A method for recovering and concentrating waste acid from titanium dioxide production

Abstract

This invention discloses a method for recovering and concentrating waste acid from titanium dioxide production, comprising the following steps: S1, adding a flocculant to the waste acid, stirring, flocculating and agglomerating, and filtering to obtain waste acid A; S2, adding a reducing agent to the waste acid A obtained in S1 to obtain waste acid B; S3, adding a dispersing and scale inhibitor to the waste acid B obtained in S2 to obtain waste acid C, wherein the dispersing and scale inhibitor includes a dispersant, a dispersing and scale inhibitor, a corrosion and scale inhibitor, and an anticoagulant; S4, concentrating and filtering the waste acid C obtained in S3 to obtain concentrated acid. This invention removes hydrated titanium dioxide particles through flocculation and filtration using a flocculant, and reduces the titanium in waste acid A to non-hydrolyzable Ti by adding a reducing agent. 3+ Adding a dispersant and scale inhibitor prevents the ferrous sulfate monohydrate crystals from agglomerating on the carrier during waste acid concentration. The interaction of these three methods solves the scaling problem in the waste acid concentration process from the source.

Description

Technical Field

[0001] This invention relates to the field of titanium dioxide, and in particular to a method for recovering and concentrating waste acid from titanium dioxide production. Background Technology

[0002] The sulfuric acid process for titanium dioxide production generates a large amount of acidic waste gas, waste residue, and wastewater. With technological advancements and continuous equipment upgrades, some of these issues have been mitigated. For example, the application of continuous acidolysis technology and the use of alkaline water spraying acid mist on the tail gas tower effectively solves the waste gas problem generated during acidolysis. Furthermore, processes such as acid washing and alkali washing are used to process the waste residue into useful substances such as water purification agents, feed additives, iron oxide red pigments, or magnetic ferrites.

[0003] Statistics show that producing 1 ton of titanium dioxide generates 7-10 tons of waste acid with a concentration of approximately 20% and about 60 tons of dilute acid wastewater with a concentration of 5%. For the dilute acid wastewater, it is typically neutralized with calcium carbide slag to produce white or red gypsum. After filtration, the dilute acid wastewater meets the standards and can be discharged directly. Direct discharge of waste acid would cause serious environmental pollution. Furthermore, the waste acid contains unhydrolyzed titanium oxysulfate, trivalent titanium, a small amount of residual hydrated titanium dioxide (metatinic acid) from water washing and filtration, as well as large amounts of sulfates (mainly ferrous sulfate) and sulfuric acid. These solid impurities cannot be directly reused. The waste acid needs to be purified and concentrated to obtain concentrated acid with a sulfuric acid content of approximately 55%. This concentrated acid can then be effectively utilized.

[0004] During the concentration process of waste acid, as the acidity increases, a large amount of scale will form on the walls of the concentrator pipes, causing serious pipe blockage. This affects the continuous and stable operation of the waste acid concentration system, requiring periodic disassembly and flushing of the equipment, which is time-consuming and labor-intensive. The disassembly process can easily damage the equipment, severely impacting production efficiency. Furthermore, the flushing process generates a large amount of acidic water, causing serious adverse effects on the surrounding environment. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for recovering and concentrating waste acid from titanium dioxide production, which addresses the shortcomings of existing technologies and solves the problem of large amounts of scaling on the walls of the concentrator tubes during the concentration process.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for recovering and concentrating waste acid from titanium dioxide production, comprising the following steps:

[0007] S1. Add flocculant to waste acid, stir, flocculate and agglomerate, and filter to obtain waste acid A;

[0008] S2. Add a reducing agent to the waste acid A obtained in S1 to obtain waste acid B;

[0009] S3. Add a dispersing scale inhibitor to the waste acid B obtained in S2 to obtain waste acid C. The dispersing scale inhibitor includes a dispersant, a dispersing scale inhibitor, a corrosion and scale inhibitor, and an anticoagulant.

[0010] The waste acid C obtained from S4 and S3 is concentrated and filtered to obtain concentrated acid;

[0011] The weight ratios of the flocculant, the reducing agent, the dispersing scale inhibitor, and the waste acid are 0.01–0.2%, 0.1–0.6%, and 0.05–0.5%, respectively.

