Pollutants

Antimony

Cadmium

Chromium

Copper

Lead

Nickel

Zinc

Cyanide

Mercury

Maximum Raw Waste Loadings (kg/Mg)

0.0072

Waste h2so4 so2 i_i

Slaked lime

Blend tank pH 2.5-3.0

Equalization tank

Neutralization tank pH 6.2-6.5

-

Neutralization tank pH 8.0 -8.3

Holding

Holding

tank

*

tank

Backwash holding tank

Backwash

Sand filters

Final discharge to river

Sludge

Sludge landfill

Filter presses

Sludge leachate grab sample

Filtrate

Sludge holding tank

Clarifier effluent holding tank

Polymer 1

Clarifiers

Legend Sampling points

FIGURE 22.7 General wastewater treatment process flow diagram at a typical chrome pigment plant.

vo w

22.3.4 Copper Sulfate

22.3.4.1 Description and Production Process

Copper sulfate can be described as a by-product of copper refining that is obtained by crystallization of the weak liquor. The copper sulfate precipitate formed is centrifuged, dried, screened, and finally packaged in bags and drum. However, it is produced in some chemical industries by reacting copper shot with sulfuric acid, air, and water. It is widely used as an insecticide, additive nutrients (for soils that are deficient in copper), copper electroplating, wood preservation, and in petroleum refining.

22.3.4.2 Wastewater Characterization

Water use in copper sulfate plants is mainly used as contact water, noncontact cooling water, wash-downs, and as wash water, where solid copper sulfate product is required. Noncontact water is generated as steam condensate from the evaporators and is used to cool the crystallizers. Contact wastewater comes mainly from washdowns, spills, and leaks. Both types of wastewater contain relatively small quantities of copper sulfides and other heavy metals.

A typical plant production of copper sulfate indicating water use and wastewater generation is shown in the flow diagram (Figure 22.8).

Results of raw waste load found in verification sampling for a copper sulfate plant are given in Table 22.11.

22.3.4.3 Wastewater Treatment Process

Prominent among the heavy metals found in the wastewater generated in the copper sulfate industry are copper, arsenic, cadmium, nickel, antimony, lead, chromium, and zinc (Table 22.11). They are traced to the copper and acids sources used as raw materials. These pollutants are generally removed by precipitation, clarification, gravity separation, centrifugation, and filtration. Alkaline precipitation at pH values between 7 and 10 can eradicate copper, nickel, cadmium, and zinc in the wastewater, while the quantity of arsenic can be reduced through the same process at a higher pH value.

Wastewater treatment in the copper sulfate industry can further be improved, particularly the removal of the toxic metals, through sulfide precipitation, ion exchange, and xanthate processes. Addition of ferric chloride alongside alkaline precipitation can improve the removal of arsenic in the wastewater.

22.3.5 Hydrofluoric Acid

22.3.5.1 Description and Production Process

Hydrofluoric acid is produced from fluorspar (CaF^) and sulfuric acid. These raw materials are continuously fed in an externally fired kiln to produce calcium sulfate and hydrogen fluoride gas. The gas, which is a high boiling compound called drip acid, is precooled to a condensate consisting of primarily of fluorosulfonic and unreacted sulfuric acids. The hydrogen fluoride is further condensed in the refrigeration unit of the plant and may be further diluted to remove the residual impurities; the anhydrous form of the gas is stored in tanks. The hydrofluoric acid content of the waste gas is often scrubbed with water; at the refrigeration unit, it is scrubbed with sulfuric acid and then recycled to the plant.

Hydrofluoric acid is an important refrigerant and it is used as a bulking agent in foam industries. It is widely used in the aluminum production industry, nuclear fuel, steel, petroleum refining, and fluoride salt production.

22.3.5.2 Wastewater Characterization

Water is mainly used as noncontact cooling water, scrubbing water, and in the transportation of gypsum as slurry to the wastewater treatment facility. The water in the heat exchangers is used to

Wash water from Cu wash down Wash water

Sulfuric acid •

Settled filter cake

Legend Sampling points

(1) Screening sampling

(2) Verification sampling

Spills, leaks, cleanup, cooling water

Cu shot ►

Oxidation

Filter

Wash

Air -►

reaction

Spills, leaks, cleanup, cooling water

Cooling water 1

Copper Salts Sulfates Hazardous

Noncontact cooling water

FIGURE 22.8 Flow diagram of a typical plant production of copper sulfate indicating water use and wastewater generation.

Noncontact cooling water

Discharge

FIGURE 22.8 Flow diagram of a typical plant production of copper sulfate indicating water use and wastewater generation.

TABLE 22.11

Summary of Raw Waste Loadings Found in Screening and Verification Sampling of Copper Sulfate

Pollutants Maximum Raw Waste Loadings (kg/Mg) x 10-3

Antimony 1.2a

Arsenic 97

Cadmium 3.5

Copper 4600

Lead 1.8

Nickel 250

Zinc 27

Chromium 0.08

Selenium <0.024

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. a 1.2 (kg/Mg) x 10-3 = 0.0012 kg/106 g.

cool the gas; it seldom contains pollutants and as such it is recycled. The scrubber water is the major source of wastewater in the plant and often contains hydrogen fluoride, silicon hexafluoride, and hexafluorosilicic acid. The scrubbing process in the distillation unit generates acidified wastewater and the slurry process contributes to solid wastes, of gypsum, in the wastewater.

