Precipitation Of Sludge Research Articles

Degradation of phenol by metal-free electro-fenton using a carbonyl-modified activated carbon cathode: Promoting simultaneous H2O2 generation and activation

The low yield of hydrogen peroxide, narrow pH application range, and secondary pollution due to iron sludge precipitation are the major drawbacks of the electro-Fenton (EF) process. Metal-free electro-Fenton technology based on carbonaceous materials is a promising green pollutant degradation technology. Activated carbon cathodes enriched with carbonyl functional groups were prepared using a two-step annealing method for the degradation of phenol pollutants. The •OH in the activation process of H2O2 were identified using the EPR test technique. The action mechanism of carbonyl groups on H2O2 activation was investigated in conjunction with density functional theory (DFT) calculations. The EPR tests demonstrated that the modified activated carbon could promote the in-situ activation of H2O2 to •OH. And the results of material analysis and DFT showed that C=O could facilitate the activation of hydrogen peroxide through the electron transfer mechanism as an electron-donating group. Electrochemical tests showed that both the oxygen reduction activity and 2e−ORR selectivity of the modified activated carbons were significantly improved. Compared with the original activated carbon cathode and EF, the degradation efficiency of phenol in the ACNH-1000/GF cathode was increased by 58.10% and 45.61%, respectively. Compared with EF, ACNH-1000/GF metal-free electro-Fenton effectively expands the pH application range, and is proven to be less affected by solution initial pH, while avoiding secondary pollution. The metal-free electro-Fenton system can save more than a quarter of the cost of EF system. This study has a deep understanding of the reaction mechanism of the carbonyl modified activated carbon, and provides valuable insights for the design of metal-free catalysts, so as to promote its application in the degradation of organic pollutants.

Rapid Treatment and Reaction Mechanism of Complexation Plating Effluents by Molybdenum Disulfide/Copper Sulfide+Peroxosulfate Process

AbstractThe removal of complexed heavy metals in complexation plating effluents is always a problem. Here molybdenum disulfide/copper sulfide (MoS2/CuS) composite was synthesized to activate peroxosulfate (PMS) to treat complexation nickel simulated plating effluents. MoS2/CuS showed excellent PMS activation effect. The removal efficiency of 50 ppm Ni‐EDTA by MoS2/CuS+PMS process and Ni‐citrate by MoS2/CuS process reached 99.7 % and 100 % for 20 min treatment, respectively. The excellent performance of MoS2/CuS+PMS process for heavy metal complexes removal is attributed to two ways: oxidative degradation and catalytic decomplexation. Oxidative degradation is to degrade complexing agent into small molecules through .SO4− produced by PMS activated directly by the synergy between MoS2, CuS and Ni‐EDTA. Catalytic decomplexation is to replace Ni2+ in Ni‐EDTA by the metal active sites of MoS2/CuS to release Ni2+ and remove it. These two degradation pathways make the MoS2/CuS+PMS process has excellent removal efficiency. In addition, the MoS2/CuS+PMS process eliminates the dependence of the Fenton reaction on iron‐based reagents and avoids the production of iron sludge precipitation. These findings provide new insights for the development of metal sulfide activating PMS to treat complexation plating effluents.

Experimental evaluation of henna extract as a corrosion inhibitor additive in sandstone acidizing and its side effect on rock/fluid system