[0012] During the concentration process of existing waste acid, as the acidity increases, a large amount of ferrous sulfate monohydrate precipitates out (this substance is not a good crystal). At the same time, as the temperature rises, titanium oxysulfate in the waste acid will hydrolyze and mix with a small amount of residual hydrated titanium dioxide, causing scaling on the concentrator tube wall.

[0013] In step S1 of this invention, after adding the flocculant, the hydrated titanium dioxide particles in the waste acid rapidly flocculate and agglomerate. At this point, the hydrated titanium dioxide particles become larger, allowing them to be filtered and trapped by the filter membrane. Without the addition of the flocculant, the filter membrane is easily clogged, making filtration impossible.

[0014] Adding a reducing agent causes the Ti in the titanium oxysulfate in waste acid A to... 4+ Reduced to non-hydrolyzable Ti 3+ Meanwhile, the concentrated acid contains Ti. 3+ It acts as a partial reducing agent in the acidolysis process (the subsequent titanium dioxide production process), reducing the amount of reducing agent used.

[0015] Adding a dispersing and scale inhibitor disperses the ferrous sulfate crystals in waste acid B, preventing the aggregation of monohydrate ferrous sulfate crystals during waste acid concentration. This further prevents ferrous sulfate from precipitating on the concentrator tube wall and causing scale buildup during waste acid concentration.

[0016] As can be seen from the above, the present invention removes hydrated titanium dioxide particles by adding a flocculant for flocculation and filtration, and reduces titanium in waste acid A to non-hydrolyzable Ti by adding a reducing agent. 3+ Adding a dispersant and scale inhibitor prevents the ferrous sulfate monohydrate crystals from agglomerating on the carrier during waste acid concentration. The interaction of these three methods solves the scaling problem in the waste acid concentration process from the source.

[0017] In a preferred embodiment of the present invention, the flocculant in S1 is composed of an organic flocculant and an inorganic flocculant. The organic flocculant is at least one of chitosan, sodium alginate, and polydimethylammonium chloride, and the inorganic flocculant is at least one of polyaluminum chloride, aluminum sulfate, and aluminum chloride. The weight ratio of the organic flocculant to the inorganic flocculant is (0.005-0.2):(0.002-0.05).

[0018] Conventional processes use only polyacrylamide as a flocculant, resulting in slow filtration rates and small flocculated particles. This invention combines organic and inorganic flocculants, leading to better flocculation, reduced viscosity, and a 20% increase in filtration rate. In this invention, if the weight ratio of organic to inorganic flocculants is too low, insufficient bridging leads to poor flocculation; conversely, if the weight ratio is too high, the flocculants cannot spread properly, negatively impacting flocculation.

[0019] In a preferred embodiment of the present invention, the reducing agent in S2 is composed of an organic reducing agent and an inorganic reducing agent. The organic reducing agent is at least one of zinc formaldehyde sulfoxylate and sodium hydroxymethylmethanesulfinate dihydrate, and the inorganic reducing agent is sodium sulfite or sodium dithionite. The weight ratio of the organic reducing agent to the inorganic reducing agent is (0.2-0.6):(0.05-0.3).

[0020] Conventional reducing agents often use aluminum powder, which requires 70-80℃ to achieve reduction. The reaction is incomplete, and improper storage of aluminum powder can lead to explosions. The organic and inorganic reducing agents used in this invention are environmentally friendly and safe, and can undergo reduction at temperatures above 10℃. Experiments have confirmed that they can reduce Ti... 4+ Completely reduced to non-hydrolyzable Ti 3+ .

[0021] In a preferred embodiment of the present invention, the mass ratio of the dispersant, scale inhibitor, corrosion and scale inhibitor, and anticoagulant in the scale inhibitor is (0.05-0.4):(0.006-0.04):(0.002-0.04):(0.0008-0.008).

[0022] The waste acid concentration of this invention is above 20%. Commonly used dispersants such as sodium silicate and sodium hexametaphosphate cannot be used under acidic conditions with a concentration of 20%. The high-efficiency dispersing and scale inhibitor selected in this invention is a combination of dispersant, dispersing and scale inhibitor, corrosion and scale inhibitor, and anticoagulant, which has a high efficiency of dispersing and sedimentation inhibition.

