Chromium is present in the wastewaters of a number of industries, including: stainless steel manufacturing, protective coatings on metal, magnetic tapes, chrome plating, tanneries, textile dyes production, pigments and paint production, production of cement, paper, rubber, etc. Chromium is typically precipitated in two steps: reduction and precipitation.
In the first step (Reaction Tank #1) acid is added to lower the pH to < 3 so that the reaction may take place. The reaction is conducted in Reaction Tank #2 where hexavalent chromium (Cr+6) is reduced to trivalent chromium (Cr+3). In this step, compounds such as ferrous sulfate (FeSO4), sodium bisulfite (Na2S2O5), or sulfur dioxide (SO2) are used as reducing agents. The trivalent chromium is precipitated as Cr(OH)3.
In the second step (Reaction Tank #3), lime is typically used for the precipitation reaction. In this flow diagram, a process known as high density sludge (HDS) is employed to help promote precipitation. The heart of the HDS process is the pH in the HDS or densification tank which needs to be at a pH of 11.5 or higher. A change of state takes place at the higher pH and the process works better. The actual precipitation of chrome hydroxide takes place under mildly caustic conditions. Thus, there will always be some Cr(OH)3 in the clarifier feed, but these hydroxides will eventually be recirculated through the densification tank and undergo a change of state.
In this process, previously settled sludge is recycled to the reaction tank (HDS tank) where it is mixed with fresh lime slurry. This lime slurry coats the sludge, making it more reactive and able to form stable flocs with the incoming solids in Reaction Tank # 3.
The effluent concentration is 0.2 ppm Cr at pH 7.5. If additional metals are present, the pH is raised to approximately 10 to promote further precipitation.
To further facilitate removal of additional metals, carbonate co-precipitation is sometimes used. Carbonate precipitation takes place only if free carbonate ions (CO2-3) are present and this occurs only if the pH is high. Some wastewaters, especially those with lead, cadmium, nickel, etc. which can form insoluble carbonates that can be used in carbonate precipitation, may already contain enough carbonates to allow precipitation to occur.
Alternatively, inorganic carbonates such as soda ash (Na2CO3) can be added. High pH’s also promote the precipitation of the metals as hydroxides. Hence, carbonate precipitation is often a co-precipitation. As mentioned above, carbonate precipitates settle and can be dewatered more easily than the corresponding hydroxide precipitates.
Note that pH values above 10 promote the formation of metal hydroxy complexes that can increase the metal solubility and reduce the precipitation effectiveness.
The treated water is then filtered by either pressure or gravity media filters depending on the logistics of the final disposal.
The sludge from this process typically goes to a conventional thickener and filter press for dewatering prior to disposal. Dewatering produces an average 20:1 reduction in sludge volumes. The water from the dewatering system is returned for processing. Variations of this process can be used to precipitate most heavy metals.
Acid mine drainage (AMD) is typically characterized by low pH and high dissolved iron. The acid mine drainage may also contain high amounts of CO2 which forms carbonic acid and further depresses the pH.
There are four chemical reactions that represent the chemistry of pyrite weathering to form acid mine drainage. An overall summary reaction is as follows:
2 FeS2+7 O2+2 H2O → 2 FeSO4-+2 H2SO4
Pyrite + Oxygen + Water → Ferrous Sulfate + Sulfuric Acid
The acid mine drainage waste is characterized by red water. The simplest treatment is neutralization and clarification. The ideal neutralization first combines one of the reactants with previously precipitated solids. This blend is then mixed with the other reactant. This seeding provides the opportunity for crystal growth.
It also significantly reduces the reaction time. The final pH range of most neutralization reactions is 6–9. Many heavy metals precipitate as hydroxides within this pH range. However, if these heavy metal hydroxides are subjected to a pH > 11.5 for a few minutes, they convert to a crystal-like particle that clarifies, thickens, and filters more effectively than the original hydroxide.
