A steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering and its preparation method

Abstract

The present invention discloses a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering and a preparation method thereof, belonging to the technical field of soil remediation; all the raw materials of the passivator proposed by the present invention are solid wastes and can be used as fertilizers, causing no secondary pollution to the soil, having a wide source and a simple preparation process; the steel slag powder for sintering can replace nano iron metal powder and be used as the main functional component of the passivator to achieve a similar arsenic passivation effect, and at the same time can significantly reduce the cost of the passivator. The prepared passivator can be used for the remediation of arsenic-contaminated soil. Through the complexation and coprecipitation of iron oxides, the reduction rate of available arsenic in the arsenic-contaminated soil in mining areas can reach more than 95%.

Description

Technical Field

[0001] The present invention relates to the technical field of soil remediation, and particularly relates to a steel slag-based passivator for arsenic-contaminated mine soil for sintering and a preparation method thereof. Background Art

[0002] At present, the treatment of arsenic-contaminated soil is an urgent problem to be solved. Arsenic is the world's number one teratogenic and carcinogenic toxin. When its content in the human body is higher than 60 mg, it will cause death. Arsenic in the soil is easily absorbed by various plants and then accumulates in the human body, causing serious harm to the human body.

[0003] To improve this situation, many scholars have already carried out relevant research and proposed various treatment methods such as phytoremediation, passivation treatment, and microbial treatment. Moreover, many types of materials that can be used for arsenic-contaminated soil remediation have been discovered, among which iron-containing materials are widely used. Chinese Patent CN 201910459345.6 discloses a passivator for arsenic-cadmium co-contaminated soil, its preparation method and usage method. The passivator includes earthworm manure with a specific gravity of 45% - 50% and zero-valent iron powder with a specific gravity of 50% - 55%. The maximum reduction rate of available arsenic can reach 68%, but the reduction effect of arsenic is not significant enough. Chinese Patent CN202111043974.4 discloses a passivation and remediation method for arsenic-cadmium co-contaminated agricultural land. The compounding of zero-valent iron and FeSO4·7H2O can achieve a reduction rate of arsenic in alkaline soil reaching 97%, but the raw material cost is relatively high. Chinese Patent CN 201110309622.9 discloses a heavy metal-polluted acidic soil remediation material, its preparation method and uses. After using biomass power plant ash, slag powder, steel slag powder, fly ash, and bentonite as raw materials to remediate the contaminated soil, the content of heavy metals such as arsenic in the soil meets the requirements of the secondary soil environmental quality standard in GB15618-1995, but the problem of secondary pollution of the soil caused by the addition of multiple raw materials needs further evaluation. Chinese Patent CN 201110309622.9 discloses a remediation agent for arsenic-contaminated soil prepared from trivalent metal salts, metal oxides, and mud. It can reduce the concentration of leached arsenic by more than 99.8%, and the effect is significant, but the cost of metal salts is high and it is also easy to cause soil acidification. Chinese Patent CN 201511010971.5 discloses a method for remediating arsenic-contaminated soil. After mixing the stabilizing agent FeSO4·7H2O, CaO, and water with arsenic-contaminated soil, adding the curing agent cement, water, water glass, and bentonite, and using the combination of the stabilizing agent and the curing agent for in-situ injection to perform solidification and stabilization treatment on the heavy metal-contaminated soil at the site. After treatment, the leaching concentration of heavy metal arsenic in the soil is lower than the Class IV water standard of the "Groundwater Quality Standard" (GB / T14848-93), but the addition of cement is easy to cause soil compaction and is not conducive to the growth of plants.

[0004] In summary, traditional iron-based passivators have problems such as high raw material costs, complex preparation processes, and secondary pollution in the remediation of arsenic-contaminated soils. Therefore, inventing a low-carbon, highly efficient, and low-cost arsenic-contaminated soil remediation agent has become a difficult problem that urgently needs to be solved in engineering applications. Summary of the Invention

[0005] The purpose of the present invention is to provide a passivator for arsenic-contaminated soil with low cost, simple process, low-carbon environmental protection, and remarkable remediation effect, as well as a preparation method thereof, to solve the problems of high raw material costs, complex preparation processes, and secondary pollution existing in the remediation of arsenic-contaminated soil by traditional iron-based passivators.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering, the passivator is composed of the following raw materials in parts by weight: 50-90 parts of steel slag powder for sintering, 0-50 parts of ferrous sulfate heptahydrate particles, and 5-15 parts of biomass materials.