A typical plant production of hydrofluoric acid indicating water use and wastewater generation is shown in the flow diagram (Figure 22.9).

Results of raw waste load found in verification sampling for a hydrofluoric acid plant are given in Table 22.12.

22.3.5.3 Wastewater Treatment Process

Heavy metal pollutants such as zinc, lead, nickel, mercury, chromium, arsenic, copper, and selenium are predominantly found in the raw wastewater generated in the hydrofluoric acid plant; these are traceable to the raw materials. These pollutants are also found in the scrubber and washdown wastewaters in the plant. The wastewater resulting from leaks and spills of the drip acid contains fluorosulfonate complex. These pollutants are generally treated by alkaline precipitation, settling, filtration, and clarification. Drip acid and hydrofluoric acid spill wastewaters are combined and treated with aluminum fluoride. The gypsum content of the wastewater is removed in the gypsum pond and the overflow from this pond is neutralized and linked to other waste stream lines for final pH adjustment before discharge. Some other wastewater treatment processes applicable to a hydrofluoric acid plant include sulfide precipitation for effective removal of zinc, nickel, lead, copper, and antimony. The xanthate process and ion exchange are also potent treatment processes in this industry.

22.3.6 Hydrogen Cyanide

22.3.6.1 Description and Production Process

Hydrogen cyanide is an important product of the Andrussaw process. In this process, air, ammonia, and methane are reacted over a platinum catalyst at a high temperature to produce hydrogen cyanide. The accompanying gases in the reaction process contain ammonia, nitrogen, carbon monoxide, carbon dioxide, hydrogen, and oxygen. These gases are precooled before being scrubbed with phosphate

Vent t

Ethanol Uses

Water

FIGURE 22.9 Flow diagram for production of hydrofluoric acid indicating water use and wastewater generation.

Water

FT D

FIGURE 22.9 Flow diagram for production of hydrofluoric acid indicating water use and wastewater generation.

TABLE 22.12

Summary of Raw Waste Loadings Found in Verification Sampling of Hydrofluoric Acid

Pollutant Maximum Raw Waste Loadings (kg/Mg) x 10-2

Antimony

Arsenic

Cadmium

Chromium

Copper

Lead

Mercury

Nickel

Selenium

Thallium

Zinc

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. a 12 (kg/Mg) x 10-2 = 0.12 kg/106 g.

solution to remove unreacted ammonia. The scrubbed liquor is decomposed to obtain the phosphate solution and ammonia that are recycled to the plant. Alternatively, sulfuric acid may be used instead of the phosphate solution. Hydrogen cyanide from the ammonia scrubber effluent gases is absorbed in cold water to vent off other gases and the absorbed solution that contains hydrogen cyanide, steam, and other contaminants are sent to the distillation unit to produce a high-purity hydrogen cyanide gas. This gas can also be obtained as a by-product of the production process of acrylonitrile.

Hydrogen cyanide is an important raw material used in the production of methyl methacrylate and is widely used for Lucite, Plexiglas molding, and extrusion powders as well as coating resins. It is used widely in the agricultural sector to fumigate orchards and tree crops.

22.3.6.2 Wastewater Characterization

Water is mainly used in heat exchanger segments of units and as wash water for the equipment. Leaks and spills water is also used in the scrubber and the distillation unit; the resulting wastewater contains ammonia, hydrogen cyanide, and small amounts of organic nitriles. Scrubber purging is employed in order to avoid the buildup of impurities in other sources of wastewater in the plant. General plant wash water and rainfall runoff collectively contribute to the volume and characteristics of the wastewater in this plant.

Results of raw waste load found in verification sampling for a hydrogen cyanide plant are given in Table 22.13.

22.3.6.3 Wastewater Treatment Process

Both oxidizable cyanides and metallic complexes of ferro- and ferrycynides are the main pollutants found in a hydrogen cyanide process plant. Treatment processes generally employed in this industry include alkaline precipitation, settling, filtration, clarification, and recycling. The cyanide is oxidized to produce carbon dioxide and nitrogen. When ammonia is present in the waste stream, chlorine is used as oxidizing agents and the choice of hydrogen peroxide is hampered by high operating costs. The combined wastewater in the plant is sent to an alkaline chlorination treatment unit where sodium hypochlorite is added and the pH is adjusted to 10 with dilute caustic soda in succeeding

TABLE 22.13

Summary of Raw Waste Loadings Found in Screening and Verification Sampling of Hydrogen Cyanide

Pollutant Maximum Raw Waste Loadings (kg/Mg)

Cyanide, total 6.1

Cyanide, free 0.82

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. Note: 1 kg/Mg = 1 kg/106 g.

ponds. Sufficient chlorine and caustic soda are further added in the last pond to meet the discharged standard. Chlorine concentration resulting from excessive usage in the treatment process is removed through biological treatments, particularly aeration and trickling filtration (Figure 22.10).

Some other types of treatment processes that can be employed in the hydrogen cyanide industry include ozonation, to oxidize the wastewater chlorine. Potency of sulfur oxide is also high in the oxidation process.

Production Sodium Bisulfite

Discharge

FIGURE 22.10 General wastewater treatment process flow diagram at a typical hydrogen cyanide plant.

