METHOD FOR THE REMOVAL OF CHROMIUM IN WATER USING METALLURGICAL SLAG.

Abstract

The present invention provides a method for the removal of Cr6+ in contaminated water comprising a Cr6+ reduction step with Fe+2 ions from non-ferrous slags containing iron oxides and a Cr3+ precipitation step with OH- ions from ferrous slags containing calcium oxide (CaO) and other basic compounds in lower composition such as magnesium, manganese and aluminum oxides.

Description

METHOD FOR THE REMOVAL OF CHROMIUM IN WATER USING METALLURGICAL SLAG Technical Field The present invention relates to a novel method for removing chromium (CrBt) present in water contaminated with this element by using metallurgical slag. Background Chromium (Cr) is one of the most common environmental pollutants due to its industrial applications. It is not biodegradable, as it is a heavy metal, and is therefore a cause for concern (Jobby et al., 2018) due to the unregulated disposal of chromium waste generated by anthropogenic sources (Pradhan et al., 2017), such as the paper, electroplating, paint, dye, leather tanning, and other human activities that contaminate water supplies (Panda et al., 2017). Chromium is found in natural waters in its trivalent and hexavalent forms, the latter being highly soluble and mobile in water (Mamais et al., 2016). It exhibits high toxicity and is potentially dangerous to all forms of life because it tends to accumulate in living tissues along the food chain (Durán et al., 2018). Furthermore, when it enters the human body, whether through ingestion, contact, or inhalation, it generates harmful genetic, mutagenic, and carcinogenic effects (Mthombeni et al., 2018). Therefore, high levels of chromium in groundwater represent a threat to living organisms. In Mexico, the amendment to NOM-127-SSA11994 (modified in 2000) establishes the permissible limit for total chromium at 0.05 mg / L for drinking water. Technologies have been sought to remove Cr6+ from contaminated water that are highly efficient in bringing chromium levels within permissible limits (Jobby et al., 2018). Water treatment processes for chromium removal include: reduction-precipitation, coagulation-precipitation, redox-assisted coagulation-precipitation, electrocoagulation, adsorption with various adsorbent or bioadsorbent materials, ion exchange, and membrane technologies (reverse osmosis and nanofiltration). Of the technologies mentioned above, the most widely used is the reduction of Cr6+ to Cr3+, which is less toxic and less soluble and can be removed by precipitation as Cr(OH)3. The conversion of Cr6+ to Cr3+ is achieved using chemical reducing agents such as ferrous sulfate and thiosulfate salts (Han et al., 2016; Gang et al., 2005 (https: / / doi.org / 10.1021 / es050486p)). In addition to the use of commercial chemical reducing agents, various industrial solid wastes and byproducts, as well as natural mineral sources containing Fe(II), can be used as reducing agents, which are more economical and readily available, thus lowering operating costs (Sinha et al., 2017). Metallurgical slags have been used in various studies demonstrating many successful applications, making their simple disposal in landfills or waste dumps unacceptable (Wang, 2016). The use of slag decreases waste disposal, reduces CO2 emissions, and helps preserve natural resources (Piakat, 2018), generating financial returns instead of disposal costs (Wang, 2016). In Mexico, the criteria for reusing steel slags are found in the standard NMX-B-085-CANACERO-2005, which indicates their use in the construction industry, road construction, as a wastewater filter medium, and as a neutralizing material for the remediation of acidic soils; however, the amount of slag generated is much greater than what is currently being used (Sarfo et al., 2017). The most common forms of chromium in natural waters are trivalent chromium (Cr3+) and hexavalent chromium (Cr6+) (Moffat et al., 2018). Trivalent chromium is an essential element for humans and is found in many vegetables, fruits, meats, grains, and yeast. Hexavalent chromium occurs naturally in the environment due to the erosion of natural chromium deposits and can also be produced by industrial processes (EPA, 2017). Due to the toxicity and carcinogenic effects of Cr6+, the United States Environmental Protection Agency (US EPA) has identified it as one of the seventeen chemical elements that pose a threat to humans (EPA, 2010), classifying it as a priority contaminant for soils and natural waters (Shahid et al., 2017). Most groundwater contamination is caused by human activities. In Mexico, this is a persistent concern that continues to affect different areas of the country, such as Tultitlán in the State of Mexico, where the Cromatos de México company was located. This plant left tons of waste that leached into an aquifer. Reported studies analyzed wells near the area and showed hexavalent chromium concentrations exceeding the maximum permissible limit of 0.05 mg / L established in the MODIFICATION to NOM-127-SSA1-1994, the standard for water for human use and consumption (Gutiérrez-Ruiz, 1990). Another case is presented in Castro-Rodríguez et al., 2015, "Adsorption of hexavalent chromium in an industrial site contaminated with chromium in Mexico." DO110.1007 / s12665-0143405-4, which analyzes the contamination of groundwater with Cr6+ in a populated area of ​​the State of Mexico. Chromium contamination has also affected groundwater in various parts of the world, as is the case in Ranipet, India. There, tanneries in an industrial development area manufactured chromate compounds and, over time, accumulated a large amount of hazardous solid waste approximately 5 meters above ground level. Rainfall in the area led to infiltration into the soil, resulting in rapid migration of chromium contamination to the water table (Tamma, 2011). Table 1 below shows a compilation of countries with high concentrations of chromium in their groundwater. bZR / nn / zznz / e / Yi Table 1 High concentrations of chromium in groundwater bZR / nn / zznz / e / Yi COUNTRY TYPE OF CHROMIUM CONCENTRATION (mg / L) REFERENCE Slovenia Cr6+ 0.175 (Brilly, 2003) Bangladesh Total Cr 4.06 (Zahid, 2006) India Cr6+ 38.6 (Tamma, 2011) China Cr6+ 20 (Beaumont, 2008) Greece Cr6+ 10 (Dermatas, 2017) El Mexe, Hidalgo Total Cr 15 (CONAGUA, 2019) Ecatepec, State of Mexico Crtotal 0.6 (CONAGUA, 2019) TuItitlán, State of Mexico Cr6+ 8 (Gutierrez-Ruiz, 1990) 0.25 León Valley, Guanajuato, Mexico Cr6+ 50 (ArmientaHernández, 1995) 2.3 Mérida Aquifer, Mexico Crtotal 0.37 (Marín, 2000) 0.18 Cuautitlán-Pachuca Aquifer, Mexico Crtotal 116.42 (SEMARNAT and IMTA, 2011) 25.45 In Mexico, several government agencies are responsible for regulating water use and quality, such as the Ministry of Health (SSA), which has standards for human consumption and drinking water; the National Water Commission (CONAGUA), which manages, regulates, contracts, and protects national waters; and the Ministry of Environment and Natural Resources (SEMARNAT), which has standards for water discharge. The following official Mexican standards establish the maximum permissible limits for chromium in drinking water, artificial aquifer recharge, and wastewater discharges. The Official Mexican Standard NOM-127-SSA1-1994, modified in 2000, establishes the permissible limits of quality and treatments to which water must be subjected for its potabilization; which establishes as a permissible value for total chromium 0.05 mg / L in drinking water (SSA, 1994). The Official Mexican Standard NOM-014-CONAGUA-2003 mentions the requirements for the artificial recharge of aquifers with treated wastewater and the maximum permissible concentration for total chromium is 0.05 mg / L (CONAGUA, 2003). NOM-001-SEMARNAT-1996 establishes the permissible limits of pollutants in wastewater discharges into receiving bodies owned by the nation; in rivers, streams, canals and drains the limit for chromium is 1 mg / L monthly average and in reservoirs, lakes and lagoons it is 0.5 mg / L monthly average (SEMARNAT, 1996). The World Health Organization states that due to the carcinogenicity and toxicity of chromium (+6) by inhalation, the current reference value of 0.05 mg / L for total chromium has been questioned, but toxicological data do not support the derivation of a new value. As a practical measure, 0.05 mg / L is considered unlikely to pose significant health risks and has been retained as a provisional reference value until further information becomes available and chromium can be reassessed (WHO, 2003). Therefore, there is a need in the field for new methodologies that allow for the effective removal of Cr6+ from contaminated water, achieving the production of treated water that complies with the NOM-127-SSA1-1994 standard for water intended for human use and consumption. Brief Description of the Invention The present invention provides a method for removing chromium (Cr6+) present in water contaminated with this element using unconventional, economical and easily obtained materials in our country, such as metallurgical slags. The method comprises a Cr6+ reduction step with Fe+2 ions from non-ferrous slags containing iron oxides and a Cr3+ precipitation step with OH- ions from ferrous slags containing calcium oxide (CaO). Figures The particular features and advantages of the invention, as well as other objects thereof, will be apparent from the following description, taken in conjunction with the accompanying figure, where: Fig. 1 shows the chemical composition of iron, steel, and copper slags. Fig. 2 shows the minerals identified in copper and steel slags. Figure 3 is a graph showing the concentration of total iron leached by applying a dose of 1 g / L of copper slag to distilled water, with a particle size of 0.250 mm for 60 minutes. Figure 4 is a graph showing the concentration of total iron leached by applying a dose of 1 g / L of copper slag to distilled water, with a particle size of 0.074 mm for 60 minutes. Figure 5 is a graph showing the removal of Cr6+ with an initial concentration of 2 mg / L and different initial pH with copper and steel slags. Figure 6 is a graph showing the removal of Cr6+ with an initial concentration of 25 mg / L and different initial pH with copper and steel slags. Figure 7 is a graph showing the effect of steel slag dosage on Cr3+ precipitation and pH. bZR / nn / zznz / e / Yi Detailed Description Definitions: The following definitions are provided for the purpose of enabling a better understanding of the invention: “Approximately” – The use of this term provides a certain additional range with respect to the numerical value to which it is being applied. This additional range is ± 10%. For example, but not limited to, if “approximately 40 grams” is stated, the exact range being described and / or claimed is between 36 grams and 44 grams. It is important to note that, for the purposes of the present invention, when an interval is indicated, it is understood that any numerical value included within said interval, including its ends, is included within the scope of the present invention. As stated above, the object of the present invention is to provide a method for removing chromium (Cr6+) present in contaminated water, comprising: For the reduction stage: Mix a quantity of non-ferrous metallurgical slag with water contaminated with Chromium (Cr6+); Adjust the pH of the mixture to an acidic pH; Shake for a specific time and speed; After the agitation time has elapsed, remove the non-ferrous metallurgical slag from the mixture by generating an effluent; For the precipitation stage: Add a quantity of ferrous metallurgical slag to the effluent from the previous stage, achieving a basic pH; Stir for a determined time and speed. Once the agitation period for the precipitation stage has elapsed, the generated sludge is allowed to settle, and the effluent is subsequently filtered to obtain water that complies with the modified NOM-127-SSA1-1994 standard. A conventional sand filter can be used in this filtration stage. The chromium (Cr6+) contaminated waters that are the subject of the present invention are natural drinking waters (groundwater and surface water), as well as wastewater that has been contaminated as a result of anthropogenic activities, among which we can mention: electroplating workshops, tanneries, mining (chromite extraction), textiles, dyeing, photographic printing, pharmaceutical products, stainless steel manufacturing (Bakshi et al., 2018), wood preservation, paper manufacturing, preparation of chromium compounds (Jobby et al., 2018), and the production of pigments and paints (Pradhan et al., 