[0008] Preferably, the weight fractions of the raw materials for preparing the passivator are: 50-70 parts of steel slag powder for sintering, 30-50 parts of ferrous sulfate heptahydrate particles, and 8-12 parts of biomass materials.

[0009] As a further scheme of the present invention: the mass fraction of total iron in the steel slag powder for sintering is 40-80%, the mass fraction of metallic iron is 30-60%, the mass fraction of CaO is 5-15%, and the quality meets the requirements of the national standard "Magnetic Separation Slag Steel Powder for Sintering" GB / T 30897-2014.

[0010] Preferably, the mass fraction of total iron in the steel slag powder for sintering is 60-80%, and the mass fraction of metallic iron is 50-60%.

[0011] As a further scheme of the present invention: the ferrous sulfate heptahydrate particles are by-products of the titanium dioxide industry. Among them, the mass fraction of ferrous sulfate heptahydrate is 90-100%, and the mass fractions of heavy metals such as arsenic, lead, cadmium, mercury, and chromium are all less than 0.00002%, meeting the requirements of the national standard "Water Treatment Agent - Ferrous Sulfate" GB / T 10531-2016, and can be used as fertilizers.

[0012] As a further scheme of the present invention: the biomass material is one or more of crop straws, bamboo powders, and sawdust, and is sieved through a 100-mesh sieve after being crushed.

[0013] Preferably, the biomass material is selected to be easily available according to the crops and native plants in the area where the arsenic-contaminated soil in the mining area is located.

[0014] A preparation method of a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering, comprising the following steps:

[0015] S1. After the converter steel slag is crushed, dry-ground, and dry-magnetically separated, coarse-grained steel slag powder for sintering with a particle size less than 2 mm is obtained;

[0016] S2. Further grind the coarse-grained steel slag powder for sintering with a particle size less than 2 mm prepared in S1 by using a vertical mill or a vibration mill to obtain steel slag powder for sintering with a particle size less than 0.1 mm;

[0017] S3. Mix the steel slag powder for sintering with a particle size less than 0.1 mm prepared in S2 with ferrous sulfate heptahydrate particles according to the formulation ratio and stir strongly to disperse the ferrous sulfate heptahydrate aggregates, so that the pH value of the mixture is stabilized at 6-8;

[0018] S4. Mix the mixture prepared in S3 with a biomass material and stir strongly to make the steel slag powder for sintering and the ferrous sulfate heptahydrate particles adhere to the surface of the biomass material, thereby preparing the passivator.

[0019] An application of a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering in the field of arsenic-contaminated soil remediation, specifically: incorporating the passivator into the arsenic-contaminated soil at a ratio of 1-10% of the dry basis mass fraction of the soil, with the soil moisture content being 10-30% and the remediation time being 7-30 days.

[0020] As a further scheme of the present invention: incorporating the passivator into the arsenic-contaminated soil at a ratio of 3-6% of the dry basis mass fraction of the soil, with the soil moisture content being 15-25% and the remediation time being 20-30 days.

[0021] Compared with the prior art, the present invention provides a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering and a preparation method thereof, having the following beneficial effects:

[0022] (1) The raw materials used for preparing the passivator in the present invention are steel slag powder for sintering, ferrous sulfate heptahydrate particles, and a biomass material. Among them, the steel slag powder for sintering is a steelmaking slag, the ferrous sulfate heptahydrate particles are by-products of the sulfuric acid method for producing titanium dioxide, and the biomass material is agricultural solid waste. The cost is low and the sources are extensive. While achieving the passivation of arsenic-contaminated soil, it realizes pollution treatment with waste, is green and environmentally friendly, and has no secondary pollution to the soil.

