A preparation method of porous ceramics with uniform micro-nano pore diameters

Abstract

The invention discloses a method for preparing porous ceramics with uniform micro-nano pore size, belonging to the technical field of ceramic materials. The method comprises the following steps: mixing a gelling agent, a surfactant and water, and then forming micro-nano gel foam through a micro-nano bubble generator, adding mineral powder and a curing agent for high-speed stirring, and then sequentially performing injection molding, curing, demoulding, drying and sintering to obtain a porous ceramic material. The method utilizes a micro-nano foam generator to strengthen the uniform dispersion of micro-bubbles in a high-viscosity slurry, and can miniaturize and homogenize the pore size of porous ceramics to obtain porous ceramics with small and uniform pores, light weight and high compressive strength, breaking through the limitation that a high proportion of surfactants and high solid content are required to prepare porous ceramics by slurry foaming, thereby reducing the use of surfactants, and the process flow is short, easy to operate, and meets the requirements of industrial production.

Description

Technical Field

[0001] A method for preparing a ceramic material of the present invention particularly relates to a method for preparing a porous ceramic having a uniform micro-nano pore structure and belongs to the technical field of porous ceramics. Background Art

[0002] As a lightweight material, porous ceramics play an important role in related fields such as catalyst carriers, biomaterials, thermal insulation systems, and heat exchange due to their high specific surface area, high permeability, low density, and high heat insulation. Currently, one of the popular strategies for preparing porous ceramics from liquid slurries is mainly to incorporate external gas phases into the system through mechanical stirring. Surfactants or mineral colloids are positioned at the gas-liquid interface to stabilize the gas phase; other additives are then used to solidify the foam slurry. There are significant obstacles to preparing micro-fine and lightweight porous foam ceramics by traditional mechanical pulp stirring. The size of the bubbles in the foam slurry plays a crucial role in reducing the pore size of the sintered sample. Chinese Patent (Publication No.: CN101967064A) discloses a method for preparing a bulk photocatalytic material by a foaming method, which uses egg white as a foaming agent. However, this method requires a large amount of egg white, has low solubility, and will cause the foam slurry to be relatively viscous, making it difficult to disperse a large number of fine bubbles. Chinese Patent (Publication No.: CN101941231A) discloses a silicon nitride whisker-bonded silicon carbide porous ceramic and a preparation method thereof, which uses an organic monomer and a cross-linking agent as a gelling agent for stabilizing the foam and solidifying the green body. This method requires removing the gas in the slurry under the condition of a vacuum degree of 6×10 -2 Pa, and the presence of oxygen will have an inhibitory effect on the polymerization reaction, which increases the complexity of the preparation process. Chinese Patent (Publication No.: CN103145444B) discloses a method for preparing a low-cost heat-insulating lightweight porous mullite ceramic, which uses a cationic surfactant, dodecyl sulfate, as a foaming agent for mullite powder and prepares a porous mullite material under the condition of a solid content. The foaming agent in this method is prone to electrostatic repulsion with the mullite powder with a negative surface potential and is easily desorbed during low-temperature drying, making it difficult to stabilize the foam layer. The pore size distribution of the porous ceramics prepared by this method is approximately 200 - 500 μm.

[0003] In summary, the existing methods for preparing porous ceramic materials from foam slurries all have problems such as low foam stability, large amount of foaming agent used, large pore size, and complex processes. Summary of the Invention

[0004] Aiming at the technical drawbacks existing in the preparation of porous ceramic materials from existing foam slurries, the purpose of the present invention is to provide a method for preparing porous ceramics with uniform micro-nano pore diameters. This method uses a micro-nano generator to inflate with high pressure and combine with high-speed strong shearing to obtain saturated and stable slurry gel foams, thereby further obtaining porous ceramic materials with fine and uniform pores, and having characteristics such as light weight and high compressive strength. This method breaks through the limitation that the existing slurry foaming process requires the use of a high proportion of surfactants and high solid content, resulting in a reduction in pores, reduces the use of surfactants, and has a short process flow, is easy to operate, and meets industrial production requirements.