ABSTRACT Green inhibitors are promising alternatives to toxic corrosion inhibitors. Recently, several researches are conducted to investigate the anti-corrosive property of henna extract in different media. The current work investigates for the first time the inhibitive characteristic of the henna in protecting the N80 API steel immersed in the regular mud acid in the range of 28–75°C, beside its side effect on reservoir rock and fluid during acid stimulation. Potentiodynamic-polarization, electrochemical impedance spectroscopy, and immersion tests were employed for corrosion studies. The compatibility of the inhibitor with reservoir rock and reservoir fluid was investigated through the wettability test conducted via the sessile dope technique and the sludge test applied by the Houchin method, respectively. The results confirm the efficiency of the inhibitor in protecting the N80 steel in regular mud acid in the range of 28–75°C. The optimum concentration of henna at 28, 40, 60, and 75°C was 1.6, 1.4, 0.8, and 0.4 ppm, respectively. The consequent inhibition efficiency for all temperatures is more than 80% and increases with the temperature rising. The thermodynamic analysis confirms the chemical adsorption of henna molecules on the metal face through a spontaneous endothermic reaction. This inhibitor causes 0.96 wt% additional sludge precipitation in contact with regular mud acid, which is the minimum value in comparison with the main acid and other acidizing additives selected from a commercial package. The contact angle of n-heptane on rock samples aged with synthetic brine, dead acid, and inhibited dead acid was 41, 48, and 54°, respectively. Results illustrated that henna extract tends the samples toward the oil-wet phase. Overall, the results of this comprehensive study could secure the industrial use of this additive in sandstone acidizing.

Pore-scale insights into sludge formation damage during acid stimulation and its underlying mechanisms

Acid-oil emulsion and sludge formation are known as two major formation damage mechanisms and the reason for failure of some acid treatments. The published studies in this area focus primarily on core- to well/reservoir-scale and it is fairly unclear how acid-oil interaction at the pore-scale leads to the formation damage observed at the macro- or core-scale. In this paper, dynamic, micro-scale experiments were designed and executed to investigate the acid-induced formation damage using microfluidic approach. In addition, a series of so-called static (microscope) tests were performed in which acid-crude oil compatibility tests were conducted on a glass slide followed by microscopic observation to better analyze the micromodel results.The results show that sludge precipitation occurred right at the interface of the crude oil and acid; either with the residual oil film on the pore wall, at the acid-oil displacement menisci or acid-oil interfaces in the emulsion. Although, it is theoretically assumed that the reaction between acid and crude oil is fast, the results highlight that the damage is intensified by increasing the exposure time up to 8 hours. Besides, the acid type and concentration showed to have considerable impact on the acid-sludge and emulsion formation. The dynamic results indicate that increasing live acid concentration from 15 wt% to 28 wt% caused about 27.5% more permeability impairment due to sever pore-bridging, while spent acid did not result in any acid-sludge deposit. Furthermore, the role of ferric ion, as a transfer catalyst facilitating the two-phase contact, was visually confirmed through static tests. The dynamic tests also indicated 13% more permeability reduction when 3000 ppm ferric ion was present during 28 wt% HCl injection compared with the free-iron case.Emulsion analysis at the pore-scale showed that acid emulsion critically plugs the pore throats and intensify acid-sludge depositions. Size of acid-oil emulsion droplets was found to be distance-dependent such that as the acid permeated deeper into the porous medium the emulsions became finer and denser causing increased damage.Mitigating formation damage requires underpinning the mechanisms, the microfluidic approach can assist in identifying whether sludge precipitation or emulsion formation are the chief mechanisms and can serve as a fast, intermediate step before conducting the time intensive acidizing experiments at core-scale.

Treatment of landfill leachate evaporation concentrate by a modified electro-Fenton method

ABSTRACT Landfill leachate evaporation concentrate (LLEC) is difficult to treat due to its complex pollutant composition, which involves large amount of organic matter and inorganic salts such as scaling ions. Because of its high conductivity and high chloride-ion content, this study employed the modified electro-Fenton method with a self-developed iron-loaded cathode to treat LLEC wastewater. The operating variables were optimized according to the response surface methodology where the chemical oxygen demand (COD) removal efficiency was considered as the response based on single-factor experiments. A second-order polynomial regression model was obtained, and an application experiment revealed that it could be applied to determine LLEC treatment conditions. The removal rates of COD and colour were 100% and 99.8%, respectively, under the optimal operating conditions of an initial pH of 6, electrode spacing of 1 cm and applied voltage of 9 V. Three-dimensional fluorescence spectroscopy demonstrated that the humic acid and fulvic acid pollutants were almost completely removed. Scanning electron microscopy and energy dispersive spectroscopy analysis showed that the iron catalyst was loaded in activated carbon pores and exhibited almost no consumption during the reaction, which effectively solved the problem of iron sludge precipitation caused by electro-Fenton oxidation technology. The atomic distribution in the crystal was also analyzed by X-ray diffraction. The specific energy consumption of electrochemical oxidation was 0.498 Wh·mg−1 COD. The results indicate that the modified electro-Fenton technique with the proposed novel cathode is an effective method for treating LLEC.