[0023] In this invention, the mass ratio of dispersant, scale inhibitor, corrosion inhibitor, and anticoagulant in the dispersing and scale inhibitor is not within the range of (0.05~0.4):(0.006~0.04):(0.002~0.04):(0.0008~0.008). The double-layer theory and steric hindrance have small effects, resulting in poor dispersion and easy agglomeration of ferrous sulfate monohydrate.

[0024] In a preferred embodiment of the present invention, the dispersant is at least one of sodium methylene bis(naphthalene) sulfonate, naphthalene sulfonate, 2-naphthalene sulfonic acid, and potassium salt of styrene-maleic anhydride copolymer; the scale inhibitor is a homopolymer of 2-acrylic acid; the corrosion and scale inhibitor is at least one of a copolymer of organophosphate, organophosphorus carboxylate, and polycarboxylate sulfonate; and the retarder is hydroxyphenyl methyl cellulose.

[0025] In a preferred embodiment of the present invention, the organophosphate is at least one of aminotrimethylene phosphate, ethylenediaminetetramethylene phosphate, and hydroxyethylidene diphosphonate.

[0026] In a preferred embodiment of the present invention, the size of the individual particles after flocculation and agglomeration in S1 is 0.07-1.0 mm. S1 is filtered through a ceramic membrane. After flocculation and agglomeration, the size of individual hydrated titanium dioxide particles changes from 0.2-1.0 μm to 0.07-1.0 mm. The ceramic membrane filter traps the flocculated hydrated titanium dioxide particles and does not clog the ceramic membrane.

[0027] In a preferred embodiment of the present invention, the stirring time in S1 is 5-15 minutes.

[0028] In a preferred embodiment of the present invention, S4 specifically includes the following steps:

[0029] Waste acid C is concentrated under vacuum until ferrous sulfate monohydrate forms at the bottom of the flask. The ferrous sulfate monohydrate is then filtered off to obtain concentrated acid containing 50-55% sulfuric acid by mass.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] This invention removes hydrated titanium dioxide particles through flocculation and filtration by adding a flocculant, and reduces titanium in waste acid A to non-hydrolyzable Ti by adding a reducing agent. 3+ Adding a dispersant and scale inhibitor prevents the ferrous sulfate monohydrate crystals from agglomerating on the carrier during waste acid concentration. These three methods work together to solve the scaling problem at its source during waste acid concentration. Additionally, the concentrated acid contains Ti... 3+ In subsequent titanium dioxide production processes, it can act as a reducing agent, reducing the amount of reducing agent used. This invention facilitates the continuous and stable operation of concentration equipment, improves production efficiency, saves costs, and is environmentally friendly and energy-saving. Attached Figure Description

[0032] Figure 1 , Figure 2 , Figure 3 This is a photograph of the vacuum concentrator tube wall after vacuum concentration in Comparative Example 4.

[0033] Figures 4-6 This is a photograph of the tube wall of the vacuum concentrator in Embodiment 1 of the present invention.

[0034] in, Figures 1-3 Ferrous sulfate monohydrate adhered to the wall of the vacuum concentrator, and the ferrous sulfate monohydrate agglomerated into clumps. Figures 4-6 The ferrous sulfate monohydrate crystals are relatively fine, and there is no material adhering to the tube wall of the vacuum concentrator. Detailed Implementation

[0035] The waste acid treated in the example has the following composition: H2SO4: 22.3%, FeSO4: 13.1%, TiO2 (soluble): 0.36%, TiO2 (solid): 0.51%.

[0036] Example 1

[0037] (1) Add 0.008% chitosan and 0.002% aluminum chloride to the waste acid tank according to the weight ratio of waste acid, stir for 10 minutes to flocculate the hydrated TiO2 in the waste acid, and obtain clear waste acid A by filtration through a ceramic membrane. The TiO2 (solid) content in waste acid A is 0.001%.

[0038] (2) Add 0.5% formaldehyde-zinc sulfate and 0.05% sodium sulfite according to the weight ratio of waste acid A, so that the Ti in waste acid A is reduced. 4+ Restored to Ti 3+ The tetravalent titanium in waste acid A was completely reduced to trivalent titanium, resulting in waste acid B. The content of trivalent titanium (calculated as TiO2) was 0.36%.