[0005] To achieve the above technical purpose, the present invention provides a method for preparing porous ceramics with uniform micro-nano pore diameters, which includes the following steps:

[0006] 1) Stir and mix a gelling agent, a surfactant and water to form a homogeneous solution;

[0007] 2) Pass the homogeneous solution into a micro-nano bubble generator to circulate and flow while inflating and then spray out to form micro-nano gel foams; the rate of the circulating flow is 2 - 20 L / min; the conditions for inflation are: the gas flow rate is 100 - 250 mL / min, and the time is 5 - 10 min;

[0008] 3) Add mineral powder to the micro-nano gel foams and stir at a high speed of 1000 - 1400 r / min for 10 - 30 min to obtain slurry gel foams, and then add a curing agent to the slurry gel foams and stir at a high speed of 1000 - 1400 r / min for 15 - 30 min to obtain stabilized slurry gel foams;

[0009] 4) Pass the stabilized slurry gel foams through injection molding, curing, demolding, drying and sintering in sequence to obtain porous ceramic materials.

[0010] In the prior art, the foaming of pulp uses shearing air as the source of bubble generation. Since the solubility of air is relatively low, this is the main reason why it is difficult to achieve micro-nano foaming in the natural environment. The key of the present invention lies in forming micro-nano gel foam by means of a micro-nano bubble generator and adopting a high-speed shearing technique to mix mineral powder with the micro-nano gel foam, so as to strengthen the uniform dispersion of the micro-nano foam in the high-viscosity pulp and form a stabilized pulp gel foam. More specifically, under the action of the gas pressure in the micro-nano bubble generator, the homogeneous solution containing a gelling agent and a surfactant undergoes ultra-high-speed rotation of the gas-liquid mixture, and then is ejected through a nozzle. High-speed and strong shearing and high-frequency pressure changes are generated at the gas-liquid contact interface to form a micro-nano gel foam under artificial extreme conditions, so that the gas content in the gel reaches a supersaturated state. Then, under the shearing action of high-speed stirring, the addition of mineral powder and a curing agent can ensure the uniform mixing of the mineral powder while using the rapid curing effect of the curing agent to enable the micro-nano bubbles to be stably and uniformly dispersed in the pulp until curing and forming before aggregation, growth and dissipation, thereby further micro-refining and homogenizing the pore diameter of the porous ceramic, and obtaining a porous ceramic material with fine and uniform pores, and having the characteristics of light weight and high compressive strength.

[0011] Another innovation of the present invention lies in: first forming a pulp gel foam with uniformly dispersed micro-nano foam by using a gelling agent, a surfactant and mineral powder, and then finally introducing a curing agent for stabilization. If the curing agent is introduced in advance, it will affect the gelling effect of the micro-nano foam, making it difficult to form a uniform and stable gel network, resulting in uneven pore wall thickness, being unfavorable for the forming of the porous ceramic body, and greatly reducing the strength of the porous ceramic. After preparing the pulp gel foam first and then introducing the curing agent, the rapid curing effect of the curing agent is used to enable the micro-nano bubbles to be stably and uniformly dispersed in the pulp until curing and forming before aggregation, growth and dissipation, so that a porous ceramic material with fine and uniform pores, and having the characteristics of light weight and high compressive strength can be obtained.

[0012] As a preferred solution, the surfactant includes at least one of sodium dodecyl sulfonate, dodecylamine, cetyltrimethylammonium bromide and plant protein. The plant protein is further preferably saponin or sapindus triterpenoid saponin. The surfactants selected in the present invention are all commercially available industrial pure or analytical pure. They are conventional surfactants in the process of preparing porous ceramic materials from foam pulp. The type of surfactant will affect the gas-liquid-solid distribution characteristics, and the preferred surfactant is beneficial to strengthening the uniform dispersion of micro-nano bubbles and mineral powder.

[0013] As a preferred solution, the gelling agent includes at least one of carboxymethyl cellulose, soluble starch and sodium alginate. The gelling agents selected in the present invention are all commercially available industrial pure or analytical pure.

[0014] As a preferred embodiment, the curing agent includes at least one of polyvinyl alcohol, polyvinyl ether, isobutene-maleic anhydride copolymer, and polyacrylate. The curing agents selected in the present invention are all commercially available industrial pure or analytical pure. By optimizing the types of gelling agents and curing agents, the gas-liquid-solid interfacial tension can be controlled, which is beneficial to controlling the stable and uniform dispersion of micro-nano bubbles in the pulp until the green body is cured before aggregation, growth, and dissipation.