Discharge

FIGURE 22.10 General wastewater treatment process flow diagram at a typical hydrogen cyanide plant.

22.3.7 Nickel Sulfate

22.3.7.1 Description and Production Process

Nickel sulfate can be produced from either pure or impure sources. The pure source involves the reaction of pure nickel or nickel oxide powder (combined or separately) with sulfuric acid to produce nickel sulfate that is filtered and crystallized to produce a solid product. The impure raw material may be spent industrial liquor that contains a high percentage of nickel sulfate. The impurities in the liquor are precipitated by sequential treatment with oxidizers; lime and sulfides can later be filtered out. The treated liquor, which is a pure solution of nickel sulfate, can be packaged in a drum or further crystallized and dried to produce solid nickel sulfate. Nickel sulfate is used mainly in the metal plating industries. Other uses include dyeing and printing of fabrics and production of patina, an alloy of zinc and brass.

22.3.7.2 Wastewater Characterization

Generally water is used, in a nickel sulfate plant for process reaction, cooling of reactor, crystallization, plant washdown of spills, pump leaks and general cleanup. The water used in the process reaction is for preliminary preparation of the spent plating solution. In other units, especially where impure nickel raw material is used, the wastewater is often recycled. Wastewaters from this plant contain contact and noncontact water, which predominantly contain nickel as a major impurity. A typical plant production of nickel sulfate is shown in the flow diagram (Figure 22.11). Results of raw waste load found in verification sampling for a nickel sulfate plant are given in Table 22.14.

22.3.7.3 Wastewater Treatment Process

The source and nature of raw materials employed in the production process of nickel sulfate determine the type and quantity of pollutants generated in a typical nickel sulfate production plant. However, nickel is the significant pollutant in some plants (Table 22.14), while copper shows prominence in others. As stated in the production process, most heavy metals in the spent liquor, an impure source of raw material, are precipitated as sludge before using the pure liquor for the production of nickel sulfate. Generally, wastewater generated in the production process is treated through alkaline precipitation at pH between 9 and 10, followed by filtration and settling before discharge into the environment. The sludge generated in the plant is disposed of or used in a landfill.

General wastewater treatment process flow diagram at a typical nickel sulfate plant is shown in Figure 22.12.

Precipitation of nickel and other heavy metals, besides chromium, as metallic sulfide, followed by separation by settling and filtration of the wastewater containing the metals, is an improved treatment process of wastewater in the nickel sulfide industry.

22.3.8 Sodium Bisulfite

22.3.8.1 Description and Production Process

Sodium bisulfite is the product of a reaction between sodium carbonate, sulfur dioxide, and water. The slurry product contains crystals of sodium bisulfite that can be packaged in its liquid form or processed further through thickening, centrifuging, and drying to form anhydrous sodium metabisulfite. Sodium bisulfite is an important photographic chemical. It is also widely used in organic chemicals, textiles, food processing, tanning, and paper production industries.

22.3.8.2 Wastewater Characterization

Wastewater generated in most sodium bisulfite plants is mainly the process water that is used in making slurry of sodium carbonate. Washdowns and general cleanup water are other sources. The

Sulfate Bisulfate Diagram
FIGURE 22.11 Flow diagram of a typical plant production of nickel sulfate.

filter wash, wastewaters from other units of the plant, and wash water are combined and neutralized with caustic soda (50%) to a high pH of 9-10. The mixture is then passed through an aeration tank between 8 and 17 retention times to facilitate the conversion of bisulfate waste to sulfite, which are eventually oxidized to sulfate with air. The treated wastewater passes through the primary and secondary settling ponds before being discharged. The noncontact water is mainly used in cooling the centrifuge. The wastewater from this plant is relatively very low.

TABLE 22.14

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Nickel Sulfate

Pollutant Maximum Raw Waste Loading (kg/Mg) x 10-4

Antinomy

Arsenic

Cadmium

Chromium

Copper

Lead

Mercury

Nickel

Selenium

Thallium

Zinc

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. a 2 (kg/Mg) x 10-4 = 0.0002 kg/106 g. BDL, below detection limit.

Other nickel waste

NiSO4 process water

Monitoring shed weir box

Sump

R

Filter press

Sand filter

Backwash

Solid to the NiSO4 process

Backwash tank

Mixing tank

Weir box

Final discharge

H2SO4

FIGURE 22.12 General wastewater treatment process flow diagram at a typical nickel sulfate plant.

A typical plant production of sodium bisulfite indicating water use and wastewater generation is shown in the flow diagram (Figure 22.13).

Results of raw waste load found in verification sampling for a sodium bisulfite plant are given in Table 22.15.

22.3.8.3 Wastewater Treatment Pollutant

Presence of heavy metals in the wastewater coming from a sodium bisulfite plant is least expected since the raw materials used in its production do not bear traces of these pollutants. Table 22.15 clearly shows that raw wastewater from this plant contains low concentration of the heavy metals. Of all the investigated pollutants, dissolved zinc, which is suspected to be from corrosion of galvanized metal or zinc compounds used in the industry, has the highest concentration. Generally, the toxic metal pollutants in this plant are precipitated by treating the wastewater with lime, sodium carbonate, and caustic soda, followed by settling and filtration processes before discharging the treated wastewater. Other treatments applicable to the wastewater from this plant include sulfide precipitation that effectively precipitates zinc from the solution, the ion exchange process that removes other ions, and the xanthate process.