[0023] (2) Both the steel slag powder for sintering and the ferrous sulfate heptahydrate particles can be used as soil fertilizers; the biomass material can increase the specific surface area of the steel slag-based passivator. Attaching the steel slag powder for sintering and the ferrous sulfate heptahydrate particles to the surface of the biomass material is beneficial to improving the mixing uniformity of the steel slag-based passivator in the soil layer and enhancing the passivation effect, and at the same time providing some organic substances for the soil.

[0024] (3) The metal iron content in the steel slag for sintering is relatively high, which can replace common nano-metal iron powder as a passivator for arsenic-contaminated soil remediation and achieve the same remediation effect. At the same time, on the one hand, CaO in the steel slag powder for sintering can neutralize the hydrogen ions generated by the hydrolysis of ferrous sulfate heptahydrate, adjust the soil pH value, effectively avoid soil acidification, and provide a suitable pH value for the ecological remediation of mining area soil. On the other hand, the dissolved calcium ions combine with arsenic in the soil to form insoluble Ca-As products, significantly reducing the content of available arsenic.

[0025] (4) The present invention utilizes the synergistic effect between two kinds of iron-based solid wastes to achieve the complexation and precipitation of soil arsenic through the hydrolysis of ferrous sulfate and the redox reaction of iron in the steel slag for sintering, so as to achieve the purpose of highly efficient and stable passivation of arsenic in the soil. Description of the Drawings

[0026] Figure 1 It is a comparison of the passivation effect of the available arsenic in arsenic-contaminated soil with different dosages of the steel slag-based passivator in Example 2 of the present invention;

[0027] Figure 2 It is a comparison of the change of the passivation effect of the available arsenic in the soil with the repair time in Example 2 of the present invention;

[0028] Figure 3 It is a comparison of the change of the leaching concentration of arsenic in the soil with the repair time in Example 2 of the present invention;

[0029] Figure 4 It is the SEM-EDS spectrum of the arsenic-containing soil after being repaired with the steel slag-based passivator in Example 2 of the present invention. Specific Embodiments

[0030] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0031] Example 1:

[0032] The test soil was collected from the Lajiapo Mining Area in Chehe Town, Nandan County, Guangxi. The soil was naturally air-dried, ground and sieved through a 2 mm sieve. The prepared steel slag powder for sintering was mixed with ferrous sulfate heptahydrate particles and biomass sawdust to prepare steel slag-based passivators according to the four ratios of A, B, C, D and E in Table 1 and fully mixed with the soil. After the soil was evenly mixed, it was repaired at a constant temperature (temperature 25±1°C), and the water content was maintained at 20%. After 30 days of repair, samples were taken to measure the content of available arsenic.

[0033] The steel slag used in this example is converter steel slag. The total iron mass fraction of the steel slag for sintering prepared by the above steps is 78%, the metallic iron mass fraction is 50%, the CaO mass fraction is 7%, and the average particle size is less than 0.1 mm.

[0034] The ferrous sulfate heptahydrate used in this example is a by-product of the titanium dioxide industry, and the mass fraction of ferrous sulfate heptahydrate is 99%.

[0035] The biomass material used in this example is biomass sawdust, which is passed through a 100-mesh sieve after being crushed.

[0036] Taking the soil without adding a passivator as a blank control, in the control group soil, the available arsenic content is 7.02 mg / kg, and the TCLP leaching concentration of arsenic is 12.03 μg / L.

[0037] Table 1: Passivation effects of steel slag-based passivators with different ratios on arsenic in soil (parts by weight)

[0038]