[0015] As a preferred embodiment, the particle size distribution of the mineral powder is 400-1000 mesh, and the Al2O3 content is 10-44 wt%, and the SiO2 content is 15-60 wt%. As a more preferred embodiment, the mineral powder includes at least one of coal gangue, coal slime, red mud, lithium slag, kaolin, mullite, feldspar, quartz, copper tailings, iron tailings, and lead-zinc tailings.

[0016] As a preferred embodiment, the mass ratio of the gelling agent and the surfactant to the water is (0.005-0.03):(0.002-0.015):1. The dosages of the gelling agent and the surfactant affect the dispersion effect of bubbles and foams. If the proportion of the gelling agent is too high and / or the content of the surfactant is too low, the foaming effect is affected, resulting in a low porosity of the ceramic; if the content of the surfactant is too high, large bubbles are easily formed and aggregated, and micro-nano bubbles and uniformly dispersed micro-nano pores cannot be formed, and the strength of the porous ceramic is low. If the proportion of the gelling agent is too low, the micro-nano pores are difficult to maintain stably. The gelling agent and the surfactant in the present invention are dissolved in an appropriate amount of water in advance before being added.

[0017] As a preferred embodiment, the mass ratio of the mineral powder to the micro-nano gel foam is (0.6-1.2):1. The mass of the mineral powder mainly affects the density and porosity of the ceramic. If the introduction ratio of the mineral powder is too high, the formed porous ceramic has a lower porosity, a larger density, and the pores are not uniform and rich enough, and the properties such as heat preservation and sound insulation are poor; if the ratio is too low, the structure is prone to collapse during the curing process and is not easy to form, and the solid-phase connection of the pore walls of the fired porous ceramic is weak and the strength is low.

[0018] As a preferred embodiment, the mass ratio of the curing agent to the slurry gel foam is (0.06-0.30):1. If the proportion of the curing agent is too high, the micro-nano pore structure will be damaged and the cost will be greatly increased; if the proportion is too low, the fluidity and stability of the slurry during injection molding will become poor, the slurry is prone to agglomeration or precipitation, and defects are easily formed after the green body is demolded. The curing agent is measured by its dry basis mass.

[0019] As a preferred embodiment, the curing conditions are: curing at a temperature of 30-80 °C for 8-24 h.

[0020] As a preferred embodiment, the drying conditions are as follows: drying for 1 - 3 h at a temperature of 100 - 120 °C.

[0021] As a preferred embodiment, the sintering conditions are as follows: sintering for 0.5 - 2 h at a temperature of 1250 - 1400 °C.

[0022] The gelling agent involved in the present invention is added in the form of a solution. The gelling agent and deionized water are mixed at a mass ratio of (0.005 - 0.03):1, and magnetically stirred in a water bath to prepare a homogeneous solution.

[0023] The surfactant involved in the present invention is added in the form of a solution. The surfactant and deionized water are mixed at a mass ratio of (0.001 - 0.02):1, and magnetically stirred in a water bath to prepare a homogeneous solution.

[0024] The curing agent involved in the present invention is added in the form of a solution. The curing agent and deionized water are mixed at a mass ratio of (0.0125 - 0.04):1, and magnetically stirred in a water bath to prepare a homogeneous solution.

[0025] The temperature of the magnetic stirring in the water bath involved in the present invention is 30 - 45 °C, and the stirring speed is 200 - 500 r / min.

[0026] Compared with the prior art, the technical solution of the present invention has the following positive effects:

[0027] (1) By using a micro-nano bubble generator to generate micro-nano gel foam, the present invention enhances the uniform dispersion of micro-bubbles in high-viscosity pulp, further micro-refines and homogenizes the pore size of the formed porous ceramics. Using a micro-nano bubble generator to prepare micro-nano porous ceramics breaks through the limitation that the pore reduction caused by the use of a high proportion of surfactants and high solid content in the pulp foaming process, and reduces the use of surfactants.

[0028] (2) Using micro-nano gel foam as the base liquid, the present invention is applicable to the selection of various mineral powders and has good universality.