Revitalization of Batu Songgan Traditional Village in Sustainable Development

Customary villages are traditional autonomous institutions, traditional cultural communities, indigenous law communities, and local identities. Batu Songgan Village is one of the traditional villages which is included in the Bukit Rimbang Wildlife Reserve in Bukit Baling in Riau. Local cultural values that are beginning to be neglected in life today are an important issue. Revitalization of the village or the traditional village is a process or way reactivate potentials in order to conserve the indigenous villages. An important value possessed by indigenous peoples in their relationship with the exploration and exploitation of nature . But the reality is a decline in the functions of traditional villages in maintaining environmental sustainability, resulting in ecosystem damage that causes a number of problems, such as flooding, siltation of rivers due to precipitation of sludge, decreasing productivity of crops, fisheries and so on. local communities, the effort to maintain ecosystems in an integrated manner is a strategic effort in the context of sustainable the traditional Village management. Based on this important issue, this study aims to see how the revitalization of the traditional Batu songgan village in sustainable development. Analysis of this study using library research methods (library research). The revitalization research of Batu Songgan traditional village has been carried out with 3 approaches namely physical intervention, economic rehabilitation and social / institutional revitalization. Based on the results of the study, it is known that physical intervention is still weak or not good, while economic rehabilitation and social / institutional revitalization are good enough but still need significant improvement.

Analysis and Treatment of the Deterioration Products of Fire-Resistant Oil

The 300 MW unit of a power plant is resistant to fuel, and its color is reddish brown. The acid value and resistivity are in line with the standard requirements. However, a large amount of sludge appears in the same grade of oil, causing the filter to be blocked and unable to supply oil. After the oil is filtered by the online oil filtering device, the degraded anti-fuel oil and the new oil are mixed and there is still sludge. In order to solve this problem, the components in the anti-fuel oil were tested and the acid value, moisture, resistivity, particle size and sludge precipitation test were tested. The cause of the sludge in the mixed oil was found and the treatment method was found. The oil samples were adsorbed by diatomaceous earth, activated clay and strong polar silica-alumina, and the parameters were analyzed after treatment. It was found that both activated clay and polar silica-alumina adsorbent have better purification functions, but the polar silica-alumina adsorbent is better and effectively solves the problem of fuel resistance degradation.

Simulation Study of Sludge Precipitation in Spent Fuel Reprocessing

A simulation technology with electrolyte thermodynamic model has been developed to evaluate the precipitation behavior in reprocessing solution. In this study, we have chosen the model of OLI system Inc. which combines an excess Gibbs energy model for mixed-solvent electrolyte systems with a comprehensive treatment of chemical equilibrium. The simulation results of precipitation were compared with the experiment data from non-radioactive simulated HLLW and Pu-Zr-Mo solution, and they showed good agreement each other. Most of the precipitation species was zirconium molybdate hydrate (ZMH) from the both data. This study has shown the thermodynamic simulation model is one of the useful tools to estimate the behavior of precipitation species from the reprocessing solution.

