KR100806200B1 - Solidifying agent of highly water-containing sludge - Google Patents

Abstract

A solidifying agent for sludge with a high water content is provided to realize excellent cost efficiency and effects of accelerating solidification and reducing water content. A solidifying agent for sludge with a high water content comprises a product obtained by combustion of 40-70 wt% of petroleum coke including 80-88 wt% of carbon, 2-3 wt% of hydrogen, 5-10 wt% of oxygen, 1-1.5 wt% of nitrogen and 4-5.5 wt% of sulfur, and a calorific value of 8,000 kcal/kg, and 30-60 wt% of limestone having an apparent density of 2.8745 g/ml, a specific surface area of 15.450 m^2/g, an average pore diameter of 169 angstroms, a porosity of 15.8% and a particle diameter of 0.1-3 mm, at 850-950 deg.C.

Description

High function sludge hardener {SOLIDIFYING AGENT OF HIGHLY WATER-CONTAINING SLUDGE}

The present invention relates to a solidifying agent that can be easily utilized as a cover agent, green soil, etc. by drying and solidifying sludge or sludge having a high moisture content, and more specifically, using anhydrous gypsum or quicklime, which are generally used in the manufacture of a solidifying agent, separately. Petroleum coke and limestone containing sulphide instead of combustion in a fluidized bed boiler or lime kiln, and using the desulfurized combustor independently as a solidifying agent, or by adding sulfuric acid or iron sulfate to the combustor, or The moisture content was obtained by adding the sulfuric acid or iron sulfate, and an additive composed of at least one selected from incineration ash, fly ash, steel slag dust, diatomaceous earth, and olivine as solidifying agent. It is added to high sludge and sludge to dry and solidify high-functional sludge solidifying agent It is about.

Sewage sludge with high water content, food sludge, and sedimentary sludge from the shore or lake are typical environmental pollutants, which have been treated by ocean dumping, landfill, etc., and now restore the environment contaminated by the pollutants. There are many ways to make it happen.

For example, sewage sludge discharged more than 6000 tons per day has been banned from landfills since July 2003, and even dumping at sea, which was the easiest to dispose of, was allowed only until 2011 as a London dumping treaty.

In particular, Korea, which has a narrow territory, should urgently seek efficient and safe treatment methods to prevent pollution of the surrounding environment and to prevent contamination of the leachate from sludge.

Currently, local governments around the country, including metropolitan landfills, are preparing treatment facilities that can solidify high-function sludge and use it as a cover agent.In this regard, solidification of high-density sludge is efficiently performed, resulting in excellent productivity and economic efficiency. Development of hardeners is critical.

In the conventional solidification treatment with an alkali-based solidifying agent, there is a problem in that the odor is generated by ammonia gas due to the high pH of the solidified material. Therefore, an efficient solidifying agent is required without generating ammonia during the solidifying treatment. In addition, in order to utilize the solids as a cover agent and rust soil, heavy metal leaching inhibition, permeability coefficient, compressive strength, and the like are required.

In order to solve the problems as described above, the present invention is to provide a solidifying agent with economical efficiency and excellent economic loss effect by promoting the solidification and petroleum coke and limestone combustion material generated as a by-product in a fluidized bed boiler for the purpose. do.

In order to achieve the above object,

The present invention has an elemental composition of 80 to 88% by weight of carbon, 2 to 3% by weight of hydrogen, 5 to 10% by weight of oxygen, 1 to 1.5% by weight of nitrogen, and 4 to 5.5% by weight of sulfur, and a calorific value of 8,000 (㎉ / kg) Phosphorus petroleum coke (a1 ') 40-70 weight%,

Apparent density 2.8745g / mL, Specific surface area 15.450㎡ / g, Average pore diameter 169Å, Porosity 15.8%, Limestone with particle size 0.1 ~ 3mm ") Use a combustor combusted from 30 to 60% by weight at a temperature of 850 ~ 950 ℃, or

A solidifying agent of the first mixture obtained by adding 10 to 70% by weight of the mixture (a2) composed of any one or two selected from sulfuric acid or iron sulfate to 30 to 90% by weight of the combustion material (a1). Or as

60 to 85% by weight of the first mixture (A), incinerator; Fly ash with a specific gravity of 1.80 and 1.0 moisture; Steelmaking sludge dust having a powder degree of 3500 cm 2 / g; Diatomaceous earth with a pore of 10 to 30 µm, a porosity of 73.05%, and a water absorption of 70.55%; Olivine crushed into a size of 300 to 400mesh after heating for 10 to 15 seconds at a temperature of 400 ℃ with a heat dryer; The main technical configuration is to use the second mixture, which is added by mixing 15 to 40% by weight of the additive (B), which is composed of any one or two or more kinds selected from among them, is mixed.