2017). The effluents from most industries are discharged directly into bodies of water or fields without any treatment, resulting in pollution and destruction of the ecosystem (Vendruscolo et al., 2017). Metallurgical slags are byproducts generated from metal smelting operations; that is, they are materials produced in parallel with, or as a consequence of, the production of a primary product (iron and steel), through the combination of molten material with the gangue, or valueless portion of the mineral. They are mainly composed of calcium silicates and ferrites, combined with oxides, fluxes (carbonates and silicates), aluminum, iron, manganese, calcium, magnesium, limestone, ash, furnace lining, and other chemical elements added to the furnace during smelting (SEMARNAT, 2009) (SEMAR-CANA, 2006) (EUROSLAG, 2018). Slags are classified into two groups: ferrous slags from primary iron and steel production, and non-ferrous slags from the production of base metals such as nickel, zinc, copper, and some precious metals. The chemical composition of slag is a result of the ore, fluxes, fuel, and furnace conditions (Piatak, 2018). Blast furnace slag is primarily composed of silica and alumina from the original iron ore (acidic clay gangue from the iron material and sulfur ash from the coke), with calcium and magnesium oxides (both basic compounds) from the more or less dolomitic limestones used as fluxes. These components are also used in basic oxygen furnace (BOF) and electric arc furnace (EAF) processes, so these slags are similar in composition to basic furnace (BF) slag, but with a higher iron and manganese content (Proctor et al., 2000).Figure 1 shows the chemical composition of iron, steel, and copper slags, while Figure 2 identifies the minerals present in copper and steel slags. The preferred slags for use in the present invention are selected non-ferrous slags, for example, from copper slags (EC) and selected ferrous slags, for example, from steel slags (EA) or iron. One of the characteristics of steel slag is its ability to alkalize aqueous solutions, increasing the pH due to its content of calcium, magnesium, aluminum, iron, and manganese oxides, which, upon contact with water, become hydroxides. The use of this type of slag in the method of the present invention advantageously allows for avoiding the use of additives or additional components that fix the pH at basic levels, since the use of this type of slag allows the pH to be adjusted to the basic levels required to achieve Cr3+ precipitation. The pH of the mixture in the reduction step is between approximately 2 and approximately 6. The preferred pH in this step is in the range of approximately 2 to approximately 3, with approximately 2 being the most preferred. The acidic pH is obtained by adding a strong or weak acid that allows the pH to be reduced to the required levels. Examples of acids that can be used are sulfuric acid, hydrochloric acid, nitric acid, etc. When using sulfuric acid, it can be used at a 50% concentration, and the concentration used will depend on the strength or weakness of the acid. The particle size of the slag is an important factor for the reduction and precipitation of chromium (Cr6+) and chromium (Cr3+), respectively. The particle size of non-ferrous slag, for example, copper slag, ranges from approximately 0.037 mm to approximately 0.250 mm, preferably from approximately 0.048 mm to approximately 0.174 mm. In one embodiment of the invention, the particle size is 0.048 mm, 0.111 mm, or 0.12 mm. The formula that governs the reduction reaction of Cr6+ to Cr3+ is as follows: Cr2Oy~ + 6Fe2++ 14H+-> 2Cr3++ 6Fe3++ 7H2O (Han et al., 2016) (Equation 1) To reduce 2 moles of chromium Cr6+, 6 moles of Fe2+ are required. bZR / nn / zznz / e / Yi / 6 mol Fe2+\ f 1 mol Cr6+\ ( 55.84α Fe \ „ „ „ Q Fe / -------tt--------- ----= 3.22 / r 6+Equation 2 \2moZCr6+7 V 51.99 g Cr J \1 mol Fe2+J / g Crb+ The stoichiometric mass ratio is: 3.22 g / Fe2+ / Cr6+ To determine the copper slag dosage with respect to the initial Cr6+ concentration, the stoichiometric relationship obtained with Equation 2 is used. In one embodiment of the invention, the dosage of non-ferrous slag, for example, copper slag, ranges from approximately 1 g / L to approximately 35 g / L, preferably from approximately 10 g / L to approximately 20 g / L. In one embodiment of the invention, the concentration is approximately 15 g / L or approximately 16.7 g / L. The time that the non-ferrous slag, for example copper slag, is in contact with the water contaminated with Chromium (Cr6+) in the reduction stage ranges from approximately 50 minutes to 24 hours, preferably between 55 minutes and 6 hours, more preferably 60 minutes. The stirring speed in the reduction stage ranges from approximately 100 rpm to 250 rpm. In one embodiment of the invention, the speed is 150 rpm. Once the contact time between the non-ferrous slag and the water has elapsed during the reduction stage, the non-ferrous slag is removed from the mixture by sedimentation of the coarse particles and filtration of the fine particles, resulting in an effluent. This ensures that the slag is not present in the precipitation stage. As will be evident to a technician in this field, the filter mesh size used will be that which allows for the removal of the non-ferrous slag. The amount of ferrous slag, for example, steel slag, used for the precipitation step is from approximately 8 g / L to approximately 130 g / L of slag in reduced solution, preferably from approximately 10 g / L to approximately 100 g / L, more preferably from approximately 50 g / L to approximately 90 g / L. In one embodiment of the invention, a dosage of 70 g / L of slag is used. The contact time during the precipitation stage between the ferrous slag, for example, steel slag, and the reduction solution resulting from the reduction stage is between approximately 30 minutes and approximately 6 hours, preferably between approximately 50 minutes and approximately 3.5 hours, more preferably approximately 3 hours. The stirring speed used during the second precipitation stage ranges from approximately 150 rpm to approximately 300 rpm, more preferably 250 rpm. The particle size of the ferrous slag is in the range from approximately 0.037 mm to approximately 0.30 mm (400 to 48 mesh, respectively), preferably