[0039] It can be obtained from Table 1 that the available arsenic contents in the soil passivated by the steel slag-based passivators A, B, C, D, and E with different ratios are 2.26, 1.15, 1.95, 0.33, and 1.11 mg / kg respectively, and the TCLP contents of arsenic are 6.68, 2.48, 3.44, 2.28, and 3.25 μg / L respectively. The five different ratios of steel slag-based passivators for sintering can all reduce the arsenic content in the contaminated soil, but the passivation effects of using only the steel slag for sintering, the composite of the steel slag for sintering and ferrous sulfate heptahydrate, and the composite of the steel slag for sintering and biomass sawdust are all weaker than the mixture of the steel slag for sintering, ferrous sulfate heptahydrate, and biomass sawdust. Especially when the mass fraction ratio of the steel slag for sintering: ferrous sulfate heptahydrate: biomass sawdust is 5:4:1, it shows the best passivation effect, and the reduction rate of available arsenic is over 95%. The composite of ferrous sulfate heptahydrate and biomass can further improve the passivation effect of the steel slag-based passivator. This is mainly because the activity of iron in the steel slag for sintering is activated by ferrous sulfate heptahydrate, increasing the action intensity on arsenic in the soil, and the biomass sawdust further increases its adsorption area, thus enhancing the passivation effect of arsenic.

[0040] Comparative Example 1:

[0041] Without adding ferrous sulfate heptahydrate and biomass to the passivator, only compare the remediation effects of different types of converter steel slags on arsenic-contaminated soil. Take converter steel slags with different total iron contents, metallic iron contents, and CaO contents as passivators (numbered A1, A2) and compare with the steel slag for sintering in Example 1 as a passivator (numbered A). See Table 2 for details.

[0042] Table 2 Remediation effects of converter steel slag with different iron and calcium contents on arsenic - contaminated soil

[0043]

[0044] Comparing the remediation effects of different steel slags in Table 2, the available arsenic content in the soil and the TCLP leaching concentration of As after remediation with A1 are 4.34 mg / kg and 8.31 μg / L respectively, and the available arsenic content in the soil and the TCLP concentration of As after remediation with A2 are 7.87 mg / kg and 9.26 μg / L respectively, which are much higher than the available arsenic and the TCLP leaching concentration of arsenic in the soil after remediation with sintering steel slag. It can be seen that the passivation effect of sintering steel slag on arsenic is significantly better than that of the steel slag with 30% total iron mass fraction, 10% metallic iron mass fraction, and 30% CaO mass fraction, and the steel slag with 15% total iron mass fraction, 5% metallic iron mass fraction, and 35% CaO mass fraction. Obviously, the sintering steel slag with relatively high mass fractions of total iron and metallic iron obtained by crushing and dry magnetic separation of converter steel slag has a good arsenic passivation effect.

[0045] Comparative Example 2:

[0046] To further compare the remediation effects of steel slags with different iron contents and CaO contents combined with ferrous sulfate heptahydrate and biomass sawdust on soil, take the best passivation effect ratio of 5:4:1 by mass fraction of sintering steel slag: ferrous sulfate heptahydrate: biomass sawdust in Example 1, and equally replace the sintering steel slag with the steel slags numbered A1 and A2 in Comparative Example 1, with other conditions the same as in Example 1. See Table 3 for details.

[0047] Table 3 Remediation effects of passivators based on converter steel slag with different iron and calcium contents on arsenic - contaminated soil

[0048]

[0049] It can be seen from Table 3 that among the passivators prepared by mixing converter steel slags with different iron and calcium contents with ferrous sulfate heptahydrate and biomass sawdust respectively, the available arsenic content of No. D - A1 is 2.44 mg / kg, and the TCLP leaching concentration of arsenic is 3.56 μg / L; the available arsenic content of No. D - A2 is 3.07 mg / kg, and the TCLP leaching concentration of arsenic is 5.42 μg / L. Compared with using converter steel slag alone as a passivator, the passivation effect is significant; however, there is still a large gap compared with the passivation effect of the sintering steel slag - based passivator. It can be seen that for the sintering steel slag - based passivator to achieve a significant arsenic passivation effect, the converter steel slag used must have relatively high mass fractions of total iron and metallic iron.

[0050] Comparative Example 3:

[0051] To compare the passivation effects of steel slag for sintering and biomass sawdust with different particle sizes on arsenic-contaminated soil, the ratio of the best passivation effect numbered D in Example 1 was taken (the mass fraction ratio of steel slag for sintering: ferrous sulfate heptahydrate: biomass sawdust was 5:4:1). Steel slag for sintering with particle sizes less than 1 mm and 2 mm, and biomass sawdust passing through 40-mesh and 10-mesh sieves were selected, and other conditions were the same as those in Example 1. See Table 4 for details.