Hereinafter, the technical configuration will be described in more detail.

The solidifying agent is suppressed from drying and gas generation due to heat of neutralization by acid alkali neutralization reaction, calcium and magnesium are combined to increase granulation and strength, and have an earthy color due to oxidation function. In the mixing process, the water content of the sludge is reduced by the hydration reaction (exotherm), and heavy metal dissolution is suppressed by the effect.

In addition, in the curing process by the solidifying agent, the separation and elution is suppressed by the ion exchange reaction, the strength is increased by the pozzolanic reaction, the strength is increased and stabilized by the carbonation reaction, and the soil engineering is improved by the effect. Inhibition of harmful pathogens and inhibition of reslurrying occurs.

The solidification principle of the solidifying agent is that when the solidifying agent is added to the sludge containing water, organic matter, minerals and odors, an exothermic reaction by the solidifying agent occurs, thereby reducing the amount of water contained in the sludge. It absorbs and absorbs moisture contained in organic matter, moisture in inorganic matter, odors, etc. in a short time.

The combustor used as the solidifying agent is a petroleum coke and limestone burned at a temperature of 850 ~ 950 ℃, the petroleum coke is a porous, glossy solid fuel made by violent pyrolysis of petroleum residue, a large amount of sulfide It is contained, and is usually in the form of anhydrous gypsum and calcined quicklime by desulfurization upon combustion at a temperature of 850 to 950 ° C. in a fluidized bed boiler or lime kiln together with limestone having a particle diameter of less than 3 mm. Accordingly, the present invention is characterized in that it utilizes a combustion material that is generated as a by-product of a fluidized bed boiler or lime kiln without separately using anhydrous gypsum and quicklime used in the manufacture of a solidifying agent.

The desulfurization rate according to the temperature of the petroleum coke is shown in Table 1 below, and the method for calculating the desulfurization rate is as follows.

Table 1: of petroleum coke with temperature Desulfurization Rate

Temperature (℃) 850 900 950 Desulfurization Rate (%) 18.50 16.94 19.22

The desulfurization rate is somewhat lower at 900 ° C. of the desulfurization rate, because carbon and sulfur components form new C-S bonds in this temperature range. On the whole, it shows an increase in temperature, but the desulfurization rate is very low.

In order to increase the desulfurization rate, it is important to gradually increase the temperature, since the rapid temperature increase causes a significant portion of the pores of the petroleum coke.

Desulfurization mechanism of petroleum coke in fluidized bed combustion is limestone (CaCO ₃ ) is used as a desulfurization agent to remove SO ₂ discharged from petroleum coke combustion in the fluidized bed, the reaction equation is as follows.

CaCO ₃ + SO ₂ + 1/2 O ₂ ⇔ CaSO ₄ + CO ₂

At this time, the calcining reaction is an endothermic reaction in which limestone is decomposed in the form of CaO and CO ₂ . In a fluidized bed boiler, when the partial pressure of CO ₂ is lower than the equilibrium pressure of limestone firing, that is, CaCO ₃ injected into the furnace under atmospheric fluidized bed combustion conditions is calcined in the form of CaO and CO ₂ as follows.

CaCO ₃ ⇔ CaO + CO ₂ ΔH = 183 kJ / gmol

The direct reaction of CaCO ₃ with SO ₂ at the operating temperature of 850 to 950 ° C. in the fluidized bed boiler is achieved when the partial pressure of CO ₂ is higher than the equilibrium pressure, and CaCO ₃ is CaO and CO _{2 for} absorption of SO ₂ at normal pressure. Decompose to In addition, CO ₂ generated during the calcination reaction serves to create and expand pores in limestone, thereby increasing the surface area for desulfurization.

Hereinafter, look at the configuration of the solidifying agent.