[0039] (3) Add 0.1% sodium methylene bisnaphthalene sulfonate dispersant, 0.01% 2-acrylic acid homopolymer dispersant scale inhibitor, 0.012% polycarboxylic acid potassium sulfonate copolymer corrosion inhibitor and 0.003% hydroxyphenyl methyl cellulose retarder according to the weight ratio of waste acid, so that the ferrous sulfate monohydrate precipitated at high temperature is in a dispersed state and does not form scale on the vessel wall, thus obtaining waste acid C.

[0040] (4) Waste acid C was concentrated in a flask under vacuum (temperature 65℃, vacuum degree 0.08MPa) until no brown ferrous sulfate monohydrate crystals formed at the bottom of the flask. The mass concentration of sulfuric acid in the concentrated acid was 53.2%. The filtered ferrous sulfate monohydrate crystals were fine and without agglomerated lumps. It is evident that this treatment of waste acid, through pre-concentration to filter out ferrous sulfate monohydrate, followed by vacuum concentration, ultimately yields concentrated acid with approximately 55% sulfuric acid content. No scaling occurred on the pipe walls during the waste acid concentration process.

[0041] Example 2

[0042] (1) Add 0.1% sodium alginate and 0.05% aluminum chloride to the waste acid tank according to the weight ratio of waste acid, stir for 5 minutes to flocculate the hydrated TiO2 in the waste acid, and obtain clear waste acid A by filtration through a ceramic membrane. The TiO2 (solid) content in waste acid A is detected to be 0.001%.

[0043] (2) Add 0.3% sodium hydroxymethylmethanesulfonate dihydrate and 0.25% sodium sulfite according to the weight ratio of waste acid to reduce the Ti content in waste acid A. 4+ Restored to Ti 3+ The tetravalent titanium in waste acid A was completely reduced to trivalent titanium, resulting in waste acid B. The content of trivalent titanium (calculated as TiO2) was 0.36%.

[0044] (3) Add 0.25% sodium naphthalene sulfonate, 0.05% 2-naphthalene sulfonic acid, 0.035% 2-acrylic acid homopolymer, 0.03% polycarboxylic acid sulfonate copolymer and 0.007% hydroxyphenyl methyl cellulose according to the weight ratio of waste acid, so that the ferrous sulfate monohydrate precipitated at high temperature is in a dispersed state and does not form scale on the vessel wall, thus obtaining waste acid C.

[0045] (4) Waste acid C was concentrated in a flask under vacuum (temperature 65℃, vacuum 0.08MPa) until no brown ferrous sulfate monohydrate crystals formed at the bottom of the flask. The concentration of sulfuric acid in the concentrated acid was 53.2%. The filtered ferrous sulfate monohydrate crystals were fine and without agglomerated lumps. It is evident that this treatment of waste acid, through pre-concentration to filter out ferrous sulfate monohydrate, followed by vacuum concentration, ultimately yields concentrated acid with approximately 55% sulfuric acid content. No scaling occurred on the pipe walls during the waste acid concentration process.

[0046] Example 3

[0047] (1) Add 0.15% polydimethylammonium chloride and 0.05% aluminum sulfate to the waste acid tank according to the weight ratio of waste acid, stir for 5 minutes to flocculate the hydrated TiO2 in the waste acid, and obtain clear waste acid by filtration through a ceramic membrane. The TiO2 (solid) content in waste acid A is detected to be 0.001%.

[0048] (2) Add 0.2% sodium hydroxymethylmethanesulfinate dihydrate and 0.08% sodium dithionite as reducing agents according to the weight ratio of waste acid A, so that Ti in waste acid A... 4+ Restored to Ti 3+ The tetravalent titanium in waste acid A was completely reduced to trivalent titanium, resulting in waste acid B. The content of trivalent titanium (calculated as TiO2) was 0.36%.

[0049] (3) Add 0.05% potassium styrene-maleic anhydride copolymer, 0.006% 2-acrylic acid homopolymer, 0.008% polycarboxylic acid sulfonate copolymer, and 0.0008% hydroxyphenyl methyl cellulose according to the weight ratio of waste acid, so that the ferrous sulfate monohydrate precipitated at high temperature is in a dispersed state and does not form scale on the vessel wall, thus obtaining waste acid C.