[0029] (3) The porous ceramics prepared by the present invention have fine and uniform pores, and have the characteristics of lightweight and high compressive strength. The pore size distribution is 0.5 - 23 μm, the density is 0.41 - 0.67 g / cm 3 , and the compressive strength is 16.3 - 19.1 MPa.

[0030] (4) The method for preparing porous ceramics in the present invention has a short process flow, is easy to operate, and meets industrial production requirements.

[0031] In summary, the process for preparing porous ceramic materials provided by the present invention can adjust the pore structure by changing the foam generation method. It has the characteristics of a short process and easy operation, and can obtain porous ceramic materials with fine and uniform pores, as well as characteristics such as light weight and high compressive strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 SEM image of the micro-nano porous ceramic prepared in Example 1.

[0033] Figure 2 Comparison diagram of the micro-nano gel foam and water foam prepared in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] The following further describes the content of the present invention in detail with specific examples, and does not limit the protection scope of the claims.

[0035] In the following detailed embodiments:

[0036] The gelling agent, surfactant, and curing agent are of industrial purity or analytical purity.

[0037] Example 1

[0038] Step 1: Add the gelling agent to deionized water according to the mass ratio of carboxymethyl cellulose: deionized water of 0.015:1, and stir magnetically in a water bath to prepare a dispersed homogeneous solution for use.

[0039] Step 2: Add the surfactant to deionized water according to the mass ratio of cetyltrimethylammonium bromide: deionized water of 0.001:1, and stir magnetically in a water bath to prepare a dispersed homogeneous solution for use.

[0040] Step 3: Add the curing agent to deionized water according to the mass ratio of polyvinyl alcohol: deionized water of 0.0125:1, and stir magnetically in a water bath to prepare a dispersed homogeneous solution for use.

[0041] Step 4: Mix the carboxymethyl cellulose solution, cetyltrimethylammonium bromide solution, and an appropriate amount of deionized water according to the mass ratio of carboxymethyl cellulose: cetyltrimethylammonium bromide: deionized water of 0.008:0.002:1 (total amount of water). Pass the aqueous solution containing the gelling agent and surfactant through a micro-nano bubble generator. The circulating flow rate is 10 L / min, the circulating flow time is 5 min, and the inflation gas flow rate is 100 mL / min. After spraying, a micro-nano gel foam is obtained.

[0042] Step 5: With the mass ratio of coal gangue powder to micro-nano gel foam being 0.6:1, use a high-speed stirrer for secondary stirring. Stir at 1400 r / min for 10 min. The coal gangue powder has an Al2O3 content of 39 wt%, an SiO2 content of 50 wt%, other components of 11 wt%, and an average particle size of 600 mesh;

[0043] Step 6: With the mass ratio of polyvinyl alcohol (by dry basis mass) to pulp gel foam being 0.06:1, add polyvinyl alcohol to the foam pulp and perform high-speed shear stirring at 1200 r / min for 15 min to obtain stabilized pulp gel foam;

[0044] Step 7: Inject the stabilized pulp gel foam slurry into a mold, cure at 60 °C for 12 h, and demold to obtain a porous material blank;

[0045] Step 8: Dry the porous material blank at 100 °C for 2 h and sinter at 1400 °C for 0.5 h to obtain a porous ceramic micro-nano material;

[0046] The pore size distribution of the sintered sample is in the range of 0.9 - 25 μm, and the density is 0.48 g / cm 3 , and the compressive strength is 19.1 MPa.

[0047] Example 2

[0048] Step 1: With the mass ratio of carboxymethyl cellulose to deionized water being 0.02:1, add the gelling agent to deionized water and perform magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0049] Step 2: With the mass ratio of dodecylamine to deionized water being 0.0015:1, add the surfactant to deionized water and perform magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0050] Step 3: With the mass ratio of polyacrylate to deionized water being 0.02:1, add the curing agent to deionized water and perform magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0051] Step 4: Mix the carboxymethyl cellulose solution, dodecylamine solution, and appropriate amount of deionized water with the mass ratio of carboxymethyl cellulose:dodecylamine:deionized water being 0.008:0.002:1 (total water amount). Pass the aqueous solution containing the gelling agent and surfactant through a micro-nano bubble generator. The circulating flow rate is 15 L / min, the circulating flow time is 5 min, the inflation gas flow rate is 200 mL / min, and micro-nano gel foam is obtained after spraying;