from approximately 0.048 mm to approximately 0.25 mm (325 to 60 mesh, respectively), more preferably 0.048 mm (325 mesh). The pH for the precipitation stage should be adjusted between approximately 6 and approximately 9. Preferably, the pH is in the range between approximately 7 and approximately 8. This pH is obtained with the ferrous slag used. bZR / nn / zznz / e / Yi It is important to note that metallurgical slags must be conditioned before use in the removal method of the present invention. This conditioning is achieved by sieving the slags to separate the constituent particles by size, so that the weight of each size can be determined and the amount that can be used without further grinding can be known. Optionally, the slag can be further ground to reduce the particle size, followed by screening with different mesh sizes. Grinding can be carried out in various devices designed for this purpose, such as a ball mill, and screening in a Ro-tap system with different mesh sizes, for example, 140, 200, 325, and 400 (0.105 mm, 0.074 mm, 0.048 mm, and 0.037 mm). EXAMPLES The slag used in the different experiments was obtained from mining companies located in the north of the country. The slag was conditioned according to the method described above. Example 1 Tests with copper slag at different initial pH These tests experimentally determined the amount of iron that copper slag can leach as a function of pH and in the absence of Cr6+. Therefore, the tests were conducted in the laboratory using two different particle sizes. The tests were performed in duplicate in a batch reactor and consisted of determining the experimental dose of copper slag by measuring the amount of iron released at different initial pH values ​​and with two particle sizes: 60 mesh (large) and 200 mesh (fine). In 500 mL Erlenmeyer flasks, 250 mL of the test solution was prepared, consisting of distilled water, and the pH was adjusted with sulfuric acid or sodium hydroxide. The initial pH values ​​used were 2, 3, 4, 5, and 6.Then, 1 g / L of copper slag was added, and the flasks were placed on an orbital shaker at a constant speed of 150 rpm for 60 minutes. Six samples were taken every 10 minutes, and total iron was measured. The 1 g / L dose of copper slag was chosen because it was necessary to determine how much total iron leached and thus calculate, using stoichiometry, the experimental dose. Figure 3 shows copper slag with a mesh size of 60 (0.174 mm), and Figure 4 shows copper slag with a mesh size of 200 (0.074 mm). Comparing the two graphs, it can be observed that the 200-mesh slag releases more iron at different initial pH values ​​compared to the 60-mesh slag. For the 200-mesh size, the slag at pH 2, 3, 4, 5, and 6 leached iron in amounts of 6.21 mg / L, 5.07 mg / L, 3.69 mg / L, 3.18 mg / L, and 1.93 mg / L, respectively. In contrast, the 60-mesh slag clearly only leaches up to 5.18 mg / L of iron in 60 minutes at pH 2, and at the other pH values, it leaches iron in amounts lower than 1.53 mg / L. These results are associated with the fact that a decrease in particle size increases the contact surface of the slag with the acidic medium, and the more acidic (pH=2) the aqueous solution is, the more it will benefit the leaching of a greater amount of iron. bZR / nn / zznz / e / Yi Example 2 Reduction of Cr6' with copper slag In 250 mL Erlenmeyer flasks, 200 mL of the test solution were prepared with an initial Cr6+ concentration ranging from a minimum of 2 mg / L to a maximum of 50 mg / L at pH 2. The pH was adjusted with sulfuric acid. These concentrations correspond to the values ​​shown in Table 1 for elevated chromium concentrations in groundwater. Table 2 shows the doses of copper slag used for the reduction of Cr6+ to minimum and maximum concentrations. Based on the results of Cr6+ concentration after reduction, it is verified that the theoretical dose of 0.25 g / 200 mL at an initial concentration of 2 mg / L reduced Cr6+ by 1.91 mg / L (95.1%), leaving 0.09 mg / L unaddressed. In contrast, the theoretical dose of 6.22 g / 200 mL at an initial concentration of 50 mg / L only reduced Cr6+ by 24.83 mg / L (50.32%). Table 2. Results of Cr6+ reduction with copper slags bZR / nn / zznz / e / Yi Concentration Dose EC Initial Concentration % Cr6+ (g) in 200 mL Final Cr6+ Reduction (mg / L) (mg / L) 2 0.25 0.09 95.1 50 6.22 25.17 50.32 Since it is necessary to increase the percentage reduction to 100% of the initial maximum concentration, it is proposed to change the concentration from 50 mg / L to a smaller one and keep the dose of 6.22 g / 200 mL of copper slag constant. To determine the required concentration, the following equation was used: / 50 mq / L \ ---—— 50.32% = 25.16mg / £ \ 100% / Table 3 shows the result obtained by changing the maximum initial concentration to 25 mg / L. The percentage reduction achieved was 97.9%, therefore, the reduction did increase when the initial chromium concentration was lowered. This test establishes that the maximum initial Cr6+ concentration should be 25 mg / L for subsequent exploratory tests with a copper slag dose of 31.1 g / L. Table 3. Cr6+ reduction result with an initial concentration of 25 mg / L Initial Cr6+ concentration (mg / L) EC Dose Concentration % (g) in 200 Final Cr6+ Reduction mL (mg / L) 6.22 0.525 97.9 To obtain the milligrams of Cr6+ present in 200 mL of the 2 mg / L Cr6+ solution. mg of Cr6+= (0.2 Γ 2 -p-} = 0.4 mg of Cr6+ To obtain the total milligrams of iron needed to reduce Cr6+ present in 200 mL, the stoichiometric mass ratio bZR / nn / zznz / e / Yi is taken into account: 3.22 g Fe2+ / g Cr6+ 3220 mg Fe 1000 mg Cr6+) 0.4 mgCr6+ 1.29 mg Fe To determine how much copper slag is needed to reduce 0.4 mg Cr6+ present in 200 mL with copper slag with a particle size of 0.25 mm, 5.18 mg of Fe is leached at pH 2 in 60 minutes. Additionally, the theoretical total milligrams of Fe required to reduce Cr6+ are considered: 1.29 mg Fe. 1000 mg escoriax / \g 5.18 mg of Fe / VlOOOmg 1.29 mg Fe = 0.25 g copper slag A similar analysis is performed for the concentration of 50 mg / L. Example 3 Reduction kinetics of Cr6' with copper slags Reduction kinetics tests were performed to determine the contact time required to reduce Cr6+ to Cr3+ at different initial pH values ​​in a batch reactor in duplicate. In ten 250 