The most commonly used neutralization agent is lime. Lime is added to previously precipitated solids in a blend tank, commonly called the densification tank. The neutralization flowsheet with this high pH feature is a high-density sludge (HDS) flowsheet.
Excess CO2 dissolved in the acid mine drainage stream can be stripped out using a surface aerator. Lowering the CO2 levels can raise the pH as much as one point and lower the amount of lime required for pH adjustment. This step also begins to oxidize iron and manganese and assists in their precipitation.
After stripping, the HDS slurry from the densification tank and the acid mine drainage stream are mixed in the reaction/aeration tank(s). The combination of aeration, high pH, and mixing causes the iron, manganese, and other heavy metals (if present) to precipitate to the fullest extent possible at the set pH level.
Treated water flows to a thickener for sludge thickening and clarification of the water. The metal precipitates as sludge, and a portion of the sludge is recycled to the sludge densification tank. The remainder of the sludge goes to disposal. Generally, the sludge will also contain gypsum and unreacted lime, which enhance the resistance to re-acidification and metal mobilization. A gravity sand filter may be used to “polish” the stream prior to discharge, depending on permit limits.
Depending on the site conditions, the thickened waste sludge may be redirected to another portion of the mine, dewatered and deposited prior to disposal in a landfill, or concentrated to paste and stacked.
Since acid mine drainage comes from abandoned mines these sites are often in mountainous, uninhabited areas where access to the site may be difficult. Many systems have been built with ease of operation being paramount. In fact, the systems are often built for operation with no onsite operator. This has led to the use of caustic for neutralization. Caustic is a liquid and is much easier to feed than lime. However, it does not make the same crystals that lime does, so the precipitate is difficult to settle. Caustic-fed systems are much more liable to upsets and the precipitation tanks should be designed for longer holding times. Also, caustic is much more expensive than lime.
Pile runoff basins have historically been used for the removal of suspended solids (coal fines) prior to discharge to wastewater streams. However, it was soon found that other contaminants were in this waste stream. Sulfur compounds contained in coal were oxidized by rainwater and produced sulfuric acid. This is analogous to the production of “acid rain” drainage from mine tailings. Water tinged with sulfuric acid leached other impurities from the coal, producing an acidic runoff contaminated with heavy metals.
The desire to eliminate all types of basins and ponds has prompted industry to use more refined treatment systems. Acidic streams containing iron, aluminum, and sulfate are treated for removal of these contaminants. Another driver for segregating and treating this stream is ever-decreasing mercury limits. These limits make it desirable to treat the stream prior to comingling with more conventional wastewater streams.
Wastewater characteristics change with rain events. Streams start out highly acidic but dilute with added rain. Dissolved metal levels also decrease the longer it rains.
Some heavy metals are removed as hydroxides by raising the pH. The pH is raised by adding chemicals, typically lime or caustic. Since the waste stream flow rate is small, pH adjustment is usually accomplished through caustic addition rather than lime slurry.
For heavy metals effluent requirements that cannot be met by precipitating the metals as hydroxides, sulfide can be added in addition to the pH adjustment. Metal sulfides have lower solubility than metal hydroxides. In these cases, organosulfides or sodium sulfides dosed into the stream precipitate as heavy metal sulfides. These compounds effectively remove mercury down to parts per trillion levels.
The addition of ferric chloride neutralizes charged particles, promoting flocculation and enhancing clarifier performance. Ferric chloride also precipitates mercury and organic matter. Polymer addition yields larger flocs, further enhancing clarifier performance. The wastewater is clarified by a CONTRAFAST® Clarifier, while pH is normalized with hydrochloric acid.
Raw water, recirculated sludge, and treatment chemicals enter the center draft tube. They are mixed and recirculated within the reactor by a variable speed impeller. The impeller accelerates solids formation and densification. A high-velocity upflow port prevents settling and moves water to the settling chamber.
The water passes under a baffle then upward through settling tubes and into the effluent collection launder. Dense sludge settles to the basin floor where it is continually scraped and further thickened prior to removal. Gravity filtration may be used to achieve even lower suspended solid levels prior to water discharge. In this case, filter backwash is returned to the front of the wastewater treatment system.
