BACKGROUND

Regulatory agencies place discharge limits on pollutants such as fluoride and cyanide. Facilities have used commodity coagulants to remove these contaminants with mixed results. Some common coagulants include ferrous sulfate, ferrous chloride, ferric sulfate, ferric chloride, Potassium Aluminum Sulfate (Alum), Aluminum chloride, Aluminum Sulfate, Polyaluminum Chloride, and Aluminum Chlorohydrate, Sodium Aluminate, Calcium chloride, Magnesium Hydroxide, Magnesium Chloride, Lime, Bentonite Clay, Modified Starches, Tannins and Lignins. It is common to use slurries of insoluble inorganic coagulants or solutions of soluble coagulants to remove total dissolved solids (TDS) and total suspended solids (TSS), which include dissolved target elements for removal.

In almost all cases, an excess of coagulant must be used to remove target elements. For example, to remove 50 ppm of fluoride it can take 1500-2000 ppm of alum at a pH of 6.6-8 to reduce fluoride to < 10 ppm. Also, to remove 10-20 ppm of total cyanide, it can take 500-1500 ppm of ferrous or ferric sulfate in pH ranges of 6-8 and 4.5-7.0 respectively. As chemical demand increases, pH control supplements also increase due to the acid content of the coagulants.

Excess coagulant feed results in higher chemical costs and generates excess sludge, which must be disposed of. Sometimes this excess sludge must be disposed of as hazardous waste. Additionally, some inorganic coagulants create sludge that is hard to dewater, further increasing disposal costs. Sludge containing Ce is typically compact while sludge containing A1 tends to hold more water. Aluminum coagulants can also adversely affect the microbial population in activated sludge, especially protozoa and rotifers. Some coagulants, such as FeC13 can cause corrosion to equipment.

Alkalinity may be added to waste streams in order to help biological processes such as nitrification/denitrification. Coagulants such as ferrous, ferric and aluminum consume and are consumed by, alkalinity which can affect nitrification processes and creates additional demand for the product. In contrast, cerium is not affected by alkalinity. SUMMARY

In treating waste fluid streams, such as waste water form coke plants, it would be advantageous to decrease the amount of coagulants and pH adjustment additives, such as Fe2+/Fe3+/A12+ and caustic respectively, which are used to remove cyanide and fluoride.

In one aspect, this disclosure provides a method of treating waste water containing a concentration of fluoride and cyanide to reduce the concentration of fluoride and cyanide. The method includes adding to the water at least one rare earth metal salt and at least one coagulant; and then reducing the concentration of fluoride and cyanide in the waste water.

DETAILED DESCRIPTION OF EMBODIMENTS

In one aspect, this disclosure is directed to the unexpected discovery of a rare earth metal salt, such as CeC13, which demonstrates a synergy with other coagulants (such as iron salts and aluminum salts) in removing both cyanide and fluoride. This removal can be accomplished in combined or series precipitation steps. Overall pH control additive usage can be minimized since co-precipitation or series precipitation can be accomplished in similar pH control ranges (such as 5-7) and due to the lower total demand for coagulant additives. Although the examples described herein use cerium salt (CeC13), it would be expected that other rare earth metal salts such as lanthanum salts would show similar synergy with other coagulants.

Treating systems with a rare earth and another coagulant has an enhanced effect when parameters are controlled, such as pH or alkalinity. This synergy can be taken advantage of to decrease chemical treatment costs or to optimize treatment. Some advantages include: decrease in chemical consumption, decrease in overall sludge generation, decrease in corrosion of equipment, prevention of interference with biological processes and decreased consumption of alkalinity.

It has further been discovered that at various pHs and/or alkalinities there is a synergy that allows for treatment of wastewater that costs significantly less by using coagulant/rare earth in combination, when compared to either rare earth salts or inorganic coagulant alone. While rare earth salts have been used to remove TSS and TDS in wastewater, they are expensive and thus their cost to treat is expensive as well.