mL Erlenmeyer flasks, 200 mL of the test solution prepared with distilled water and an initial Cr6+ concentration of 25 mg / L were added to each flask at different initial pH values ​​(2, 3, 4, 5, and 6), using sulfuric acid and sodium hydroxide to adjust the pH. The dosage of copper slag with a particle size of 0.25 mm (60 mesh) used was 31.1 g / L. The flasks were placed on an orbital shaker and mixed at a constant speed of 150 rpm for 24 hours. Samples were taken at different time intervals, and residual Cr6+ was measured. It is important to mention that the copper slag dose of 31.1 g / L was chosen from the previous result with the initial concentration of 25 mg / L of Cr6+ and with this slag dose a reduction of 97.9% was obtained. Table 4 shows the 24-hour reduction kinetics with an initial Cr6+ concentration of 25 mg / L. After 24 hours at pH 2, the Cr6+ concentration was 0.27 mg / L, representing a 98.92% reduction. This was followed by pH 3, although with a lower percentage reduction of 37.52% and a concentration of 15.62 mg / L. Finally, at pH 4, 5, and 6, the concentrations were 21.13 mg / L, 21.66 mg / L, and 21.79 mg / L, respectively. Within 60 minutes, the chromium reduction was as follows: pH 2: 0.86 mg / L; pH 3: 20.26 mg / L; pH 4: 21.57 mg / L; pH 5: 21.64 mg / L; and pH 6: 21.76 mg / L. The very low reduction results observed for pH values ​​of 3, 4, 5, and 6 could be due to the slow release of Fe2+ ions from the copper slag and the depletion of protons involved in the Cr6+ reduction reaction by the released iron. Based on the results obtained, 60 minutes was selected as the contact time needed to reduce Cr6+ to Cr3' at pH 2. In the reduction process by Fe2+ with Cr6+, the redox reaction is usually complete and has fast kinetics due to the large difference in their reduction potentials under acidic conditions (Qin et al., 2005); the final products of this reaction are coprecipitates of Fe3+ and Cr3+ in acidic medium. Table 4. Copper slag reduction kinetics Time (min) Cr6+ CONCENTRATION in mg / L pH 2 pH 3 pH 4 pH 5 pH 6 0 25 25 25 25 25 5 22.18 24.01 24.28 23.73 23.96 10 21.65 22.77 23.31 23.39 23.27 15 17.48 22.33 22.94 22.94 22.75 20 10.62 21.85 23.14 22.28 22.70 25 10.26 21.52 22.44 21.92 22.73 30 7.39 20.87 22.21 21.94 22.86 35 5.93 20.71 22.45 21.88 22.28 40 4.64 20.86 22.31 21.78 22.20 45 3.61 20.75 22.15 21.83 22.21 50 2.64 20.62 21.76 21.89 22.31 55 1.50 20.38 21.79 21.96 22.16 60 0.86 20.28 21.57 21.64 21.76 90 0.36 19.94 21.65 21.66 21.69 120 0.30 19.86 21.69 21.79 21.81 180 0.30 19.37 21.38 21.76 21.82 240 0.29 18.09 21.21 21.65 21.73 300 0.28 19.02 21.31 21.72 21.76 360 0.27 17.63 21.47 21.78 21.64 720 0.27 18.03 21.29 21.64 21.94 1440 0.27 15.62 21.13 21.66 21.79 bZR / nn / zznz / e / Yi Example 4 Precipitation kinetics of Cr(OH)3 with steel slags The precipitation kinetics were performed after the reduction kinetics were completed, so the same 10 flasks with the reduced solution were used, but the copper slag was removed before adding the steel slag with a particle size of 0.25 mm (60 mesh) and a dose of 8 g / L; the flasks were shaken at 150 rpm for 24 hours; samples were taken at different time intervals and residual Cr6+ was measured. The results of the 24-hour precipitation kinetics with steel slag are presented in Table 5, starting from the effluent of the chromium reduction stage. Notably, at pH 2 the Cr6+ concentration was 0.17 mg / L, at pH 3 after 24 hours the concentration was 15.17 mg / L, and finally, at pH 4, 5, and 6, the concentrations were 20.47 mg / L, 20.25 mg / L, and 20.15 mg / L, respectively. The Cr6+ concentrations after 60 minutes are: pH 2 0.19 mg / L, pH 3 15.29 mg / L, pH 4 20.98 mg / L, pH 5 20.91 mg / L, and pH 6 20.82 mg / L. After 60 minutes, the amount of residual Cr6+ present in the water remains constant, so it is not necessary to extend the precipitation time to 90 or 120 minutes. Therefore, 60 minutes was determined to be the optimal contact time.Precipitation is important because during reduction, iron enters the solution by dissolving from the slag and must be removed along with the reduced chromium by precipitation methods (Kiyak et al., 1999). Table 5. Precipitation kinetics with steel slag Time (min) Cr6+ CONCENTRATION in mg / L pH 2 pH 3 pH 4 pH 5 pH 6 0 0.27 15.62 21.13 21.66 21.79 10 0.27 15.53 21.1 21.59 21.64 20 0.26 15.44 21.1 21.5 21.57 30 0.24 15.32 21.07 21.46 21.48 40 0.23 15.3 21.02 21.33 21.3 50 0.22 15.28 20.98 21.15 21.12 60 0.19 15.29 20.98 20.91 20.82 120 0.19 15.24 20.86 20.8 20.71 180 0.19 15.21 20.7 20.62 20.48 360 0.18 15.19 20.63 20.5 20.45 720 0.19 15.19 20.52 20.34 20.29 1440 0.17 15.17 20.47 20.25 20.15 bZR / nn / zznz / e / Yi Example 5: Chromium removal tests with copper and steel slags Chromium removal tests were performed in synthetic water with copper and steel slag at different initial pH levels. All tests were performed in triplicate in a batch reactor. In 250 mL flasks, 200 mL of the test solution were prepared, consisting of distilled water and initial Cr6+ concentrations of 2 and 25 mg / L, at different initial pH values ​​(2, 3, 4, 5, and 6), adjusted with sulfuric acid or sodium hydroxide. For a minimum initial Cr6+ concentration of 2 mg / L, the dosage of 60-mesh copper slag was 1.25 g / L, and for the maximum initial concentration of 25 mg / L, the dosage was 31.1 g / L. The flasks were placed on an orbital shaker at 150 rpm for 60 minutes, after which the total residual iron and residual Cr6+ were measured. To carry out the precipitation, it was necessary to remove the copper slag from the flasks and a dose of 8 g / L of steel slag with a particle size of 0.25mm (60 mesh) was added for all tests and stirred at 150 rpm for 60 minutes, after 60 minutes the final pH and final Cr6+ concentration were measured. Figure 5 shows the removal of chromium from metallurgical slags with an initial chromium concentration of 2 mg / L at different initial pH values ​​(2, 3, 4, 5, and 6). At pH 2, the final concentration was 0.01 mg / L, with 1.99 mg / L removed, almost 100%. At pH 3, the final concentration was 20.74 mg / L, with 1.26 mg / L of chromium removed. At pH 4, 5, and 6, the final concentrations were 1.74 mg / L, 1.75 mg / L, and 1.78 mg / L of chromium, respectively. Figure 6 shows the removal with an initial chromium concentration of 25 