Thickened CONTRAFAST® solids are dewatered with recessed chamber filter presses or belt presses without the need for a separate thickening unit. Press choice is determined by sludge volume.
Flue gas desulfurization removes sulfur dioxide from fossil fuel flue gases. Wet-scrubbing transfers the pollutants to a liquid which is treated before waterway discharge. The scrubbing solution is usually lime and a concentrated solution of calcium sulfate is produced. Blowdown is required to keep the solution below saturation so that scaling does not occur.
Desaturating the stream of metals and gypsum is important to prevent scaling on equipment and is performed by dilution and lowering the temperature (remember that calcium salts are inversely soluble). The pH of the wastewater stream is then raised to between 8-10 using calcium hydroxide (Ca(OH)2) or sodium hydroxide (NaOH). Dissolved metals form hydroxides which precipitate as solids.
The lime or caustic is added to precipitate gypsum from the stream. Sludge is recycled from the downstream clarifier to provide seed for gypsum crystallization.
Some heavy metals are removed as hydroxides as pH is raised. Small waste stream pH adjustment is normally accomplished through caustic addition rather than lime slurry. The use of caustic saves capital costs and reduces sludge production.
Organosulfides or sodium sulfides may be added to further precipitate heavy metals. Metal sulfides have much lower solubility than metal hydroxides. These compounds are also very effective in removing mercury down to parts per trillion levels.
Ferric chloride is added to neutralize charged particles, allowing flocs to form and enhancing clarifier performance. This may also precipitate other metals and organic matter. Polymer addition aids in larger floc formation, further enhancing clarifier performance. The wastewater is clarified by a WesTech Flocculating Clarifier. A rake lift is provided since inlet solids can be as high as 2%. The pH is adjusted to normal using hydrochloric acid (HCl). HCl is used because no additional sulfate needs to be added.
The metal precipitates must now be removed from the waste stream. Since there is a relatively low amount of solids, it is necessary to use a Solids CONTACT CLARIFIER™ for this purpose. The Solids CONTACT CLARIFIER™ has an impeller-driven sludge recycle stream. This draws sludge from the tank bottom through a draft tube into the reaction well. This impeller acts as a high flow, low shear pump. The recycle stream is sized to 10 times the inlet flow and has suspended solids of 10,000 ppm. Incoming particles contact previously flocculated solids, yielding high removal rates. Blowdown sludge from the Solids CONTACT CLARIFIER™ is recycled to a mix tank in the feed stream. This promotes additional floc formation and solids removal.
Gravity media filtration may be used if a low suspended solids level is required prior to wastewater discharge. In this case, filter backwash is returned to the front of the wastewater treatment system.
The clarifier sludge typically contains 3-5 weight percent of solids. This contains inert material and precipitated metals which are pumped to a thickener to increase the solids percentage. Volume dewatering requirements determine the choice of recessed chamber filter presses or belt presses.
While the majority of waste streams generated from acid mine drainage (AMD) are characterized by low pH (2-4) and high dissolved iron (1,000 – 10,000 ppm), there are some AMD streams which are not contaminated to these extremes. These streams may have relatively high pH levels (5-7) and dissolved iron levels as low as several hundred ppm.
With water sources from industrial applications, especially hydraulic fracturing (“fracking”) operations, becoming harder to obtain, these marginal quality water streams are becoming more attractive for reclaim and use as process makeup waters.
Depending on the quantity of water required, water quality of the existing mine pools, and the water quality required for the process, there are a number of treatment options. This process flow diagram depicts one possible treatment design for flow rates of 500 gpm or less.
Dissolved ferrous iron (Fe2+) is first oxidized to ferric iron (Fe3+), which readily forms the insoluble iron hydroxide complex Fe(OH)3. In addition to providing the oxygen required to precipitate the iron, the use of surface aerators allows CO2 to be stripped from the water. This increases the pH of the water stream as well.