[0050] (4) Waste acid C was concentrated in a flask under vacuum (temperature 65℃, vacuum 0.08MPa) until no brown ferrous sulfate monohydrate crystals formed at the bottom of the flask. The concentration of the concentrated acid was 53.2%. The filtered ferrous sulfate monohydrate crystals were fine and without agglomerated lumps. It is evident that this treatment of waste acid, through pre-concentration to filter out ferrous sulfate monohydrate, followed by vacuum concentration, ultimately yields concentrated acid with approximately 55% sulfuric acid content. No scaling occurred on the pipe walls during the waste acid concentration process.

[0051] Example 4

[0052] (1) Add 0.15% polydimethylammonium chloride, 0.008% chitosan and 0.02% aluminum sulfate to the waste acid tank according to the weight ratio of waste acid, stir for 15 minutes to flocculate the hydrated TiO2 in the waste acid, and obtain clear waste acid by filtration through a ceramic membrane. The TiO2 (solid) content in waste acid A is detected to be 0.001%.

[0053] (2) Add 0.35% sodium hydroxymethylmethanesulfinate dihydrate and 0.08% sodium sulfite as reducing agents according to the weight ratio of waste acid, so that Ti in waste acid A... 4+ Restored to Ti 3+ The tetravalent titanium in waste acid A was completely reduced to trivalent titanium, resulting in waste acid B. The content of trivalent titanium (calculated as TiO2) was 0.36%.

[0054] (3) Add 0.05% sodium methylene bisnaphthalene sulfonate, 0.006% homopolymer of 2-acrylic acid, 0.008% copolymer of potassium polycarboxylate sulfonate and 0.0008% hydroxyphenyl methyl cellulose according to the weight ratio of waste acid, so that the ferrous sulfate monohydrate precipitated at high temperature is in a dispersed state and does not form scale on the vessel wall, thus obtaining waste acid C.

[0055] (4) Waste acid C was concentrated in a flask under vacuum (temperature 65℃, vacuum 0.08MPa) until no brown ferrous sulfate monohydrate crystals formed at the bottom of the flask. The concentration of the concentrated acid was 53.2%. The filtered ferrous sulfate monohydrate crystals were fine and without agglomerated lumps. It is evident that this treatment of waste acid, through pre-concentration to filter out ferrous sulfate monohydrate, followed by vacuum concentration, ultimately yields concentrated acid with approximately 55% sulfuric acid content. No scaling occurred on the pipe walls during the waste acid concentration process.

[0056] Example 5

[0057] (1) Add 0.1% sodium alginate, 0.008% polydimethylammonium chloride and 0.035% aluminum sulfate to the waste acid tank according to the weight ratio of waste acid, stir for 15 minutes to make the hydrated TiO2 in the waste acid flocculate, and obtain clear waste acid by filtration through ceramic membrane. The TiO2 (solid) content in waste acid A is detected to be 0.001%, indicating that all TiO2 (solid) in the waste acid has been filtered out and there is basically no solid TiO2.

[0058] (2) Add 0.3% sodium hydroxymethylmethanesulfinate dihydrate and 0.08% sodium sulfite as reducing agents according to the weight ratio of waste acid, so that the Ti in the waste acid... 4+ Restored to Ti 3+ All tetravalent titanium in the waste acid was reduced to trivalent titanium, resulting in waste acid B. The content of trivalent titanium (calculated as TiO2) was found to be 0.36%.

[0059] (3) Add 0.05% sodium methylene bisnaphthalene sulfonate, 0.006% homopolymer of 2-acrylic acid, 0.006% copolymer of potassium polycarboxylate sulfonate, 0.002% organophosphorus carboxyl and 0.0008% hydroxyphenyl methyl cellulose according to the weight ratio of waste acid, so that the ferrous sulfate monohydrate precipitated at high temperature is in a dispersed state and does not form scale on the vessel wall, thus obtaining waste acid C.

[0060] (4) Waste acid C was subjected to a vacuum concentration test in a flask (temperature 65℃, vacuum degree 0.08MPa). A brown ferrous sulfate monohydrate crystal formed at the bottom of the flask, indicating a concentrated acid concentration of 53.2%. The filtered ferrous sulfate monohydrate crystals were fine and free of agglomerated lumps. This demonstrates that by pre-concentrating and filtering out ferrous sulfate monohydrate, and then further vacuum concentrating, a concentrated acid with approximately 55% sulfuric acid content can be obtained. No scaling occurred on the pipe walls during the waste acid concentration process.