mg / L at different initial pH values. At pH 2, 24.34 mg / L of chromium was removed, leaving 0.66 mg / L. At pH 3, the concentration was 14.06 mg / L, and 10.94 mg / L was removed. At pH 4, 5, and 6, the concentrations were 20.8 mg / L, 21.2 mg / L, and 21.5 mg / L, respectively. At pH 4, the concentration removed was 4.2 mg / L; at pH 5, 3.8 mg / L; and at pH 6, 3.5 mg / L.As the pH increases, removal decreases; therefore, it is important that the removal be carried out at a very acidic pH (pH=2) to maximize total iron leaching and facilitate Cr6+ reduction. Table 6 shows the results of this experiment. Table 6. Chromium Removal with Copper and Steel Slags with Different pH bZR / nn / zznz / e / Yi Cr (VI) Concentration (mg / L) Initial pH Final pH Fe (mg / L) Cr (VI) Reduction (mg / L) Final Cr (VI) (mg / L) % Elimination 2 2.06 2.39 5.53 0.07 0.01 99.50 25 2.05 2.41 6.01 2.93 0.66 97.37 2 3.07 4.51 0.67 1.40 0.74 62.99 25 3.07 4.35 2.84 17.60 14.06 43.76 2 4.06 7.22 0.13 1.90 1.72 14.19 25 4.04 6.14 1.64 22.80 20.80 16.8 2 5.13 8.09 0.12 1.90 1.75 12.5 25 5.13 6.33 2.91 23.29 21.20 15.2 2 6.03 8.70 0.18 1.96 1.78 11.0 25 6.01 6.43 2.88 23.80 21.50 14.0 Example 6: Copper removal tests with real water Table 7 presents the results of the 17 tests and their duplicates performed with synthetic water and real water contaminated with chromium. The results of the tests with both types of water were similar. Test 13 achieved the highest percentage reduction in both water samples, with 98.55% (synthetic) and 98.89% (real); the test with the lowest reduction was test 4, which only reached 50.52% (synthetic) and 45.77% (real). The difference in reduction between the two experiments is almost 50%. To observe the influence of copper slag particle size, tests 11 and 12 were conducted with the same initial Cr6+ concentration (15 mg / L) and EC dose (10 g / L), but with different slag sizes of 0.048 mm (fine) and 0.174 mm (large). The reduction of Cr6+ was greater with the fine particle size (test 12), at 97.29% (synthetic) and 96.91% (real), and less with the large particle size (test 11), at 59.31% (synthetic) and 63.77% (real). It can be concluded that a finer particle size is more effective for Cr6+ reduction. Erdem et al. (2004) state that particles smaller than 200 mesh are sufficient to reduce all Cr6+ ions in a short time. The influence of the copper slag dose variable can be observed in tests 16 and 17. With the same initial Cr6+ concentration (15 mg / L), EC size (0.111 mm), and different EC doses of 3.30 g / L (test 17) and 16.7 g / L (test 16), the greatest reduction is observed in test 16, with 98.24% (synthetic) and 97.58% (actual), which has the highest slag dose. Therefore, test 17, containing the lowest dose, shows a smaller reduction, with 62.54% (synthetic) and 73.27% (actual). It is asserted that the initial Cr6+ concentration can be completely reduced by increasing the dosage of the reducing agent (copper slag). Erdem et al. (2005) concur that increasing the dose increases the percentage of Cr6+ reduction over a certain contact time. To assess the influence of the initial Cr6* concentration variable, tests 13 and 15 were compared using the same EC dose (10 g / L), EC size (0.111 mm), and different initial Cr6+ concentrations of 4.96 mg / L (test 15) and 5 and 25 mg / L (test 13). With the initial concentration of 25 mg / L, the reduction was 98.55% (synthetic) and 98.89% (real), and with the concentration of 4.96 mg / L, it was 94.39% (synthetic) and 94.39% (real). There was very little difference in the percentage reduction between the two tests. With this comparison of the two tests, it can be said that the initial concentration variable of Cr6+ is not as significant as the dose and size of copper slag, which do influence the reduction of hexavalent chromium. It is important to mention that only in this study is the initial chromium concentration variable not significant because the range of copper slag doses used were sufficient for the chromium concentrations used. Han et al. (2016) mention that the dependence of the Cr6+ reduction efficiency on its initial concentration is due to the limited amount of Fe2+ in the constant mass slag. bZR / nn / zznz / B / Yi Table 7. Results of the optimization of the Cr6+ reduction stage with synthetic and real water bZR / nn / zznz / e / Y No. VARIABLES SYNTHETIC WATER REAL WATER Exp. Conc. Dose Size Cr6+ Fe % Cr6+ Fe % Initial EC of EC residual residual Residual reduction residual Reduction (mg / L) (mm) (g / L) (mg / L) (mg / L) of Cr6+ (mg / L) (mg / L) of Cr6+ 3.57 98.15 0.335 3.85 97.77 2 9 0.149 14 0.235 3.99 97.38 0.364 3.94 95.96 3 21 0.074 14 0.463 3.799 7.707.76 97.59 4 21 0.149 6 8.212 3.61 60.90 11.18 3.53 46.72 5 9 0.074 6 0.235 3.94 97.38 0.606 3.99 93.27 1966.149. 3.74 79.82 1.902 3.79 78.87 7 21 0.074 6 0.463 3.75 97.79 0.378 3.77 98.20 8 9 0.074 14 0.421 3.79 0.23 4.23. 96.43 9 21 0.149 14 3.212 3.45 84.70 2.130 3.88 89.86 10 15 0.111 10 0.321 3.52 97.86 0.321 3.98 97.17 1.15 0.15 6.104 3.62 59.31 5.434 3.85 63.77 12 15 0.048 10 0.406 3.44 97.29 0.463 5.2 96.91 13 25 0.111 10.36 3.56 3.45 0.278 4.06 98.89 14 15 0.111 10 0.278 3.8 98.15 0.335 4.02 97.77 15 4.96 0.111 10 0.278 5.29 94.39 4.27 4.39 16 15 0.111 16.7 0.264 4.05 98.24 0.364 4.14 97.58 17 15 0.111 3.30 5.619 3.96 62.54 4.010 3.96 73.27 ID 15 0.111 10 0.364 4.64 97.58 0.364 3.84 97.58 2D 9 0.149 14 0.264 4.09 97.07 0.535 3.86 94.06 3D 21 0.074 14 0.620 4.17 97.05 0.463 4.04 97.79 4D 21 0.149 6 10.39 3.99 50.52 11.38 3.66 45.77 5D 9 0.074 6 0.307 3.98 96.59 0.620 3.98 93.11 6D 9 0.149 6 1.930 3.93 78.55 2.172 3.71 75.86 7D 21 0.074 6 0.506 3.94 97.59 0.435 3.95 97.93 8D 9 0.074 14 0.378 4.09 95.80 0.349 3.85 96.12 9D 21 0.149 14 4.337 3.95 79.35 2.358 4.01 88.77 10D 15 0.111 10 0.335 3.73 97.77 0.321 3.91 97.86 11D 15 0.174 10 6.089 4.35 59.40 6.018 3.89 59.88 12D 15 0.048 10 0.435 4.59 97.10 0.392 4.27 97.39 13D 25 0.111 10 0.976 4.12 96.10 0.378 4.17 98.49 14D 15 0.111 10 0.349 4.4 97.67 0.349 4.03 97.67 15D 4.96 0.111 10 0.321 4.54 93.53 0.335 4.45 93.24 16D 15 0.111 16.7 0.335 4.05 97.77 0.364 4.05 97.58 17D 15 0.111 3.30 4.309 4.33 71.27 3.839 3.85 74.41. This table shows that the copper slag dose had a greater positive effect on both waters as its value increased; this indicates that increasing the dosage will result in