Investigation of Primary Fe-Bearing Compounds in Secondary Aluminium Alloys by Differential Scanning Calorimetry

The precipitation of primary Fe-rich intermetallics (sludge) in AlSi9Cu3(Fe) type alloys has been investigated for different Fe, Mn and Cr contents and cooling rates. Differential scanning calorimetry was used in order to assess the nucleation temperature and the enthalpy of sludge particles as well as to follow their evolution. The results show that the sludge nucleation temperature and the release of latent heat during sludge formation are functions of the initial concentrations of Fe, Mn and Cr in the molten alloy, i.e. the sludge factor, and the cooling rate. Being able to predict the temperature of sludge formation will support the foundries to set the right molten metal temperature in order to prevent sludge precipitation during the entire high-pressure die-casting process.

Experimental Investigation of Acid-induced Sludge Precipitation: Using Acid Additives in Iran

Dilute hydrochloric and hydrofluoric acid are used in the fracturing and acidizing of compact oil formations to dissolve the undesirable carbonate and silica deposits or scales, which interfere with the passage of oil in tubing or in the formation itself. A typical acid formulation is based on 15% hydrochloric for carbonate reservoirs and 12/3 wt% hydrochloric/hydrofluoric (regular mud acid) for sandstone reservoirs. But higher acid strengths are also used. Contact of acid with the crude oil generates a precipitate known as acid-induced sludge. Acid-induced sludge is becoming an increasing cause of oil well stimulation treatment failure. Acid stimulation experimental tests were conducted on the Marun crude oil sample from the Marun oil field for quantifying the amount of sludge precipitation as a consequence of an acidizing job. The potential of sludge precipitation by acid additives, such as iron ion concentration, standard dosage of mutual solvent, corrosion inhibitor, corrosion aid, suspended agent, and surfactant of the Marun crude oil sample, were experimentally investigated. The results show that, by increasing the concentration of acid and iron ion, the potential of sludge precipitation increases. The threshold of acid concentration for serious sludge precipitation is 17% for hydrochloric and 12/3% for mud acid. Iron ions promote the sludge precipitation. A 7,500-ppm iron ion is confirmed to be the threshold for serious acid-induced sludge. The standard dosage of all additives used in the current study (additives were prepared from Iranian Cymanband Company) was found to cause sludge precipitation.

The Silting-Up Prevention in the Geothermal Absorbent Openings of Geothermal Energy Plant Pyrzyce

Abstract The paper presents precipitation results from cold thermal water deposits that are the main cause of clogging in absorbent geothermal wells and borehole areas. As a result of physical and chemical analysis, laboratory tests and observation of the operation of a geothermal installation, a new method was developed to prevent the precipitation of sludge from cooled thermal water. The method being a modification of soft acidising was tentatively named as a super soft acidising method

Field Development and Productivity Improvement Offshore Mexico

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 121928, ’Field Development and Productivity Improvement in Offshore Mexico: An Engineering and Lab oratory Synergistic Approach to Carbonate Fracture Acidizing,’ by Antonio Inda and Octavio Steffani, Pemex, and Eduardo Soriano and Fernando Robles, Halliburton, originally prepared for the 2009 SPE European Formation Damage Conference, Scheveningen, The Netherlands, 27-29 May. The marine light-crude project offshore Mexico is a group of 12 oil fields 75 km off the coast of southern Mexico. The fields have been under development since 2003, are high-temperature carbonate formations producing gas and condensate with permeability that ranges from 1 to 6 md, and are naturally fractured. These fields were experiencing considerable drawdown, which slowed down the return on investment and, in one case, could have led potentially to well abandonment. Established practice dictates that carbonate formations in Mexico are treated best by matrix stimulation rather than fracture-stimulation techniques. The full-length paper demonstrates the effective implementation of a production-optimization design and execution methodology that has translated into productivity increase. Introduction The use of various fracturing methods for stimulation of wells has become a common procedure in the oil and gas industry. Fracture treatments are performed on wells with varying potential to help increase production and reduce the pressure drop on the formation face. Many hydrocarbon-bearing carbonate formations are routinely stimulated by fracture acidizing, and the use of fracture acidizing to enhance the production of carbonate formations continues to be an effective process. To achieve a successful fracture-acidizing treatment, three fundamental issues must be addressed: (1) reactivity control, (2) fluid-loss control, and (3) conductivity generation. Stimulation Challenges Most of the wells in Field A are producing in low-permeability formations ranging from 1 to 50 md. Bottomhole static temperatures (BHSTs) in these wells are often above 300°F. This high temperature enhances acid reactivity and limits penetration into the formation. Established practice dictates that carbonate formations in offshore Mexico are best treated by matrix stimulation rather than fracture-stimulation techniques. This has been established and practiced during the last 20 years of development of fields offshore Mexico on the basis of previous formations being high-permeability carbonates ranging from 100 to 5,000 md. Traditional acid-stimulation jobs in this area were mixtures of solvents/hydrochloric acid (HCl)/organic blends, designed to overcome sludge precipitation and high BHST as a way to distribute acid along the producing intervals. Often, viscosified acids are used as diverter systems with some regular success; however, high reservoir pressure in these fields limit the pumping rates most of the time. Recently, an acid-fracturing campaign started in these fields reported outstanding results and is described in the full-length paper.