Burning material  When used independently

Petroleum coke with an elemental composition of 80 to 88% by weight of carbon, 2 to 3% by weight of hydrogen, 5 to 10% by weight of oxygen, 1 to 1.5% by weight of nitrogen, and 4 to 5.5% by weight of sulfur and having a calorific value of 8,000 (kg / kg) Combustion materials burned with limestone with a particle diameter of 0.1 to 3mm in a fluidized bed boiler or a calcining furnace are used as a solidifying agent.

As described above, the combustion temperature of petroleum coke and limestone is 850 ~ 950 ℃, if the combustion temperature falls below 850 ℃ problem occurs that does not reduce the volatile content of petroleum coke, and exceeds 950 ℃ In this case, petroleum coke burns more than necessary, so it is desirable to maintain the combustion temperature at 850 ~ 950 ℃.

In addition, the limestone has an apparent density of 2.8745 g / ml, a specific surface area of 15.450 m 2 / g, an average pore diameter of 169 mm, porosity of 15.8%, and a particle diameter of 0.1 to When the particle diameter is less than 0.1 mm, problems of dust generation and processing cost increase when the particle size is less than 0.1 mm, and when the particle size exceeds 3 mm, the reaction with sulfite gas is reduced, so the particle size of the limestone is 0.1. It is desirable to maintain ˜3 mm.

In addition, when the specific surface area of the limestone is less than 15.450 m 2 / g or exceeds the average pore diameter of 169 kPa, the combustion time and the temperature are increased to increase the cost, thereby increasing the cost. It is preferable to use limestone having a specific surface area and an average pore size.

With combustion materials  When using a mixture of sulfuric acid or iron sulfate

Petroleum coke with an elemental composition of 80 to 88% by weight of carbon, 2 to 3% by weight of hydrogen, 5 to 10% by weight of oxygen, 1 to 1.5% by weight of nitrogen, and 4 to 5.5% by weight of sulfur and having a calorific value of 8,000 (kg / kg) And 30 to 90% by weight of the burned material of which limestone having a particle diameter of 0.1 to 3 mm was burned in a fluidized bed boiler or a calciner at a temperature of 850 to 950 ° C.,

10 to 70% by weight of the mixture composed of any one or two selected from sulfuric acid or iron sulfate is used as a solidifying agent.

As described above, the combustor may be used independently as a hardener, but may be used by adding any one or a mixture of sulfuric acid or iron sulfate to the combustor.

As described above, the combustor was desulfurized by burning petroleum coke and limestone containing sulfide at a temperature of 850 to 950 ° C. in a fluidized bed boiler or lime kiln, and the combustion temperature of the petroleum coke and limestone was 850. If the temperature is lower than 占 폚, the volatilization of petroleum coke cannot be reduced. If the temperature is higher than 950 占 폚, the petroleum coke may be burned more than necessary. Therefore, it is preferable to maintain the combustion temperature at 850 to 950 占 폚.

In addition, the combustor is used in 30 ~ 90% by weight relative to the total weight of the solidifying agent, when less than 30% by weight of the solidification capacity of the solidifying agent is lowered, when used in excess of 90% by weight As the amount of waste sulfuric acid is reduced, the pH is high, which hinders the natural environment such as forest reclamation, and the waste resource recycling side is reduced, so that the combustion material is used in an amount of 30 to 90% by weight based on the total weight of the solidifying agent. It is preferable.

The mixture of any one or two selected from sulfuric acid or iron sulfate is used in the range of 10 to 70% by weight based on the total weight of the solidifying agent in consideration of the appropriate pH of the combustion material.

The sulfuric acid may be selected from those having a purity of 40 to 99% according to the acidity of iron sulfate and eluting heavy metals, and used sulfuric acid or purified sulfuric acid.

The use of waste sulfuric acid is a very important problem in terms of resource recycling and environmental improvement. In the case of utilizing waste sulfuric acid, sulfuric acid having a purity of 95 to 99% and waste sulfuric acid are mixed, and acidity and waste sulfuric acid are recycled. In consideration of this, the composition is used in the ratio of 40-50 vol% of sulfuric acid with purity of 95-99% and 50-60 vol% of spent sulfuric acid. In addition, the waste sulfuric acid is obtained by pulverizing sulfuric acid sludge, which is a strong acid having a pH of about 2 washed with a semiconductor factory substrate, at a high temperature of 70 ~ 90 ℃.