In one aspect, this disclosure thus employs a combination of a rare earth metal salts and/or aluminum salts and iron salts in coagulant applications for removing cyanide and/or fluoride. Coagulants are used, for example, in water treatment applications to remove analytes from water streams, e.g., waste water streams. In this regard, metal salts and other coagulating agents are typically used in water treatment to effect a charge disruption that neutralizes repulsive forces in particles, which causes the particles or analyte to aggregate or floe, which enables them to be removed by filtration, clarification or other means.

In accordance with this disclosure, at least one rare earth metal salt can be combined with any other coagulants, including one or more of ferrous sulfate, ferrous chloride, ferric sulfate, ferric chloride, Potassium Aluminum Sulfate (Alum), Aluminum chloride, Aluminum Sulfate, Polyaluminum Chloride, and Aluminum Chlorohydrate, Sodium Aluminate, Calcium chloride, Magnesium Hydroxide, Magnesium Chloride, and Lime. The at least one rare earth metal salt can include, for example, cerium and lanthanum salts.

The treatment can be accomplished by adding the iron salts together with the rare earth metal salts and/or aluminum salts to a water stream that is allowed to accumulate in a tank, pond, or other holding area. The rare earth and coagulant can be added to the water separately or together as a blend or in a series of coagulating steps. The water can then be mechanically stirred or mixed to allow the target analyte of interest (e.g., phosphate, fluoride and/or cyanide) to aggregate. The aggregate can then be separated from the water, e.g., by filtration or clarification, which will remove the analyte from the water. Multiple tanks or holding ponds (e.g., 2 to 10) can be arranged in series to improve the removal. The time period to achieve effective aggregation that can be removed may be in the range of 10 minutes to 4 hours, 20 minutes to 2 hours, and 30 minutes to 1 hour.

As indicated above, the rare earth metal salt shows a synergy with coagulants so that the combination is more effective at removing cyanide and fluoride than would be expected based on the performance of each separately. As demonstrated in the Examples below, the optimum ratio depends on the target analyte being removed, and the pH or alkalinity. The rare earth metal salt can be added in a weight ratio to total coagulant additives, i.e., on a dry basis, rare earth salt/(rare earth salt + other coagulants), that is in the range of 0.05-0.95, from 0.1-0.9, from 0.3-0.7, or from 0.4-0.6.

Before treatment, cyanide or fluoride may be present in the water in amounts in the range of from 1 ppm to 1,000 ppm, from 2-10 ppm, from 30 to 200 ppm, and from 50 to 100 ppm. Preferably, the combination of (i) the dosage of the rare earth and coagulant, and (ii) the pH and/or alkalinity, are selected or controlled such that at least 50 wt.% of the target analyte is removed from the water, and preferably at least 75 wt.%, at least 90wt.%, at least 95 wt. %, and at least 99 wt. %. After treatment, the analyte can be present in amounts of less than 20 ppm, less than 10 ppm, less than 6 ppm, or from 10 ppb to 3 ppm.

The total dosage of rare earth and coagulant can be, on a dry basis, in the range of, for example, from 20 ppm to 4,000 ppm in the water, from 100 ppm to less than 1,500 ppm, from 150 ppm to less than 1,000 ppm, and from 200 ppm to less than 750 ppm. The total dosage of rare earth, on a dry basis, can be in a range of from 10 ppm to 2,500 ppm, from 20 ppm to 1,000 ppm, from 50 ppm to 750 ppm, and from 100 ppm to 500 ppm. A ratio of the total dosage of rare earth and coagulant to the amount of fluoride in the water before treatment may be from 5 to 30, 10 to 30, less than 25, or less than 20.

Where an aluminum salt or an iron salt is used as the coagulant, it can be present in an amount of, on a dry basis, from 20 ppm to 2000 ppm, from 100 ppm to 400 ppm, or from 200 ppm to 300 ppm. As indicated above, using less of these types of salts can decrease sludge and sludge removal efforts. In some aspects, the amount of aluminum salt that is added can be less than 1,000 ppm, less than 500 ppm, or less than 250 ppm.