22.3.9 Sodium Dichromate

The chemical reactions involving chromites, limestone, and soda ash produce sodium chromate, which when reacted with sulfuric acid produces sodium dichromate. Chromites ore, which primarily consists of ferrous chromite and small amounts of aluminum, silica, and magnesia, is finely powdered in the plant and mixed with soda ash before being calcined in rotary kilns. The kiln product is dissolved using hot water and the solution formed is filtered through the filtration process; the filtrate is then evaporated to produce a concentrated solution of sodium chromate. Sulfuric acid is then reacted with the concentrated solution of sodium chromate to produce sodium dichromate and sodium sulfate. The latter is crystallized and the former is filtered out of the boiling solution. The filtrate is sent to the multiple-effect evaporators for further concentration and later to the water cooling crystallizer where sodium dichromate is crystallized; this is followed by centrifugation, drying, and packaging. The aluminum found in the thickener overflow is hydrolyzed before being precipitated out as aluminum hydrate slurry, which is finally sent out of the plant. Sodium dichromate is an important raw material used in the production of chromic acid and chrome pigments. It is also used in the tannery and metal plating industries as a corrosion inhibitor.

22.3.9.1 Wastewater Characterization

Generally, water is used in this plant to cool, leach, filter wash, scrub, heat, and washdown. The unre-acted ore is slurred and sent, along with chromium and other impurities originally present in the ore, to the treatment plant. The boiler blowdown, which is sometimes contaminated with chromium escaping from the process area, adds to the volume of wastewater coming from the plant. The non-contact cooling water from the plant contains dissolved sulfate, chloride, and chromate; thus it is sent to a wastewater treatment plant. The scrubber water may be used to slurry the ore or discharged.

Results of raw waste load found in verification sampling for a sodium dichromate plant are given in Table 22.16.

22.3.9.2 Wastewater Treatment Process

Hexavalent chromium and metals such as zinc and nickel that are present as impurities in the chro-mites ore are predominant pollutants associated with the sodium dichromate plant. They are generally removed through alkaline precipitation, clarification, filtration, and settling processes. The wastewater is treated with sodium sulfide to reduce hexavalent chromium to trivalent chromium,

VO hO

Alkaline slurry

To atmosphere

Scrubber

NaHS03

Na2C03

Na2C03

Radiating tank

Gases

Weak NaHSOo

Wet dust collection

Slurry liquor

Centrifuge

Dryer

Waste

Waste streams sampled

Drains, drips, spills, washdowns

Liquor filter

Filter wash

Liquor storage

NaHS03 liquor

Anhydrous sodium bisulfite

Outfall to river

NaOH Air

FIGURE 22.13 A typical plant production of sodium bisulfite indicating water use and wastewater generation.

TABLE 22.15

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Sodium Bisulfite

Pollutant

Arsenic

Antimony

Cadmium

Chromium

Copper

Lead

Mercury

Nickel

Zinc

Silver

Thallium

Maximum Raw Waste Loading (kg/Mg) x 10"

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. a 0.3 (kg/Mg) x 10-4 = 0.00003 kg/106 g.

which is then precipitated as chromium hydroxide out of the solution. Zinc is equally reduced and precipitated with further treatment of the wastewater with sulfite. The wastewater generated from this plant is treated in the same manner and sent to settling tanks where the precipitates and other suspended solids are settled before discharging the overflow.

Flow diagram of the general wastewater treatment process at a typical sodium dichromate plant is shown in Figure 22.14.

A more appropriate and improved method of treatment is still under study. The ion exchange and xanthate processes have not proved to be effective in treating the raw waste associated with this industry.

TABLE 22.16

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Sodium Dichromate

Pollutant Maximum Raw Waste Loading (kg/Mg) x 10-3

Chromium 3300a

Lead 0.09

Copper 0.67

Nickel 5.0

Silver 0.28

Zinc 2.5

Selenium <0.04

Arsenic <0.04

Cooling tower blowdown

Waste mud slurry

-Sodium sulfide -

(^Settling and dewatering landfill areas

Treated basin

Discharge basin

Reactor

Surface runoff

Settling pond

Legend /Waste streams sampled

Treated effluent

FIGURE 22.14 Flow diagram of the general wastewater treatment process at a typical sodium dichromate plant.

22.3.10 Sodium Hydrosulfite

22.3.10.1 Description and Production Process

22.3.10.1.1 Formate Process

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant.

22.3.10.1.2 Zinc Process

Sodium hydrosulfite can also be produced through the zinc process, where pure sulfur dioxide from oleum or liquid sulfur dioxide is bubbled through zinc dust (suspended in water), in a well-agitated reactor. The zinc hydrosulfite formed flows into another similar reactor where it is reacted with a calculated amount of caustic soda to produce sodium hydrosulfite. The mixture is filtered to collect zinc hydroxide residue, while the filtrate is further processed in the clarifier and later crystallized out. The crystals are filtered in a vacuum filter, washed with ethyl alcohol, and finally dried in a vacuum dyer before being packaged. This process is losing patronage because of environmental reasons. Sodium hydrosulfite is widely used as a reducing agent in the textile dyeing, wood pulp bleaching, vegetable oil, and soap industries.