The said iron sulfate selects and uses any 1 type of ferrous sulfate monohydrate or ferrous sulfate hexate. The ferrous sulfate monohydrate has a molecular formula FeSO ₄ Crystalline solid inorganic compound which is H ₂ O, whose composition is 18.4% Fe and TiO ₂ 5.0%, Cu 0.005%, Mn 4.0%, water insoluble 0.2%, hydrochloric acid insoluble 0.1%, adhered water 5.0%,

The ferrous sulfate 7 hydrochloride has a molecular formula FeSO ₄ 7H ₂ O is a crystalline solid inorganic compound, its composition is Fe 18.4%, Fe ₂ (SO ₄ ) ₃ 0.5%, TiO ₂ 0.5%, Cu 0.005%, Mn 0.6%, water insoluble content 0.2%, hydrochloric acid insoluble content 0.1%, adhesion moisture 5.0%.

The sulfuric acid or iron sulfate is mixed in the range of less than 70% by weight based on the total weight of the solidifying agent in consideration of ammonia gas generation, heat generation, granulation, use, etc. with respect to the petroleum coke combustor mixed with limestone, The pH of the topic is about 4-12.

The sulfuric acid or iron sulfate is used in 10 to 70% by weight relative to the total weight of the solidifying agent, when used in less than 10% by weight has a problem that hinders forest greening, and exceeds 70% by weight When used, the solidification capacity of the solidifying agent falls, and there is a problem in that it affects forest greening due to acid, so the sulfuric acid or iron sulfate (B) is 10 to 70% by weight based on the total weight of the solidifying agent. It is preferable to use.

With combustion materials  When using sulfuric acid or iron sulfate and additives

Petroleum coke with an elemental composition of 80 to 88% by weight of carbon, 2 to 3% by weight of hydrogen, 5 to 10% by weight of oxygen, 1 to 1.5% by weight of nitrogen, and 4 to 5.5% by weight of sulfur and having a calorific value of 8,000 (kg / kg) And limestone having a particle diameter of 0.1 to 3 mm, 30 to 90% by weight of the combustion material (a1) combusted at a temperature of 850 to 950 ° C. in a fluidized bed boiler or a calciner, and any one or two selected from sulfuric acid or iron sulfate. 60 to 85 wt% of the first mixture (A) formed by mixing 10 to 70 wt% of the mixture (a2) formed by mixing;

Incinerator (b1), Specific gravity 1.80, Fly ash (b2) within 1.0 Moisture, Steelmaking slag dust (b3) with a powder degree of 3500 cm2 / g, Porosity 10-30 μm, Porosity 73.05%, Absorption rate 70.55% Phosphorus diatomaceous earth (b4), an additive (B5) selected from a mixture of one or two or more selected from olivine (b5) pulverized to a size of 300 to 400 mesh after heating for 10 to 15 seconds at a temperature of 400 ℃ with a thermal dryer. ) 15 to 40% by weight is used as a solidifying agent.

The content of the combustion material, sulfuric acid, or iron sulfate is the same as described above, and is formed by mixing the combustion material (a1) and a mixture (a2) of any one or two selected from sulfuric acid or iron sulfate. The first mixture (A) is used in the range of 60 to 85% by weight relative to the total weight of the solidifying agent, when less than 60% by weight, the problem of the solidification capacity of the solidifying agent falls, and exceeds 85% by weight When using, since the cost rises, it is preferable to use the first mixture (A) at 60 to 85% by weight based on the total weight of the solidifying agent.

The additive (B) is composed of one or two or more selected from incineration material (b1), fly ash (b2), steelmaking slag dust (b3), diatomaceous earth (b4), and olivine (b5), Looking at each component of the additive (B) is as follows.

The incineration ash (b1) is any one or a mixture of two or more selected from waste incineration ash, sewage sludge incineration ash, industrial by-product incineration ash, and paper ash incineration ash. Looking at the chemical composition through (accession number TE 04 1044) is shown in Table 2 below.

Table 2: Test results of chemical composition of paper ashes

Test Items result Test Methods  Chemical composition (%) SiO ₂ 33.8  KS L 4007 '96 Al ₂ O ₃ 17.5 CaO 30.4 SO ₃ 0.6 KS L 5120 '99

The fly ash (b2) is produced as a by-product in a thermal power plant, the main chemical components are SiO ₂ , Al ₂ O ₃ , Fe ₂ O ₃ and other CaO, MgO, unburned carbon powder, the powder of the fly ash In addition, as an important factor governing pozzolanic activity with the content of SiO ₂ , the fly ash used in the present invention has a powder density of 3,000 cm ² / g.

