Method for resource utilization of converter slag and construction waste

Abstract

A converter slag and construction waste resource processing method, comprising: a gradient crushing unit, which comprises a rod mill with a variable-diameter rod assembly, the diameter of the variable-diameter rod assembly gradually changes from Φ80mm to Φ50mm along the material running direction, and the inner wall of the rod mill cylinder is provided with a guide groove with a spiral angle of 25°-35°; a temperature control module integrated in the double-layer water-cooled interlayer structure of the rod mill, the cooling medium flow rate in the interlayer is 2.5±0.2m³ / h, and the crushing temperature is controlled at 55-60℃; a multi-stage magnetic separation unit, which comprises a first-stage magnetic separation device and a second-stage magnetic separation device, wherein the magnetic field strength of the first-stage magnetic separation device is 1.0-1.2T in the front section and 0.3-0.5T in the rear section, and the second-stage magnetic separation device adopts a high-frequency pulse magnetic field with a frequency of 75±5Hz; and a waste cooperative treatment unit, which is provided with a micro-powder-waste feeder with a dynamic mixing ratio and an alkali activator atomizing spraying device, and the mixing ratio adjustment precision is ±2%.

Description

Technical Field

[0001] This invention relates to the field of solid waste resource utilization technology, specifically a method for the resource utilization treatment of converter slag and construction waste. Background Technology

[0002] In today's society, with the rapid advancement of industrialization and urbanization, the amount of converter slag and construction waste generated is increasing day by day, and their treatment has become a key problem that urgently needs to be solved in the fields of environment and resource utilization.

[0003] Converter slag is an inevitable byproduct of the iron and steel smelting process. According to statistics, approximately 100-150 kilograms of converter slag are produced for every ton of steel produced. This converter slag is rich in valuable elements such as iron, calcium, and magnesium, and also possesses certain potential reactivity. However, for a long time, large quantities of converter slag have simply been stockpiled or crudely landfilled. This not only occupies a large amount of valuable land resources, causing waste, but may also lead to the leakage of heavy metals from the slag, polluting the surrounding soil and groundwater, posing a serious threat to the ecological environment. More importantly, the abundant resources contained in converter slag are being wasted and not effectively utilized, violating the concept of sustainable development.

[0004] At the same time, the amount of construction waste generated is staggering. With the continuous development of urban construction, old city renovation, and infrastructure projects, construction waste is constantly being generated. It is estimated that demolishing 10,000 square meters of old buildings generates approximately 7,000-12,000 tons of construction waste. This construction waste is complex in composition, mainly including concrete blocks, brick and tile fragments, waste wood, and metals. Currently, most construction waste is also disposed of through landfills, which not only occupies a large amount of land but also damages the ecological environment around landfill sites. Furthermore, traditional construction waste disposal methods neglect recyclable resources. For example, waste concrete blocks and brick and tile fragments can be processed and used as recycled aggregates to produce recycled building materials; waste metals can be recycled and smelted, achieving resource recycling. This waste of resources stands in stark contrast to the current global resource shortage.

[0005] Traditional solid waste treatment technologies have several shortcomings in the mixed treatment of converter slag and construction waste. In the crushing stage, traditional crushing processes struggle to accommodate the differences in characteristics between hard metal slag and brittle construction waste. For hard converter slag, insufficient crushing may occur, resulting in coarse particles remaining; while for brittle construction waste, over-crushing is prone to producing excessive fine powder, affecting subsequent processing efficiency and product quality. In the magnetic separation stage, conventional magnetic separation equipment is ineffective at separating iron from converter slag and construction waste, resulting in low iron recovery rates and iron concentrate grades that are difficult to meet the standards for direct reuse as steelmaking raw materials, leading to resource waste. Regarding the preparation of building materials, when using these solid wastes to produce permeable bricks using traditional methods, the weak interfacial bonding between aggregates and cementitious materials results in an inability to balance strength and permeability, failing to meet the needs of practical engineering applications. Summary of the Invention

[0006] In view of the above situation and to overcome the defects of the prior art, the present invention provides a method for the resource utilization of converter slag and construction waste, which effectively solves the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The present invention includes: The gradient crushing unit includes a rod mill with a variable diameter rod assembly, the diameter of which gradually changes from Φ80mm to Φ50mm along the material travel direction, and the inner wall of the rod mill cylinder is provided with a guide groove with a helix angle of 25°-35°. The temperature control module is integrated into the double-layer water-cooled jacket structure of the rod mill. The flow rate of the cooling medium in the jacket is 2.5±0.2m³ / h, and the crushing temperature is controlled at 55-60℃. The multi-stage magnetic separation unit includes a primary magnetic separator and a secondary magnetic separator. The magnetic field strength of the primary magnetic separator is 1.0-1.2T at the front end and 0.3-0.5T at the rear end. The secondary magnetic separator uses a high-frequency pulsed magnetic field with a frequency of 75±5Hz. The waste co-processing unit is equipped with a micro powder-waste feeder with dynamic mixing ratio and an alkali activator atomizing spray device, with a mixing ratio adjustment accuracy of ±2%.