22.3.10.2 Wastewater Characterization

Water use in this process is for contact and noncontact use. The noncontact use is mainly for cooling, scrubbing, drying, and as washdowns and blowdowns. Reaction solution makeup and steam generation in the rotary dryers are the main processes involving contact use of water in the plant. The scrubber wastewater is recycled to the methanol recovery distillers and eventually goes to the plant wastewater stream in addition to the voluminous aqueous residue from the distillation column bottoms. This wastewater contains concentrated reaction coproducts. Dilute wastewaters resulting from leaks, spills, washdowns, and other wash water are collected and sent to the biological treatment system. A typical plant production of sodium hydrosulfite is shown in the process diagram (Figure 22.15). Results of raw waste load found in verification sampling for a sodium hydrosulfite plant are given in Table 22.17.

22.3.10.3 Wastewater Treatment Process

Zinc and other heavy metals are the major toxic pollutants associated with the zinc process of the sodium hydrosulfite process; thus the zinc process is not a favored industrial process unlike the Formate process. Heavy metal impurities such as zinc, nickel, lead, chromium, copper, and trace amounts of cyanide are pollutants resulting from the Formate process. The resulting wastes from various forms of sulfite, methyl formate, and residual methanol after the solvent recovery process are characterized by high chemical oxygen demand (COD) and low level of phenolic compounds. The heavy metals can be removed through alkaline precipitation and sulfide treatment. Chromium concentration can be controlled at a higher pH. The COD of the generated wastewater can be controlled by various forms of chemical and biological oxidation. Aeration systems such as submerged air diffusion, induced air, or mechanical surface aeration facilitate effective contact with oxygen, which oxidizes sulfite to sulfate. Organic pollutants such as formate, phenols, methanol, and chlorinated hydrocarbons are removed through trickling filtration, rotating biological discs, or activated sludge processes. Microorganisms and nutrients such as nitrogen, phosphorus, and urea are added to the wastewater to facilitate digestion and eventual removal of the pollutants as sludge. Liquid effluent is further treated with chlorine before being sent to a final tank for settling, equalization, and eventual discharge.

22.3.11 Titanium Dioxide

22.3.11.1 Description and Production Process

Production of titanium dioxide in the industry can be achieved through two different processes— the sulfate and chloride processes.

22.3.11.1.1 Sulfate Process

The ore is often dried in a continuous, direct fired rotary kiln and then ground to finely divided solid to facilitate effective reaction with sulfuric acid. The resulting product is dissolved in water and then flocculated continuously to classify the insoluble impurities such as silicon, zirconium, and unre-acted ore. The concentrated liquor is further mixed with water and heated to form titanium hydrate that is precipitated out of the mixture and filtered. The accompanying residue is mixed with water and conditioning agents, such as compounds of potassium, zinc, antimony, and calcium and phosphate salts, to improve the size, color, dispersability, and photochemical stability. This improved mixture is further filtered and calcinated to precipitate and remove residual acid and iron originally present in the ore. The resulting product of this process is titanium dioxide pigment, which is further repulped, milled, washed, and dried, depending on the end use.

A typical plant production of titanium dioxide (sulfate process) is shown in the process diagram (Figure 22.16).

22.3.11.1.2 Chlorine Process

This process involves the reaction of dried rutile ore and coke with chlorine in the chlorinator to produce titanium tetrachloride. The liquefied titanium tetrachloride is sent to the distillation unit to remove impurities and then to a fluidized bed reactor to react with oxygen to form titanium dioxide and chlorine. Solid titanium dioxide formed at this stage is vacuum degassed before being treated

VO Oi

Sodium Hydrosulfite

Sodium hydrosulfite crystal

FIGURE 22.15 Flow diagram of a typical plant production of sodium hydrosulfite.

Sodium hydrosulfite crystal

FIGURE 22.15 Flow diagram of a typical plant production of sodium hydrosulfite.

TABLE 22.17

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Sodium Hydrosulfite (Formate Process)

Pollutant Maximum Raw Waste Loading (kg/Mg) x 10-3

Arsenic

Cyanide

Cadmium

Chromium

Copper

Lead

Mercury

Nickel

Silver

Zinc

Pentachlorophenol

Phenol

Selenium

0.12a 0.039 0.033 0.56 0.19 1.0 0.02 1.6 0.16 24 0.83 0.15 0.03

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001,

U.S. Environmental Protection Agency, Washington, DC, 1982. a 0.12 (kg/Mg) x 10-3 = 0.00012 kg/106 g.

Hydrochloric Acid Process Flow Diagram
FIGURE 22.16 General production flow diagram of a typical plant production of titanium dioxide (the sulfate process).

Rutile coke

Rutile coke

Titanium Dioxide Manufacturing Process
FIGURE 22.17 General production flow diagram of a typical plant production of titanium dioxide (the chlorine process).

with alkali and a small amount of water to absorb chlorine and hydrochloric acid in the mixture. The final product is processed for handling and use. The chlorine generated and scrubbed in the production process is refrigerated, liquefied, and finally recycled. The bulk of titanium dioxide is used as a pigment in the production of paints, varnishes, lacquers, ceramics, ink, rubber, and in the paper and plastic industries.