In the case of these relatively small flows, caustic (NaOH) is used as the sodium hydroxide source. Caustic, while having its own hazardous properties, is easier to prepare and add to the stream, involves less capital cost, and produces less sludge than the addition of lime slurry which is commonly used on large AMD flows. The caustic also increases the pH to a neutral level for further treatment and subsequent use.
Polymer is added to the stream to aid in floc formation and the stream is subjected to high energy mixing either in an inline mixer or a rapid mix tank. This is done to ensure that the polymer solution is completely dispersed in the stream. The flow then enters a slow mix tank which allows the floc that is beginning to form to grow to a point where it is large enough and heavy enough to readily settle.
A WesTech SuperSettler™ lamella type unit can be used for this purpose. This unit provides a large projected area in a relatively small footprint. In addition, it has no moving parts making for easy operation and low maintenance. Solids settled in the lamella section are collected in a bottom hopper. This hopper can have a thickening mechanism or rake added as an option to thicken the sludge.
The SuperSettler™ is perfectly coupled with a SuperSand™ continuous backwash sand filter. This is due to the fact that the water can flow through both units without having to be repumped. The SuperSand™ unit creates its own filtered water backwash stream so there is no need for filtered water storage or backwash pumps. The unit generates a constant dirty backwash stream of approximately 3–5% of the inlet flow.
If low TDS water is required for downstream processes (i.e., boiler feed), then ultrafiltration and reverse osmosis can be added to the system to produce this type of high-quality effluent.
Depending on the mine site, the waste streams from each treatment step may be returned to a separate section of the mine for disposal. At installations where this is not possible due to either logistics or regulations, conventional sludge thickening and dewatering technologies may be applied.
WesTech has experience in the phosphoric acid industry includes clarification, thickening, and dewatering. Calcium fluoride and calcium phosphate slurries present specific challenges. Another challenge is found in plants with hydrofluoric acid in their discharge.
Hydrofluoric acid is extremely corrosive and is toxic to humans. At these plants, solids contact clarifiers have been applied instead of external reaction tanks and flocculating clarifiers. These units are designed to prolong the period of precipitation. Solids growth is enhanced by the high level of precipitated solids present during the reaction. This has resulted in denser underflow and lower fluoride levels in the effluent than laboratory predictions. The underflow is sent to a gravity thickener and then to filter presses.
The waste from phosphoric acid plants usually consist of gypsum pile drainage. It is highly acidic with a pH of 1.0 to 1.8.
The extremely acidic drainage requires a two stage neutralization system. Clarification occurs between stages. The first stage uses lime to capture fluoride and elevate the pH to 4.5. The second stage pH is then raised to exceed 10.5. This approach reduces fluorine levels well below the mandated levels of 25 ppm. Phosphorus levels are similarly reduced below the 35 ppm limit. Alternative single stage processes fail to reduce fluoride levels below the required maximum.
Historically, calcium fluoride (CaF2) precipitation required a 60 minute reaction time. Adding previously precipitated and thickened CaP2 solids to the lime slurry improves the process. Resulting reaction time is reduced and the sludge precipitate is denser. WesTech testing suggests the recirculated solids should be three to five times the precipitated solids. Others have advocated higher recirculation rates.
The reaction slurry contains most of the fluoride plus a good portion of the phosphate. This thickens to produce an underflow with up to 40% suspended solids. Effluent fluorine is now less than 40 ppm and is fed to the second stage. The underflow is discarded usually in ponds, but this practice is being reviewed by EPA.
The second stage recirculates precipitated solids reducing reaction time. Solids, phosphate quantity, and settling characteristics make it impractical to recirculate solids more than once per pass. The solids are thickened to 10% suspended solids. Clarified effluent is discharged to waterways and the underflow is discarded.
Second stage feed liquor and underflow contains phosphate that is essentially free of fluoride. Some of this may be recycled through the process if the plant water balance permits.