[0052] Table 4 Remediation effects of steel slag for sintering with different particle sizes and biomass sawdust on arsenic-contaminated soil

[0053]

[0054] It can be seen from Table 4 that under the condition of the best ratio, when the particle size of the steel slag for sintering was 1 mm and 2 mm, the content of available arsenic in the remediated soil was 2.68 and 4.56 mg / kg respectively, and the TCLP leaching concentration of arsenic was 5.85 and 8.08 μg / L respectively, which were much higher than 0.33 mg / kg and 2.28 μg / L of the passivator numbered D in Example 1. It can be seen that the increase in the particle size of the steel slag for sintering will significantly reduce the passivation effect on arsenic, which is not conducive to the dissolution of iron and calcium elements in the steel slag particles and their participation in the passivation reaction; similarly, when the passivator prepared from biomass sawdust passing through 40-mesh and 10-mesh sieves was used to remediate arsenic-contaminated soil, the remediation effect was significantly weaker than that of biomass sawdust passing through 100-mesh sieve. The reduction of the particle size of biomass sawdust is conducive to the dispersion of steel slag for sintering and ferrous sulfate heptahydrate particles, and can be evenly mixed and contacted with soil particles.

[0055] Comparative Example 4:

[0056] The ratio listed in Table 1 of Example 1 was taken, and commercially available nano metal iron powder was used to replace the steel slag for sintering in the steel slag for sintering-based passivator equally, and other conditions were the same as those in Example 1. The specific ratio is shown in Table 5.

[0057] Table 5 Weight parts of raw materials used for preparing the passivator by equally replacing the steel slag for sintering with nano metal iron powder

[0058]

[0059] As can be seen from Table 5, the available arsenic contents in the soil passivated by nano-metal iron-based passivators A*, B*, C*, D* and E* with different ratios are 2.25, 1.19, 2.01, 0.53 and 1.15 mg / kg respectively, and the TCLP contents of arsenic are 8.01, 2.35, 3.44, 2.31 and 4.23 μg / L respectively. Comparing with the data in Table 1, it can be known that the passivation effect of the steel slag-based passivator for sintering on arsenic in the soil is slightly higher than that of the nano-metal iron powder-based passivator. The reason is that, compared with nano-metal iron powder, the steel slag powder for sintering contains not only metallic iron, but also calcium-containing minerals and iron oxides, which are more conducive to the formation of Fe-As and Ca-As precipitates.

[0060] Example 2:

[0061] The steel slag-based passivator for sintering in Example 1 was mixed into the arsenic-contaminated soil in the mining area at 1%, 3%, 5% and 7% of the dry soil mass, and other conditions were the same as those in Example 1. The test results are as Figure 1 .

[0062] From Figure 1 the test results, it can be obtained that the content of available arsenic gradually decreases with the increase of the passivator addition amount. When the passivator dosage is 5%, the available arsenic contents in the soil passivated by the steel slag-based passivators A, B, C, D and E with different ratios for 30 days are 1.28, 1.14, 1.15, 0.33 and 0.8 mg / kg respectively, all reaching the best passivation effect on arsenic-contaminated soil. The reduction rate of available arsenic under the optimal ratio is 95%.

[0063] Comparing with the passivator prepared by combining different earthworm casts and zero-valent iron in the existing patent 1 (CN201910459345.6 "Arsenic-cadmium composite contaminated soil passivator and its preparation method and usage method"), when the addition amount is 5% of the dry soil mass, the reduction rate of available arsenic is 68%; comparing with the existing patent 2 (CN202111043974.4 "A passivation and remediation method for arsenic-cadmium composite contaminated agricultural land"), when 4% of FeSO4·7H2O is compounded with 1% of zero-valent iron, the reduction rate of available arsenic can reach 97%, but its raw material uses metallic iron powder, and the cost is relatively high. See Table 6 for details.

[0064] Table 6 Comparison of the passivation effects on arsenic when the addition amounts of different types of passivators are 5%

[0065]

[0066] Example 3:

[0067] Take the passivators with different ratios in Example 1 and the optimal passivator dosage (5%) in Example 2, and detect the available arsenic and TCLP concentration of arsenic in the arsenic-contaminated soil in the mining area after 7, 14, 30, and 90 days of remediation. Other conditions are the same as those in Example 1.