A typical plant production of titanium dioxide (the chlorine process) is shown in the process diagram (Figure 22.17).

22.3.11.2 Wastewater Characterizations

Water employed in the sulfuric process of titanium oxide production is mainly used for noncontact cooling, process reactions, and air emission control. Wastewater generated as a result of the scrubbing process in the scrubber contains titanium dioxide particulates, sulfur trioxide, sulfur dioxide, and acid mist. Water is also used in the wet milling unit of the plant, where the titanium dioxide pigment is rendered to desired size and surface character. The process requires steam and water for repulping of titanium dioxide and for making a solution of caustic soda. Wastewater from this unit contains titania, sodium sulfate, and other additives employed to achieve desired properties of the products. Furthermore, sulfates, resulting from the digestion of the ore in sulfuric acids, are dissolved in water before being sent to the clarifier or filter to remove insoluble impurities such as silica, alumina, sulfuric acid, and unreacted iron. Water is also used in the washing of the titanium dioxide at some stages; this generates weak and strong acid liquors that contain some impurities, such as iron sulfate, titania, heavy metal, and sulfuric acid. Again, a large volume of water is used in cleaning the kiln gases and in the finishing operations, thus producing wastewater that contains impurities common to other waste streams in the plant. The wastewater is treated with chlorine before being sent to a final tank for settling, equalization, and eventual discharge (Table 22.18).

In the chloride process, water is mainly employed for noncontact cooling, scrubbing of tail gases, and in the finishing operation of titanium dioxide. Water use for cooling the gases emanating from the chlorinator generates a wastewater stream containing solid particles of unreacted ore, iron cake, and some heavy metals and heavy metal chlorides, which is further mixed with water to facilitate its movement to the treatment plant. Wet scrubbing is employed to remove hydrogen chloride, chlorine phosgene, and titanium tetrachloride from the cooling chlorinator gas. Steam waste generated in the scrubber also contains titanium dioxide particles; when caustic soda is used, sodium hypochlorate forms part of the pollutants in the wastewater stream. Like in the sulfate process, a large volume of water is used in the finishing operation of this pigment. The wastewater generated is characterized by suspended solids of titanium dioxide and dissolved sodium chloride.

TABLE 22.18

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Titanium Dioxide (the Sulfate Process)

Pollutant

Maximum Raw Waste Loadings (kg/Mg)

Antimony Arsenic

0.22 0.032 0.02 3.4 0.12 0.42 0.15 0.008 0.55 <0.66

Cadmium

Chromium

Copper

Lead Nickel

Thallium Zinc

Selenium

Source: U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S.

Environmental Protection Agency, Washington, DC, 1982. Note: 1 kg/Mg = 1 kg/106 g.

In both processes, wastewater is equally generated as a result of equipment and plant washes, leaks, spills, and blowdowns (Table 22.19).

22.3.11.3 Wastewater Treatment Process

Common pollutants in a titanium dioxide plant include heavy metals, titanium dioxide, sulfur tri-oxide, sulfur dioxide, sodium sulfate, sulfuric acid, and unreacted iron. Most of the metals are removed by alkaline precipitation as metallic hydroxides, carbonates, and sulfides. The resulting solution is subjected to flotation, settling, filtration, and centrifugation to treat the wastewater to acceptable standards. In the sulfate process, the wastewater is sent to the treatment pond, where most of the heavy metals are precipitated. The precipitate is washed and filtered to produce pure gypsum crystals. All other streams of wastewater are treated in similar ponds with calcium sulfate before being neutralized with calcium carbonate in a reactor. The effluent from the reactor is sent to clarifiers and the solid in the underflow is filtered and concentrated. The clarifier overflow is mixed with other process wastewaters and is then neutralized before discharge.

TABLE 22.19

Summary of Raw Waste Loading Found in Screening and Verification Sampling of Titanium Dioxide (the Chloride Process)

Pollutant

Maximum Raw Waste Loadings (kg/mg)

Chromium Lead Nickel Zinc

A typical wastewater treatment process diagram in a titanium dioxide (the sulfate process) plant is shown in Figure 22.18.

In the titanium dioxide production plant where the chlorine process is employed, the wastewater from the kiln, the distillation column, bottom residue, and those from other parts of the plant first settle in a pond. The overflow from this pond is neutralized with ground calcium carbonate in a particular reactor, while the scrubber wastewater is neutralized with lime in another reactor. The two streams are sent to a settling pond before being discharged.

A typical wastewater treatment process flow diagram in a titanium dioxide production plant (the chlorine process) is shown in Figure 22.19.

Ion exchange as well as lime treatment is another method that can be applied in this process to reduce some of the toxic trace metals in the wastewater from the plant.

22.4 SUMMARY

The pollution prevention strategy in the inorganic chemical industry is largely dependent on the age and size of the facility and the type and number of processes involved in a particular chemical industry. The above listed pollution prevention opportunities, aimed at reducing wastes and reducing materials use, have been embarked upon by some of these industries as the primary means of improving process efficiencies and increasing the profits. Most of the pollutants encountered in the production process are inherent in the raw materials and reflect as traces at many stages. The best substitution is to use a higher-purity feedstock and this can be accomplished by obtaining higher-quality feed or by installing purification equipment. Less toxic and less water-soluble materials will reduce water contamination, and materials with less volatile materials will reduce fugitive emissions.