[0052] Step 5: Carry out secondary stirring using a high-speed stirrer at a mass ratio of lithium slag powder to micro-nano gel foam of 0.6:1, and stir for 10 min under the condition of 1400 r / min; the Al2O3 content of the lithium slag powder is 21 wt%, the SiO2 content is 57 wt%, the content of other components is 22 wt%, and the average particle size is 700 mesh;

[0053] Step 6: At a mass ratio of polyacrylate (by dry basis mass) to pulp gel foam of 0.08:1, add polyacrylate to the pulp gel foam, and carry out high-speed shear stirring at 1200 r / min for 15 min to obtain stabilized pulp gel foam;

[0054] Step 7: Inject the stabilized pulp gel foam into a mold, cure at 60 °C for 12 h, and demold to obtain a porous material blank;

[0055] Step 8: Dry the porous material blank at 100 °C for 2 h and sinter at 1250 °C for 0.5 h to obtain a porous ceramic micro-nano material;

[0056] The pore size distribution of the sintered sample is in the range of 1 - 23 μm, the density is 0.41 g / cm 3 , and the compressive strength is 16.3 MPa.

[0057] Example 3

[0058] Step 1: At a mass ratio of carboxymethyl cellulose to deionized water of 0.03:1, add the gelling agent to deionized water, and carry out magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0059] Step 2: At a mass ratio of tea saponin to deionized water of 0.02:1, add the surfactant to deionized water, and carry out magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0060] Step 3: At a mass ratio of polyvinyl alcohol to deionized water of 0.04:1, add the curing agent to deionized water, and carry out magnetic stirring in a water bath to prepare a dispersed homogeneous solution for later use;

[0061] Step 4: For the carboxymethyl cellulose solution, the tea saponin solution and an appropriate amount of deionized water at a mass ratio of carboxymethyl cellulose:tea saponin:deionized water of 0.008:0.002:1 (total amount of water), introduce the aqueous solution containing the gelling agent and the surfactant into a micro-nano bubble generator, with a circulating flow rate of 8 L / min, a circulating flow time of 10 min, and an inflation gas flow rate of 250 mL / min. After spraying, micro-nano gel foam is obtained;

[0062] Step 5: Carry out secondary stirring using a high-speed stirrer at a mass ratio of slime powder to micro-nano gel foam of 0.6:1, stir for 30 min under the condition of 1400 r / min. The slime powder contains 25 wt% of Al2O3, 47 wt% of SiO2, 28 wt% of other components, and the average particle size is 1000 mesh;

[0063] Step 6: Add polyvinyl alcohol to the pulp gel foam at a mass ratio of polyvinyl alcohol to pulp gel foam of 0.08:1, and carry out high-speed shear stirring at 1200 r / min for 15 min to obtain stabilized pulp gel foam;

[0064] Step 7: Inject the stabilized pulp gel foam into a mold, cure at 60 °C for 12 h, and demold to obtain a porous material blank;

[0065] Step 8: Dry the porous material blank at 100 °C for 2 h and sinter at 1400 °C for 0.5 h to obtain a porous ceramic micro-nano material;

[0066] The pore size distribution of the sintered sample is in the range of 0.5 - 17 μm, and the density is 0.67 g / cm 3 , and the compressive strength is 17.7 MPa.

[0067] Comparative Example 1

[0068] Other operating conditions and implementation steps are the same as those in Example 1, only a common stirrer is used to replace the micro-nano bubble generator in Step 4. Stir at a high speed of 1400 r / min for 10 min to obtain gel foam. The pore size distribution of the burned sample is in the range of 93 - 463 μm, and the density is 0.78 g / cm 3 , and the compressive strength is 5.47 MPa. The bubbles formed by using a common stirrer have a large diameter and a small density, resulting in a relatively large pore size and uneven distribution of the fired porous ceramic, and a lower compressive strength.