Whether added separately or as a blend, the rare earth metal salt and coagulants can be added as a solid salt form, or preferably as an aqueous solution. The aqueous solution can include rare earth and/or coagulant in a total amount in the range of 10 to 70 weight percent, from 20 to 60 weight percent, or from 30 to 45 weight percent, where the balance is primarily water. Thus, in some aspects, the aqueous solution can include from 2 wt. %-25 wt. % rare earth metal salt and from 2 wt. %-25 wt. % of coagulant, from 5 wt. %-20 wt. % of the rare earth metal salt and from 5 wt. %-20 wt. % of the coagulant, or from 5 wt. %-15 wt. % of the rare earth metal salt and from 15 wt. %-25 wt. % of the coagulant. Typically, the rare earth metal salt will be present in a lower amount than the coagulant. The target analyte for coke plant waste water is generally fluoride and/or cyanide and can be removed by the rare earth/coagulant combination. Another example would be the addition of iron salts and rare earth metal salts together (or in series) to remove multiple analytes (e.g. cyanide and fluoride) in the same or close pH control range

In one aspect, because the synergy of the rare earth metal salt and coagulant depend on the pH and/or alkalinity of the water, these properties can be measured or controlled to optimize the synergy. Here also, and as demonstrated by the Examples below, the particular optimum value can depend on the target analyte, the dosage, and the coagulant(s). In general, the pH can be measured (e.g., with a pH meter, pH paper, titration, etc.) and controlled so that the water is maintained within a target pH range (e.g., by adding caustic or an acid). The amount of caustic that is added to the water can be less than 250 ppm, less than 200 ppm, or less than 150 ppm, for example. Depending on the factors indicated above, the target pH range may be between 5 and 7, between 5 and 6.5, between 7.5 and 8, between 4.5 and 6.5, between 6 and 8, or between 4.5 and 5.5. Alkalinity can similarly be measured (e.g., dye indicators, titration, etc.) and controlled to be within an optimal range (e.g., by adding buffer). The control system can be automated by including on-line pH or alkalinity monitors that send measurement information to a computer, processor, or other controller, which in turn, communicates with pumps and/or valves that add acid, base, or buffer to keep the water within the target range.

In another aspect, e.g., where it may not be feasible to readily alter the pH or alkalinity of the water, the ratio and dosage of the rare earth/coagulant combination can be selected to be optimal for those properties. The ratio and dosage can be determined based on empirically determined efficacies for removing the target analyte at varying pH levels and/or alkalinity levels.

EXAMPLES

In prior methods, cyanide and fluoride removal is normally accomplished in separate precipitation/clarification (using iron and aluminum salts respectively) steps due the pH requirement needed to drive each reaction. However, when rare earth metal salts are used (possibly in combination with aluminum salts) along with iron salts, co-precipitation or series precipitation process can be accomplished. Depending on the factors indicated above, the target pH range may be between 5 and 7, between 5 and 6.5, between 7.5 and 8, between 4.5 and 6.5, between 6 and 8, or between 4.5 and 5.5. Removal of cyanide from 1-100 ppm to < 1 ppm and removal of fluoride from 10-200 ppm to < 10 ppm can be accomplished with minimum pH control additives.

Table 1

Cyanide and Fluoride Removal

Data summarized in Table 1 summarizes the chemicals required to individually remove cyanide or fluoride and to also remove them together in a combined treatment or series treatment. Effective removal of fluoride can be accomplished with aluminum salts, P8200L or combinations thereof. Effective removal of cyanide can be accomplished with iron salts. However, the most effective treatment is the co-removal of both cyanide and fluoride in a combined precipitation or series addition process controlled in a similar close pH range that minimizes pH adjustment chemical usage/cost.