The efficiency of a given reactor affects the quality of the product coming out of it. If its efficiency is high, despite the presence of impurities in the raw materials, effective reaction may greatly reduce the generation of such impurities in its output. Installations in the reactor, such as baffles, motors with high speed for agitation, multiple impellers, and pump recirculation system, and the use of improved blade design are some of the techniques to improve effective mixing. The method of feed introduction, particularly for a continuous flow system, can be improved to avoid spills and to allow thorough mixing within the residence time of the reactor. Most of the catalysts, particularly those of heavy metal origin, are oftentimes the sources of toxic metal pollutant in the inorganic chemical industries. Noble metal catalysts can be used in place of these conventional heavy metal catalysts to eliminate the wastewater contaminated with heavy metals. An active form of catalysts will reduce consumption of the catalyst and emissions and effluent generated in the processes.

Optimization processes in manufacturing industries are mainly employed to minimize loss and maximize profit. Wastes generation and the cost of removing them always lead to financial and even environmental impact on the operators of such industries. Processes in inorganic chemical industries can be optimized by the installation of computer control systems that are more efficient than usual manual control systems. Equalizing the reactor and storage tank vent lines during batch filling to minimize vent gas losses; addition of reactants and reagents in a well-ordered sequence to optimize yields and lower emissions; and well-sequenced washing operations are some of the methods of optimization of production processes in manufacturing plants involving inorganic chemicals.

Most of the reactions involving production of inorganic chemicals are exothermic, and in other processes they are heated to initiate reaction. As a result, heat generation is high and thus facilitates waste emission generation. Heat exchangers are common equipment in this industry but improvement in their efficiency will minimize emission generation. Using noncorroding tubes, reducing the tube wall thickness, increasing the effective surface area, monitoring, and prevention of fouling of the heat exchanger tubes are some of the techniques to improve the efficiencies of the heat exchangers and invariably the reduction of waste.

Larger parts of the waste generated in the inorganic chemicals production process are found in the wastewater emanating from the industry. However, if the wastewater generation could be

Weak acid pond

Sulfuric acid

Strong acid pond

Reaction

Filter

Solids to discharge/ landfill

Reactions

Filter

Reactions

Solids to discharge/ landfill

Final settling pond

Effluent

- Product wastewater

Sulfate processes waste water

Waste streams sampled

FT D

FIGURE 22.18 Flow diagram of a typical wastewater treatment process in a titanium dioxide plant (the sulfate process).

Ul Ki

Other product wastewater

Other products wastewater"

Other products wastewater"

Slurried pit solids ■

Other product wastewater

Reactor

Reactor

-►

Settling pond

■ Distillation bottom wastewater

Scrubber wastewater

Other product wastewater

Sampling points

FIGURE 22.19 Flow diagram of a typical wastewater treatment process in a titanium dioxide plant (the chlorine process).

minimized or effectively treated, less pollutant from the industries will reach the environment. Improved wastewater treatment technologies such as ion exchange, electrolytic cells, reverse osmosis, evaporation, dewatering, and improved distillation can be added or used to replace existing treatment systems. More importantly, most of the wastewater treatment streams can be recycled before or after treatment to reduce the rate and volume of discharges into the environment. Prevention or elimination of leaks and spills of reactants, products, or wastewater in the inorganic chemical industries is an effective means of pollution prevention. This can be achieved by installing seamless pumps, a leak detection program, and maintaining a rapid response solution in the case of leaks or spills.

Packaging, storage area, and safety programs are important for most of the products coming from the inorganic chemical industries since they are in transit within the premises until purchased by interested manufacturing industries. Good inventory in terms of the amount used, returning of unused, and reducing the likelihood of accidental release are some of the ways to manage the generation and emission of wastes from the storage end of the industry.30-33

REFERENCES

1. Howard, S.P., Donald, R.R., and George, T., Environmental Engineering, McGraw-Hill, New York, 1985.

2. Economic and Social Commission for Western Asia Wastewater Treatment Technologies: A General Review, United Nations, New York, 2003.

3. Wang, L.K., Shammas, N.K., Selke, W.A., and Aulenbach, D.B. (eds.), Flotation Technology, Humana Press, Totowa, NJ, 2010.

4. U.S. EPA, National Pollutant Discharge Elimination System; Secondary Treatment Standards. U.S. Environmental Protection Agency, Washington, DC, 2002. Available at http://efpub.epa.gov/npdes/ techbasedpermiting/sectreat.ofm.

5. Liu, D.H.F. and Liptak, B.G., Wastewater Treatment, Lewis, Boca Raton, Florida, 1999.

6. Water Environmental Federation and American Society of Civil Engineering Design of Municipal Wastewater Treatment plants, Volume 1, WEF manual of practice No. 8 and ASCE manual and report on Engineering Practice No. Vermont Book Press, 1992.

7. Amuda, O.S., Amoo, I.A., and Ajayi, O.O., Performance optimization of some coagulants/flocculants in the treatment of a beverage industrial wastewater, Journal of Hazardous Materials, B129, 69-72, 2006.

8. Amuda, O.S. and Alade, A., Coagulation/flocculation process in the treatment of abattoir wastewater, Desalination, 196, 22-31, 2006.