a greater reduction. Therefore, for both waters, the reduction of Cr6+ shows better results if the particle size is small, the initial concentration is lower, and the slag dose is higher. Taking the previous test as a reference, a test was conducted to determine the optimal values ​​for the Cr6+ reduction stage of the two response variables using copper slag. This was done using Multiple Response Optimization, obtained with the STATGRAPHICS software. This procedure helps determine the combination of experimental factors that simultaneously optimizes several responses. For the response variable "reduction percentage," the goal was to maximize, while for "residual Fe," the goal was to minimize, since the aim is to have the lowest possible dissolved iron and the maximum reduction percentage. Table 8 shows the optimal values ​​of the experimental factors with synthetic water and real water; for synthetic water they are: initial Cr6+ concentration of 20.17 mg / L and slag dose of 13.04 g / L with a size of 0.048 mm, for real water contaminated with chromium: initial Cr6+ concentration of 15.99 mg / L and a slag dose of 16.7 g / L with a particle size of 0.12 mm. Table 8. Optimal values ​​of the factors for the Cr6+ reduction stage with synthetic and real water bZR / nn / zznz / e / Yi Factor Low High Optimum Synthetic Water Optimum Actual Water Initial Cr6+ Concentration (mg / L) 4.96 25.04 20.17 15.99 Copper Slag Size (mm) 0.048 0.17 0.048 0.12 Copper Slag Dose (g / L) 3.3 16.69 13.04 16.7 Table 9 shows the response variables with their optimal theoretical values. For the response variable "reduction percentage," the target is 100% in synthetic water and 98.89% in real water. For "residual Fe," the target would be 3.82 mg / L in synthetic water and 3.88 mg / L in real water. Table 9. Response variables with optimal theoretical values ​​for the CrB+ reduction stage with synthetic and real water Response Optimal Synthetic Water Optimal Real Water % Reduction 100 98.89 Residual Fe 3.82 3.88 After the statistical program STATGRAPHICS CENTURION calculated the theoretical optimal values, these were checked by reproducing the theoretical optimal values ​​of the factors in the laboratory; the experiment was reproduced with one repetition. Table 10 shows the results obtained in synthetic water, where it can be seen that the experimental reduction percentage reached 100% and 99.8%, with total residual iron of 3.84 and 3.97 mg / L. With real water contaminated with chromium, a reduction of 99.86% and 99.87% was achieved, with total residual iron of 3.66 and 3.63 mg / L. These reduction percentages are 1% higher than the theoretical value, and the residual iron is lower than predicted by the theoretical value. bZR / nn / zznz / e / Yi Table 10 Experimental verification of the theoretical optimum values ​​of the Cr6+ reduction stage with synthetic water Operating Conditions Theoretical Values ​​Actual Values ​​Water Type Initial Concentration Cr(VI) (mg / L) EC Size (mm) EC Dose (g / L) Residual Fe (mg / L) % Reduction Residual Fe (mg / L) % Reduction 1 20.17 0.048 13.04 3.82 100 3.84 100 Synthetic ID 3.97 99.8 1 15.99 0.12 16.7 3.88 98.89 3.66 99.86 Actual ID 3.63 99.87 Example 7: Precipitation of Cr3' with steel slag To precipitate Cr3+, the copper slag was removed from the flasks. Then, 200 mL of the effluent from the reduction stage was placed in each flask, and the corresponding dose of steel slag with a particle size of 0.048 mm was added. The doses used were 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 g / L, with stirring at 250 rpm for 180 min. The flasks were then allowed to stand for 60 min (sedimentation time). Subsequently, a sample was taken from each flask, and pH, total Cr6+, total chromium, and total iron were measured. For dissolved iron, the samples were filtered at 0.45 µm, and for total dissolved chromium, they were filtered at 0.2 µm. Figure 7 shows the effect of steel slag dosage on the precipitation of chromium and iron in the reduced solution. As the pH of the solution increases with the addition of steel slag, the concentration of chromium and iron in the supernatant decreases. In compliance with the Maximum Permissible Limit (MPL) of the Amendment to NOM-127-SSA1-1994, the dissolved chromium was 0.04 mg / L with a steel slag dosage of 10 g / L and a pH of 6.06; in contrast, the dissolved iron was 0.24 mg / L with a steel slag dosage of 70 g / L and a pH of 7.83. This difference in dosage and pH between the chromium and iron required for precipitation is due to the different solubility product constants (Ksp) of both metals (Cr(OH)3 = 6.3 x 10^31 and Fe(OH)3 = 2 x 10^31). The precipitation of Cr3+ is related to the change in its species due to pH variation. When the Cr3+ solution is between pH 6 and 11, the predominant species is Cr(OH)3, which precipitates due to its minimal solubility in aqueous media (Mijaylova et al., 2003). This also confirms what Bonenfant et al. (2009) mention: that steel slags can alkalize solutions because they are considered basic due to their high content of CaO and other oxides (magnesium, aluminum, and manganese), and upon contact with water, they are converted into hydroxides. bZR / nn / zznz / B / Yi Table 11. Steel slag dosage tests on the precipitation of Chromium and Iron in the reduced solution Exp. Dose Slag Steel (g / L) Cr (VI) (mg / L) TOTAL TOTAL Chromium (mg / L) DISSOLVED Chromium (mg / L) SUSPENDED Chromium O (mg / L) TOTAL Fe (mg / L) Dissolved Fe (mg / L) Suspended Fe (mg / L) Final pH 0.20 4.73 4.05 0.68 2.3 1 10 0.007 0.09 0.04 0.05 4.38 3.87 0.51 6.06 2 20 0.013 0.07 0.03 0.04 3.20 3.36 30 0.015 0.08 0.04 0.04 4.19 3.93 0.26 6.36 4 40 0.012 0.07 0.04 0.03 4.11 4.07 0.04 6.55 5 50 0.010 0.060 3.060. 1.43 0.53 0.90 6.95 6 60 0.008 0.07 0.04 0.03 0.94 0.36 0.58 7.15 7 70 0.012 0.07 0.04 0.03 0.71 0.07 0.88 7.4 0.020 0.06 0.03 0.03 0.64 0.16 0.48 7.85 9 90 0.013 0.06 0.03 0.03 0.62 0.14 0.48 7.87 10 100 0.01 0.00 7.0.04 0.70 0.17 0.53 8.14 Finally, the embodiments of the invention described here are not intended to limit its scope, but only to illustrate some of the variations that fall within its spirit and scope. As will be evident to a person with average knowledge of the subject, variations or modifications that do not depart from the spirit of the invention are within its scope.