[0068] From Figure 2 Figure 3 the results, it can be obtained that as the passivation time extends, both the available arsenic and the TCLP concentration of arsenic in the arsenic-contaminated soil show a downward trend and gradually stabilize. When the passivation time reaches 30 days, the contents of available arsenic in the soil passivated by the steel slag-based passivators A, B, C, D, and E for sintering with different ratios are 1.0, 0.96, 1.25, 0.33, and 0.74 mg / kg respectively, and tend to be stable with the increase of time. After 60 days of passivation, the arsenic TCLP leaching concentrations of the passivators numbered D and E are close to 0 μg / L, and the passivation effect is significant.

[0069] Comparing with the passivator prepared by combining different earthworm castings and zero-valent iron in the existing patent 1 (CN201910459345.6 "Arsenic-cadmium composite contaminated soil passivator and its preparation method and usage method"), after 60 days of passivation, the reduction rate of available arsenic is 68%. See Table 7 for details.

[0070] Table 7 Comparison of the passivation effects of arsenic at different passivation times

[0071]

[0072] Figure 4 To obtain the SEM-EDS spectrum of the soil after remediation with the ratio of steel slag for sintering: ferrous sulfate heptahydrate: biomass in the passivator being 5:4:1, as Figure 4 shown, flaky iron-arsenic complexes and precipitates are formed on the soil surface. The EDS spectrum shows the coexistence of arsenic with iron and calcium, and it is speculated that Fe-As and Ca-As precipitates are formed, indicating that the passivator prepared by the present invention realizes the complexation and precipitation of soil arsenic, achieving the purpose of highly efficient and stable passivation of arsenic in the soil.

[0073] The above is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent replacements or changes, and should be covered by the protection scope of the present invention.

Claims

1. A steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering, characterized in that The passivating agent is composed of the following raw materials in weight fractions: 50-90 parts of sintering steel slag powder, 20-50 parts of ferrous sulfate heptahydrate particles, and 5-15 parts of biomass materials; The mass fraction of total iron in the sintering steel slag powder is 40-80%, the mass fraction of metallic iron is 30-60%, and the mass fraction of CaO is 5-15%; The biomass material is one or more of crop straw, bamboo powder and sawdust, which are crushed and then passed through a 100-mesh sieve.

2. A steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering, characterized in that, The ferrous sulfate heptahydrate particles are by-products of the titanium dioxide industry, wherein the mass fraction of ferrous sulfate heptahydrate is 90-100%, and the mass fractions of heavy metals arsenic, lead, cadmium, mercury, and chromium are all less than 0.00002%.

3. The preparation method of a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering according to any one of claims 1-2, characterized in that, The following steps are involved: S1. crushing, dry grinding and dry magnetic separation of converter slag to obtain coarse sintering slag powder with a particle size of less than 2 mm; S2, further grinding the coarse-grained sintering steel slag powder with a particle size of less than 2 mm prepared in S1 by a vertical mill or a vibration mill to obtain sintering steel slag powder with a particle size of less than 0.1 mm; S3, mixing the sintering steel slag powder with a particle size of less than 0.1 mm prepared in S2 with the ferrous sulfate heptahydrate particles according to the formula ratio, and vigorously stirring to break up the ferrous sulfate heptahydrate agglomerates, so that the pH value of the mixture is stabilized at 6-8; S4. Mix the mixture prepared in S3 with the biomass material and stir vigorously to make the sintering slag powder and ferrous sulfate heptahydrate particles adhere to the surface of the biomass material, thereby preparing a passivating agent.

4. The application of a steel slag-based passivator for arsenic-contaminated soil in mining areas for sintering, characterized in that, Used for the restoration of arsenic-contaminated soil, specifically: the passivating agent is added into the arsenic-contaminated soil at a ratio of 1-10% of the mass fraction of the soil dry basis, the soil moisture content is 10-30%, and the restoration time is 7-30 days.

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