9. Amuda, O.S., Amoo, I.A., Ipinmoroti, K.O., and Ajayi, O.O., Coaguation/flocculation process in the removal of trace metals present in industrial wastewater, Journal of Applied Sciences and Environmental Management, 10 (3), 159-162, 2006.

10. Amoo, I.A., Amuda, O.S., Ipinmoroti, K.O., and Ajayi, O.O., Performance optimization of some chemical coagulants/flocculants in the treatment of wastewater from food industry, Science Focus, 11 (1), 38-45, 2006.

11. Amuda, O.S. and Amoo, I.A., Coagulation/flocculation process and sludge conditioning in beverage industrial wastewater treatment, Journal Hazardous Materials, 141, 778-783, 2007.

12. Amuda, O.S. and Ibrahim, A.O., Industrial wastewater treatment for chemical oxygen demand (COD) sing natural material as adsorbent, African Journal of Biotechnology, 5 (16), 148-1487, 2006.

13. Amuda, O.S., Giwa, A.A., and Bello, I.A., Removal of heavy metal from industrial wastewater using modified activated coconut shell carbon, Biochemical Engineering Journal, 36, 174-181, 2007.

14. Amuda, O.S., Ojo, O.I., and Edewor, T.I., Biosorption of lead from industrial wastewater using Chrysophyllum albidum (Sapotaceae) seed shell, Bioremediation Journal, 11 (4), 183-194, 2007.

15. Dorste, R.L., Theory and Practice of Waste and Wastewater Treatment, Wiley, New York, 1996.

17. Leung, W.C., Wong, M.F., Chua, H., Lo, W., Yu, P.H.F., and Leung, C.K., Removal of heavy metals by bacterial isolated from activated sludge treating industrial effluents and municipal wastewater, Water Science Technology, 41 (12), 233-240, 2000.

18. Hussein, H., Ibrahim, S.F., Kandeel, K., and Moawad, H., Biosorption of heavy metals from wastewater using Pseudomonas sp., Environmental Biotechnology, 7 (1), 2004.

19. Preetha, B. and Viruthagiri, T., Biosorption of zinc (II) by Rhizopus arrhizuz: Equilibrium and kinetic modeling, African Journal of Biotechnology, 4 (6), 506-508, 2005.

20. Al-Garni, S.M., Biosorption of lead by gram-ve capsulated and non-capsulated bacteria, Water SA, 31 (3), 345-350, 2005.

21. Quintelas, C., Sousa, E., Silva, F., Neto, S., and Tavares, T., Competitive biosorption of ortho-cresol, phenol, chlorophenol and chromium (VI) from aqueous solution by a bacterial film supported on granular activated carbon, Process Biochemistry, 4, 2087-2091, 2006.

22. U.S. Department of Commerce, United State Industrial Outlook, 1994.

23. U.S. Department of Commerce, Bureau of the Census, 1994 Census of Manufactures, Industrial Inorganic Chemicals, 1995.

24. U.S. EPA, Treatability Manual, Technical Report EPA-600-/2-82-001, U.S. Environmental Protection Agency, Washington, DC, 1982.

25. Qasim, S.R., Wastewater Treatment Plants. Planning, Design and Operation, 2nd ed., Technomic, Lancaster, PA, 1999.

26. Kroschwitz, J.I. and Howe-Grant, M., Encyclopedia of Chemical Technology, 4th ed., Wiley and Sons, New York, 1994.

27. Büchner, S. and Winter, B., Industrial Inorganic Chemistry, VCH publishers, New York, 1989.

28. Verar Inc. Multi-media Assessment of the Inorganic Chemicals Industry, Cincinnati, OH, U.S. EPA, Industrial Environmental Research Laboratory, 1980.

29. Ward's Business Directory of United State private and public companies, Gale research incorporated, United State, 1983.

30. Nasr, F.A., Doma, H.S., Abdel-Halim, H.S., and El-Shafai, S.A., Chemical industry wastewater treatment. The Environmentalist, 27 (2), 1573-2991, 2007.

31. Wang, L.K., Shammas, N.K., and Hung, Y.T. (eds.), Advances in Hazardous Industrial Waste Treatment, CRC Press, NY, 2009.

32. Wang, L.K., Wang, M.H.S., Suozzo, T., Dixon, R.A., Wright, T.L., and Sarraino, S., Chemical and Biochemical Technologies for Environmental Infrastructure Sustainability. The 2009 National Engineers Week Conference, Albany, NY, February 5-6, 2009.

33. Wang, L.K., Vaccari, D.A., Li, Y., and Shammas, N.K., Chemical precipitation. In: Physicochemical Treatment Process, Wang, L.K., Hung, Y.T., and Shammas, N.K. (eds.), Humana Press, Totowa, NJ, p. 183. 2005.

Was this article helpful?

0 0
Going Green For More Cash

Going Green For More Cash

Stop Wasting Resources And Money And Finnally Learn Easy Ideas For Recycling Even If You’ve Tried Everything Before! I Easily Found Easy Solutions For  Recycling Instead Of Buying New And Started Enjoying Savings As Well As Helping The Earth And I'll Show You How YOU Can, Too! Are you sick to death of living with the fact that you feel like you are wasting resources and money?

Get My Free Ebook


Post a comment