Influence of additives on the impact toughness of Al–10.8% Si near-eutectic cast alloys

This article investigates the effects of melt treatment and addition of alloying elements on the impact toughness of as-cast and heat-treated Al–10.8% Si near-eutectic alloys. Increasingly precise impact behaviors are discussed in the context of differentiating between initiation and propagation energies, including the ductility index, which is the ratio of the propagation to initiation energies; total energy as a useful measure is also discussed. Details concerning the evaluation of tensile properties are reported in a separate article [Mohamed AMA, Samuel FH, Samuel AM, Doty HW. Influence of additives on the microstructure and tensile properties of near-eutectic Al–10.8%Si cast alloy. Mater Des, in press]. The concentration of elements in the alloys was changed to the following range: Fe 0.5–1 wt%, Mn 0.5–1 wt%, Cu 2.25–3.25 wt%, and Mg 0.3–0.5 wt%, while the impact toughness upon artificial aging in a temperature range of 155–240 °C for 5 h was also investigated. The results indicate that the morphology of fibrous Si in Sr-modified alloys enhances toughness because of its profound effect on crack initiation and crack propagation resistance. The combined addition of modifier and grain refiner leads to a 33% increase in the impact strength compared to the untreated alloy. In alloys containing high levels of iron, such as the RF2 (∼1% Fe, ∼1% Mn) and RF4 (∼1% Fe, ∼0.5% Mn) alloys, the addition of iron leads to an increased precipitation of sludge or β-Fe platelets, respectively; these particles also act as crack initiation sites and reduce the impact properties noticeably. In alloys already containing high levels of copper, such as the RC2 (∼3.25% Cu, ∼0.3% Mg) and RC5(∼0.3.25% Cu, ∼0.5% Mg) alloys, increasing the copper level lowers the impact properties significantly, in view of the fact that the fracture behavior is now predominantly influenced by the Al 2Cu phase rather than by the Si particles. The average crack propagation speed of impact-tested samples shows a good inverse relationship to impact energy. Crack propagation speed can thus provide a qualitative estimation of the impact energy expected for special alloy conditions.

Chromium Removal and Sorption Mechanism from Aqueous Solutions by Wine Processing Waste Sludge

Wine processing waste sludge (WPWS) has been shown to be an effective sorbent for the sorption of heavy metals (i.e., chromium and nickel), but the mechanism of removal of hexavalent chromium [Cr(VI)] by WPWS remains obscure. The aims of this study were to determine the effects of temperature, initial concentration of Cr(VI), and particle size on the removal Cr(VI) using WPWS. The characteristics of WPWS were determined, and sorption mechanism studies were also performed. The WPWS used was a deposit mixture containing considerable quantities of chemical coagulation as well as activated sludge precipitation from the settling basins of a wastewater treatment plant. Differential scanning calorimetry (DSC) analysis revealed that the WPWS comprised abundant labile carbohydrates and few aromatic structures. According to the IR spectrum, carboxylic groups were the most important functional group in WPWS, interacting with chromium species by protonation and redox reaction. All kinetic experiments were conducted a...