Claims

1. A method for removing chromium from water comprising: For the reduction stage: Mixing a quantity of non-ferrous metallurgical slag with water contaminated with Cr6+; Adjusting the pH of the mixture to an acidic pH; Stirring for a predetermined time and speed; After the stirring time has elapsed, removing the non-ferrous metallurgical slag from the mixture, generating an effluent; For the precipitation stage: Adding a quantity of ferrous metallurgical slag to the effluent produced from the previous stage, achieving a basic pH; Stirring for a predetermined time and speed.

2. The method according to claim 1, wherein the non-ferrous slag is selected from copper slag.

3. The method according to claim 1, wherein the ferrous slag is selected from steel or iron slag, preferably steel slag.

4. The method according to claim 1, wherein the non-ferrous slag has a particle size between approximately 0.037 mm and approximately 0.250 mm.

5. The method according to claim 1, comprising conditioning the metallurgical slags before their incorporation into the Cr6+ removal.

6. The method according to claim 1, wherein the pH of the mixture in the reduction step is between approximately 2 and approximately 6.

7. The method according to claim 6, wherein the pH is adjusted to approximately 2.

8. The method according to claim 6, wherein the pH is adjusted with a strong or weak acid.

9. The method according to claim 1, wherein the stirring speed employed during the second precipitation stage ranges from approximately 150 rpm to approximately 300 rpm.

10. The method according to claim 1, wherein the time that the non-ferrous slag is put into contact with the water contaminated with Chromium (Cr6+) in the reduction stage ranges from approximately 50 minutes to 24 hours, preferably from 55 minutes to 6 hours, more preferably 60 minutes.

11. The method according to claim 1, wherein the dosage of non-ferrous slag ranges from approximately 1 g / L to approximately 35 g / L, preferably from approximately 10 g / L to approximately 20 g / L. bZR / nn / zznz / e / Yi 12. The method according to claim 1, wherein the pH of the mixture in the precipitation step is between approximately 6 and approximately 9.

13. The method according to claim 1, wherein the amount of ferrous slag used for the precipitation step is from approximately 8g / L to approximately 130g / L of slag to reduced solution, preferably from approximately 10g / L to approximately 100g / L.

14. The method according to claim 12, wherein the pH is adjusted from approximately 7 to approximately 8.

15. The method according to claim 1, wherein the contact time during the precipitation stage between the ferrous slag and the reduction solution resulting from the reduction stage is between approximately 30 minutes and approximately 6 hours, preferably between approximately 50 minutes and approximately 3.5 hours.

16. The method according to claim 1, wherein the particle size of the ferrous slag is in the range from approximately 0.037 mm to approximately 0.30 mm (400 to 48 mesh, respectively), preferably from approximately 0.048 mm to approximately 0.25 mm.

17. The method according to claim 1, wherein, in order to remove non-ferrous slag from the reduction stage, the coarse particles of the mixture are allowed to settle and the fine particles are filtered out, obtaining an effluent that will be subjected to the precipitation stage.

18. The method according to claim 1, wherein once the agitation time of the precipitation stage has elapsed, the generated sludge is allowed to settle and subsequently the effluent is filtered.

19. The method according to claim 8, wherein the strong or weak acid is selected from sulfuric acid, hydrochloric acid, or nitric acid.

20. The method according to claim 11, wherein the dosage of non-ferrous slag is approximately 15 g / L or approximately 16.7 g / L.