The fly ash is composed of more than 50% of the SiO ₂ of the total chemical composition, generally using a specific gravity of 1.80, moisture within 1.0.

The steelmaking slag dust (b3) is composed of lime, Silicate, Wustite phase, etc., among the components of the furnace slag, but some form free calcium oxide (free CaO) that employs Mg, Mn, Fe, etc. in the undissolved state. After aging with water in advance, the powder is processed into a particle size of 3500 cm 2 / g.

The diatomaceous earth (b4) is made by the accumulation of debris of diatoms or single cell plants, and has its own micropores of several hundreds of micrometers, which is very light, soft, and has a lot of pores due to the nature of the raw materials, and the diatomaceous earth used in the present invention has pores. 10-30 micrometers, the thing of 73.05% of porosity, and 70.55% of water absorption are used.

The olivine (b5) is a forsterite (Mg-Oliver) and has the characteristics of high far-infrared radiation, antibacterial, anti-fungal, deodorant and anion radiation by magnesia (MgO), hardness of 6.5 ~ 7, specific gravity 3.2 ~ 3.4 In order to prevent the alteration of olivines in advance, they are heated with a heat dryer at a temperature of 400 ° C. for 10 to 15 seconds to maintain a dry state, and thus the olivines in the dry state are crushed to a size of 300 to 400 mesh. .

Additive (B) having the above components is used in the range of 15 to 40% by weight relative to the total weight of the solidifying agent, when less than 15% by weight, the problem of rising costs, 40% by weight In the case of using excessively, since the solidification capacity of the solidifying agent is deteriorated, the first mixture (A) is preferably used in an amount of 15 to 40% by weight based on the total weight of the solidifying agent.

Hereinafter, the above-described technical configuration will be described in more detail with reference to the following examples.

Example 1

Combustion material burned at a temperature of 850 to 950 ° C by burning petroleum coke and limestone in a 55:45 weight ratio for 10 kg of sewage sludge deposited in a coastal lake with 75% water content. It is as shown in Table 3.) was put into 15 ~ 30% by weight of the sludge was mixed enough for 2 minutes and the solidification progress was measured.

Table 3: Average Composition of Combustors

Item unit Result SiO ₂ % 2.45 Al ₂ O ₃ % 0.45 Fe ₂ O ₃ % 0.33 MgO % 1.42 SO ₃ % 22.6 Na ₂ O % 0.19 K ₂ O % 0.18 CaO % 59.3

The sedimentary sludge is mostly fine sand and has the properties as shown in Table 4, and it takes 28 to 60 hours to obtain a water content of 45%, which can be utilized as a cover agent, green soil, etc., by using the combustion material as a hardener. However, in the case of petroleum coke combustors, a strong alkalinity of pH 12.4, when the sewage sludge solidified, ammonia gas of 200 ppm or more was generated and could not be used alone.

Table 4: Sedimentary sludge  Constellation

division pH Water content Organic matter importance Sedimentation sludge (bottom) 8.66 79% 10.06% 1.36 Sedimentation sludge 8.37 75% 10.17% 1.42

The deposited sludge (middle and lower) of Table 4 had a dark black color prior to the addition of the solidifying agent, but changed to light gray after the solidification treatment by the addition of the solidifying agent.

In addition, the amount of solidifying agent added to the sewage sludge and the time required for solidification are shown in Table 5.

Table 5: Sedimentary Solidifying agent By input ( Sedimentation  100) Solidification time

division 100: 15 100: 20 100: 25 100: 30 Solidification time (based on 45% water content) 60hr 50hr 36hr 28hr

Example 2

To 10 kg of sewage sludge taken from the landfill of metropolitan area with water content of 80%, 0.3 kg of combustor, 0.3 kg of sulfuric acid with 98% purity, and 5 kg of ferrous sulfate hepatate were mixed as shown in Table 7. A solidifying agent having physical properties was prepared and sufficiently mixed, and then the solidified state was observed.