The fate of trace elements in fluidised bed combustion of sewage sludge and wood

Combustion tests have been carried out in a fluidised bed boiler to investigate the fate of trace elements during co-combustion of wood and municipal sewage sludge. The approach was to collect fuel and ash samples and to perform thermodynamic equilibrium calculations for gasification (reducing) and combustion (oxidising) conditions. Trace elements are found in the ash. Even most of the highly volatile Hg is captured in the bag filter ash. The bag filter ash offers higher surface area than the secondary cyclone ash and enhances the capture of Hg. There is no obvious correlation between capture and parameters investigated (sludge precipitation agent and lime addition). As, Cd, Hg, Pb, Se, Sb and Tl are predicted by equilibrium calculations to be volatile in the combustion chamber under oxidising conditions and Hg even at the filter temperature (150 °C). Reducing conditions promote, in some case more than others, the volatility of As, Cd, Pb, Sb, Se, Tl and Zn. The opposite effect was observed for Cu and Ni. Data points to the necessity of including bag filter in the gas cleaning system in order to achieve good removal of toxic trace elements.

Effect of ferrous and ferric iron impurities on the crystallization of gypsum and sludge formation in wet-process phosphoric acid

The effect of ferrous and ferric iron on the crystallization of calcium sulfate dihydrate, the rate of filtration, and the precipitation of sludge in concentrated acid were investigated. A bench scale unit was used to simulate dihydrate plant conditions. It was found that ferrous iron was oxidized to ferric, and the behavior of the two impurities in the acid was therefore very similar. At low levels of impurities, ferrous and ferric iron improved filtration, but further additions decreased it. Sludge collected from the concentrated acid after five weeks was mainly calcium sulfate hemihydrate and anhydrate. With the addition on 0.5% and 0.75% iron impurities however, the presence of (Fe,Al) 3 KH 14 (PO 4 ) 8 ·4H 2 O was evident in electron micrographs.

Iron removal from wet-process phosphoric acid by sludge precipitation

Fertilizer-grade phosphoric acid with a P2O5 concentration of 50% should contain less than 1.5% Fe2O3, otherwise, the P2O5 water solubility of the fertilizers prepared from high-iron acids will be low. High-iron Egyptian phosphate concentrates from Abu-Tartur and Sebaiya-West were leached with sulfuric acid and recycled phosphoric acid to produce 50%-P2O5 phosphoric acids containing 4.0% and 2.7% Fe2O3 respectively. The high iron contents were reduced to the industrially accepted limit of 1.5% Fe2O3 by precipitation with potassium sulfate. The factors affecting the efficiency of the precipitation process, e.g., temperature, time and K2O/Fe2O3 ratio, were systematically studied, and the optimum conditions were achieved. The clarification of the prepared phosphoric acid from iron-containing sludges was also investigated by varying the time and temperature. The utilization of the separated sludges as fertilizer and recovery ofP2O5 by pulping in water were also evaluated.