[0061] Comparative Example 1

[0062] The difference between this comparative example and Example 1 is that step (2) is omitted. Scale formed on the pipe wall during the waste acid concentration process.

[0063] Comparative Example 2

[0064] The difference between this comparative example and Example 1 is that step (3) is omitted. Scale forms on the pipe wall during the waste acid concentration process.

[0065] Comparative Example 3

[0066] The difference between this comparative example and Example 1 is that the flocculant in step (1) is polyacrylamide. The flocculated particles in step (1) are small, resulting in a slow filtration rate and easy clogging of the ceramic membrane.

[0067] Comparative Example 4

[0068] This comparative example uses waste acid to undergo a vacuum concentration test in a flask (temperature 65℃, vacuum degree 0.08MPa), ultimately obtaining concentrated acid with a sulfuric acid content of approximately 55%. During the waste acid concentration process, severe scaling occurred on the pipe wall.

Claims

1. A method for recovering and concentrating waste acid from titanium dioxide production, characterized in that... Includes the following steps: S1. Add flocculant to waste acid, stir, flocculate and agglomerate, and filter to obtain waste acid A; S2. Add a reducing agent to the waste acid A obtained in S1 to obtain waste acid B; S3. Add dispersant and scale inhibitor A to the waste acid B obtained in S2 to obtain waste acid C. The dispersant and scale inhibitor A includes a dispersant, dispersant and scale inhibitor B, corrosion inhibitor and scale inhibitor, and anticoagulant. The waste acid C obtained from S4 and S3 is concentrated and filtered to obtain concentrated acid; The weight ratios of the flocculant, the reducing agent, the scale inhibitor A, and the waste acid are 0.01–0.2%, 0.1–0.6%, and 0.05–0.5%, respectively. The flocculant described in S1 is composed of organic flocculant and inorganic flocculant. The organic flocculant is at least one of chitosan, sodium alginate, and polydimethylammonium chloride. The inorganic flocculant is at least one of polyaluminum chloride, aluminum sulfate, and aluminum chloride. The weight ratio of the organic flocculant to the inorganic flocculant is (0.005~0.2):(0.002~0.05). The reducing agent in S2 is composed of an organic reducing agent and an inorganic reducing agent. The organic reducing agent is at least one of zinc formaldehyde sulfoxylate and sodium hydroxymethylmethanesulfinate dihydrate. The inorganic reducing agent is sodium sulfite or sodium dithionite. The weight ratio of the organic reducing agent to the inorganic reducing agent is (0.2-0.6):(0.05-0.3). The dispersant is at least one of sodium methylene bis(naphthalene) sulfonate, naphthalene sulfonate, 2-naphthalene sulfonic acid, and potassium salt of styrene-maleic anhydride copolymer; the scale inhibitor B is a homopolymer of 2-acrylic acid; the corrosion and scale inhibitor is at least one of a copolymer of organophosphorus carboxylate and polycarboxylate sulfonate; and the anticoagulant is hydroxyphenyl methyl cellulose.

2. The method for recovering and concentrating waste acid from titanium dioxide production as described in claim 1, characterized in that: The mass ratio of the dispersant, dispersant B, corrosion inhibitor, and anticoagulant in the scale inhibitor A is (0.05-0.4):(0.006-0.04):(0.002-0.04):(0.0008-0.008).

3. A method for recovering and concentrating waste acid from titanium dioxide production as described in any one of claims 1-2, characterized in that: The size of individual particles after flocculation and aggregation in S1 is 0.07-1.0 mm.

4. A method for recovering and concentrating waste acid from titanium dioxide production as described in any one of claims 1-2, characterized in that: S1 is filtered through a ceramic membrane.

5. A method for recovering and concentrating waste acid from titanium dioxide production as described in any one of claims 1-2, characterized in that: The stirring time in S1 is 5-15 minutes.

6. A method for recovering and concentrating waste acid from titanium dioxide production as described in any one of claims 1-2, characterized in that: S4 specifically includes the following steps: Waste acid C is concentrated under vacuum until ferrous sulfate monohydrate forms at the bottom of the flask. The ferrous sulfate monohydrate is then filtered off to obtain concentrated acid containing 50-55% sulfuric acid by mass.

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