Table 6: Sewage Sludge  Properties

Date of acquisition color Water content pH 2006.04.23 black 80% 7.2

Table 7: Solidifying agent  Properties

importance Water content pH color Powder 1.1 1.0 8.9 Brown 2300 brain

When the solidifying agent is added, an exothermic phenomenon of 45 ° C. or more occurs due to an acid and alkali reaction, which quickly reduces the water content. Table 8 shows the properties of sewage sludge after solidification.

Table 8: After solidification Sewage Sludge Property

Curing ammonia pH Water content Uniaxial compressive strength Permeability chart Dry density after compaction color Cover test 2 days 3 ppm 8.0 47% 0.7㎏ / ㎠ 10 ^-5 cm / s 1.5 kg / cm 3 Light black Good

Example 3

0.3 kg of 98% pure sulfuric acid and 0.3 kg of ferrous sulfate heptahydrate were sufficiently mixed with 1 kg of papermaking incinerator to form a dried composition having a pH of 6, and then mixed with 1.4 kg of combustion material for 2 minutes in a mixer to prepare a solidifying agent. Thereafter, 30% of the sewage sludge having the physical properties as shown in Table 6 was added at a weight ratio to solidify the same.

As the sewage sludge and the solidifying agent are mixed, an exothermic phenomenon of 70 ° C. or more occurs due to the action of quicklime present in the coke burning material of the solidifying agent, and the drying effect is quick. Accordingly, the sewage sludge physical properties after solidification are shown in Table 8 below.

Table 8: After solidification Sewage Sludge Property

Curing ammonia pH Water content Uniaxial compressive strength Permeability chart Dry density after compaction color Cover test 2 days 5 ppm 9 45% 0.6㎏ / ㎠ 10 ^-5 cm / s 1.7 kg / cm 3 Light yellow Good

As described above, the high-functional sludge solidifying agent according to the present invention has the following effects.

first. It inhibits heavy metal elution and reslurrying, gives proper moisturizing action and forms hard ground.

second. Through granulation, soil permeability is maintained and soil erosion is prevented.

third. As the solidifying agent is mixed, the calcined ash present in the coke burning material of the solidifying agent generates an exothermic phenomenon of 70 ° C. or higher, and a rapid drying effect is initially observed.

fourth. The solidifying agent according to the present invention has the effect of promoting solidification and excellent water loss, so that sludge or sludge can be easily utilized as a covering agent, green soil, and the like.

Claims (4)

Petroleum coke with an elemental composition of 80 to 88% by weight of carbon, 2 to 3% by weight of hydrogen, 5 to 10% by weight of oxygen, 1 to 1.5% by weight of nitrogen, and 4 to 5.5% by weight of sulfur and having a calorific value of 8,000 (kg / kg) (a1 ') 40 to 70% by weight, Apparent density 2.8745g / ml, Specific surface area 15.450m2 / g, Average pore diameter 169Å, Porosity 15.8%, Grain size 0.1 ~ 3mm ") A high-function sludge solidifying agent, characterized in that it is a combustor combusted at a temperature of 850 ~ 950 ℃ 30 to 60% by weight. 30 to 90% by weight of the combustion material (a1) of claim 1, A high-functional sludge solidifying agent, characterized in that the first mixture is added by mixing 10 to 70% by weight of the mixture (a2) composed of any one or two selected from sulfuric acid or iron sulfate. 60 to 85% by weight of the first mixture (A) of claim 2, Incineration ash; Fly ash with a specific gravity of 1.80 and 1.0 moisture; Steelmaking sludge dust having a powder degree of 3500 cm 2 / g; Diatomaceous earth with a pore of 10 to 30 µm, a porosity of 73.05%, and a water absorption of 70.55%; Olivine crushed into a size of 300 to 400mesh after heating for 10 to 15 seconds at a temperature of 400 ℃ with a heat dryer; A high function sludge solidifying agent, characterized in that the second mixture is added by mixing 15 to 40% by weight of the additive (B) composed of any one or two or more selected from among them. The sulfuric acid according to claim 2 or 3, wherein sulfuric acid sludge produced in a semiconductor factory is purified using hot air convection at 70-90 ° C. to 50-60 wt% of waste sulfuric acid and 95-99% purity. A high functional sludge hardener, characterized in that the composition is mixed by having 40 to 50% by weight of sulfuric acid.