[0069] Comparative Example 2

[0070] Other operating conditions and implementation steps are the same as those in Example 2, only the amount of gelling agent added is reduced in Step 4, and the mass ratio of carboxymethyl cellulose: dodecylamine: deionized water is 0.002:0.002:1. The pore size distribution of the burned sample is in the range of 70 - 325 μm, and the density is 0.70 g / cm 3 , and the compressive strength is 7.31 MPa. Due to the too low ratio of the gelling agent, it is difficult to maintain the stability of the micro-nano foam during the curing process, and part of the micro-nano pore structure collapses, resulting in uneven pore distribution.

[0071] Comparative Example 3

[0072] Other operating conditions and implementation steps are the same as those in Example 3, except that the curing agent addition step and addition method are changed. The gelling agent, surfactant, and curing agent are simultaneously added to water to form a mixed homogeneous solution. The pore size distribution of the fired sample ranges from 125 to 645 μm, and the density is 0.59 g / cm 3 , and the compressive strength is 3.86 MPa. The simultaneous mixing of the three agents affects the gelling effect of the micro-nano foam slurry, and it is impossible to form a uniform and stable gel network. The pore wall thickness is uneven, which is not conducive to the forming of the porous ceramic green body, and the strength is greatly reduced.

Claims

1. A method for preparing a porous ceramic with uniform micro-nano pore size, characterized in that: The following steps are involved: 1) stirring and mixing the gelling agent and the surfactant with water to form a homogeneous solution; 2) passing the homogeneous solution into a micro-nano bubble generator for circulation and aeration, and then ejecting to form micro-nano gel foam; the circulation rate is 2-20 L / min; the aeration conditions are: gas flow rate is 100-250 mL / min, and the time is 5-10 min; 3) adding mineral powder to the micro-nano gel foam and stirring at a high speed of 1000-1400 r / min for 10-30 min to obtain a slurry gel foam, and then adding a curing agent to the slurry gel foam and stirring at a high speed of 1000-1400 r / min for 15-30 min to obtain a stabilized slurry gel foam; 4) The stabilized slurry gel foam is sequentially subjected to injection molding, curing, demoulding, drying and sintering to obtain a porous ceramic material.

2. The method for preparing a porous ceramic having uniform micro-nano pore size according to claim 1, characterized in that: The gelling agent comprises at least one of carboxymethyl cellulose, soluble starch and sodium alginate; and / or, The surfactant comprises at least one of sodium dodecyl sulfonate, dodecylamine, hexadecyl methylammonium bromide and plant protein; and / or, The curing agent includes at least one of polyvinyl alcohol, polyvinyl ether, isobutylene-maleic anhydride copolymer and polyacrylate; and / or, The particle size of the mineral powder is distributed in the range of 400-1000 meshes, and the Al2O3 content is 10-44wt%, and the SiO2 content is 15-60wt%.

3. The method for preparing a porous ceramic having uniform micro-nano pore size according to claim 1 or 2, characterized in that: The mass ratio of the gelling agent and the surfactant to the water is (0.005-0.03): (0.002-0.015):

1.

4. The method for preparing a porous ceramic having uniform micro-nano pore size according to claim 1 or 2, characterized in that: The mass ratio of the mineral powder to the micro-nano gel foam is (0.6-1.2):

1.

5. The method for preparing a porous ceramic having uniform micro-nano pore size according to claim 1 or 2, characterized in that: The mass ratio of the curing agent to the slurry gel foam is (0.06-0.30):

1.

6. The method for preparing a porous ceramic having uniform micro-nano pore sizes according to claim 2, characterized in that: The mineral powder includes at least one of coal gangue, coal slime, red mud, lithium slag, kaolin, mullite, feldspar, quartz, copper tailings, iron tailings and lead-zinc tailings.

7. The method for preparing a porous ceramic having uniform micro-nano pore sizes according to claim 1, characterized in that: The curing conditions are: curing at a temperature of 30-80° C. for 8-24 hours.

8. The method for preparing a porous ceramic with uniform micro-nano pore size according to claim 1, characterized in that: The drying conditions are: drying at a temperature of 100-120° C. for 1-3 hours.

9. The method for preparing a porous ceramic with uniform micro-nano pore size according to claim 1, characterized in that: The sintering conditions are: sintering at a temperature of 1250-1400° C. for 0.5-2 hours.

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