Alternative Stimulation Fluids and Their Impact on Carbonate Acidizing

Abstract Acidizing treatments are commonly used to remove near-wellbore damage and create artificial flow channels in carbonate formations. Matrix acidizing treatments are most useful when fracture acidizing is undesirable, such as when a shale break or other natural boundary must be maintained to prevent water or gas production1 or where fracture acidizing is ineffective, such as in soft chalk formations.2 Unfortunately, matrix treatments often require low injection rates to prevent fracturing the formation rock. These treatments may also be required in heterogeneous formations with zones of low conductivity (which need stimulation the most) that accept acid at low rates. It is at these low injection rates that the problem of rapid acid spending severely limits the acid penetration distance. The injection of hydrochloric acid (HCl) into carbonate formations at low rates results in face dissolution, or complete dissolution of the carbonate matrix near the wellbore. This face dissolution consumes large volumes of acid and provides negligible increases in the conductivity of the formation. Various acid systems have been used to reduce the limitations of rapid acid spending at low injection rates. A few of the acids include: weak acids, such as acetic and formic acid, which have relatively low H+ concentrations and therefore react with carbonates at a slower rate than HCl3,4; chemically retarded acids, such as oil external microemulsion systems containing HCl, that retard acid diffusion to the carbonate surface and thus allow deeper penetration of live acid2; and foamed acids (nitrogen gas and aqueous HCl) that prevent acid from spending outside the primary dissolution channel, thereby promoting the growth of wormholes.5 Although retarded and foamed acid systems can stimulate carbonate formations at lower injection rates, strong acids such as HCl induce the precipitation of asphaltic sludge from crude oil. This sludge can plug the formation and restrict production after an acidizing treatment. When ferric ions are present, this problem is even more severe.6 Thus, adequate corrosion protection becomes more essential. Acetic acid (HAc), an iron chelating agent, does not reduce sludging tendencies in the presence of ferric and ferrous iron.6 A variety of acid additives (antisludging agents, corrosion inhibitors, and iron-reducing agents) have been used to prevent the sludging problem. However, their effectiveness is limited by the need to obtain a compatible combination of additives and a lack of understanding of the complex chemistries involved in the precipitation reactions. These limitations demonstrate the need for an alternative stimulation fluid that combines the ability to stimulate at low injection rates with fluid properties that are not conducive to asphaltic sludge precipitation or corrosion problems.

Characteristics and Production Mechanism of Sulfuric Acid and Nitric-Hydrofluoric Acid Pickling Sludge Produced in Manufacture of Stainless Steel.

Stainless steel pickling sludge consists largely of surface scale from stainless steel strips, being composed mainly of Cr oxides. Moreover, the present work shows that the Cr concentration of the sludge is approximately twice that of ordinary stainless steels. The particle size of sulfuric acid sludge is 0.325 μm, while that of nitric-hydrofluoric acid sludge is 9.2 μm. Nitric-hydrofluoric acid sludge tends to coagulate in a relatively short time, while sulfuric acid sludge essentially does not tend to coagulate. The main components of sulfuric acid sludge and nitric-hydrofluoric acid sludge are FeCr2O4 and Cr2O3. In the mechanism of sludge precipitation in the pickling process, acid penetrates cracks in the scale and dissolves part of the scale and the substrate. The undissolved scale then loses the ability to bond with other scale and the substrate and settles out in the pickling tanks.

Development of Recycling System for Sludge from the Stainless Steel Nitric-hydrofluoric Acid Pickling Process.

The sludge produced from the pickling of stainless bands settles to the bottom of the pickling tank and becomes an impediment to the operation. In this research, a very high concentrations of Cr, Ni, and Fe were found in the sludge. In particular, the concentration of Cr was more than 40 mass%. It was also found that the particle size of the sludge is as small as 9.2 μm. The diameter increases by coagulation and the sedimentation velocity of the sludge is 1.4 m/h. Based on these results, a study was made on an ejector capable of preventing the sludge precipitation in the pickling tank and moving the sludge, and a sedimentation tank where the sludge would precipitate. First, 14 groups of ejectors were installed on the two sides of the pickling tank. Two circulation tanks connect with the pickling tank and each group of ejectors was installed in different circulation tanks. Continuous action of the ejectors causes the sludge to remain afloat and prevents it from settling out in the pickling tank. The sludge is moved to the circulation tank, which is also equipped with the same type of ejectors as the pickling tank to prevent the sludge from settling. Next, precipitation and separation of the sludge are performed in this sedimentation tank. After separation in the sedimentation tank, the sludge was neutralized by mixing with hydrated lime using kneading device. The neutralized sludge is loaded into a lugger bag and can be used as a raw material for steel making.