[0008] According to the above scheme: the length of the gradual section of the variable diameter rod assembly accounts for 40%-50% of the total length of the rod mill, the spacing between adjacent rods decreases by 10%-15% along the material direction, and the surface of the rod body is provided with a spiral groove with a depth of 0.5-1mm.

[0009] According to the above scheme: the high-frequency pulsed magnetic field waveform of the secondary magnetic separator is a composite waveform of square wave and sawtooth wave, the duty cycle adjustment range is 20%-80%, and the magnetic field gradient is 0.5-1.0T / cm.

[0010] According to the above scheme: the gradient crushing in step (a) adopts a two-stage crushing mode, with the coarse crushing stage crushing to 10-30mm and the fine crushing stage crushing to <5mm with a micro powder generation rate ≤8%.

[0011] A method for the resource-based treatment of converter slag and construction waste, characterized by the following steps: (a) Mix converter slag and construction waste at a mass ratio of 1:1 ± 0.1, and control the moisture content of the mixture to ≤5%; (b) Gradient crushing stage: Two-stage crushing is carried out in the rod mill. The coarse crushing stage outputs a particle size of 15-25mm, and the fine crushing stage is crushed to a particle size D90≤3mm through the compression zone of the variable diameter rod, and the proportion of micro powder with a particle size <0.15mm is ≤6%; (c) Primary magnetic separation stage: The crushed material is subjected to strong magnetic separation with a magnetic field strength of 1.0T to separate iron concentrate; the remaining material is subjected to weak magnetic separation with a magnetic field strength of 0.4T to extract iron-containing tailings. The magnetic separation airflow velocity is 8-10m / s. (d) Screening and grading stage: Non-magnetic materials are graded into graded crushed stone >10mm, manufactured sand 3-10mm and activated micro powder <3mm by double-layer vibrating screen. The stone powder content of the manufactured sand is controlled to ≤5% by negative pressure adsorption. (e) Secondary magnetic separation enhancement stage: Iron-containing tailings are fed into a ball mill equipped with high-chromium cast iron-rubber composite liners. The ball mill media filling rate is 30%±2%. After grinding, the iron concentrate is separated by a 75Hz high-frequency pulse magnetic field to obtain iron concentrate with TFe≥60%. (f) Waste co-preparation stage: The ball mill waste and active micro powder are mixed at a mass ratio of 3:7, and 6%±0.5% alkali activator is sprayed simultaneously by atomization. The mixture is then compacted by high frequency to form a permeable brick body. After curing at 50±2℃ for 24 hours, the temperature is increased to 80℃ to accelerate curing for 12 hours.

[0012] According to the above technical solution: the alkaline activator in step (f) is composed of the following components: Nano-kaolin modified slag powder: 45%-50%; Sulfoaluminate cement clinker: 25%-30%; Lithium-based bentonite: 10%-15%; Polycarboxylate superplasticizer: 1%-2%; The remainder is silica fume, and the particle size of each component is D50≤10μm.

[0013] According to the above technical solution: the crushing energy efficiency ratio (kWh / t) of the fine fragments in step (b) satisfies the following relationship: Wherein, D80 is the 80% sieve particle size (mm) of the coarse crushed product, d80 is the 80% sieve particle size (mm) of the fine crushed product, R is the micro powder generation rate (%), and the E value is controlled within the range of 18-22 kWh / t.

[0014] According to the above technical solution: the magnetic field strength and frequency of the high-frequency pulsed magnetic field in step (e) satisfy the following: Where H0=1.2T, f=75Hz, and the magnetic field duration t is matched with the material flow rate to be 0.1-0.3s.

[0015] According to the above technical solution: the permeable bricks prepared in step (e) have a compressive strength ≥30MPa and a permeability coefficient ≥1.5×10⁻⁶. -2 cm / s, and the curing temperature is maintained at 50±5℃ for 48 hours.

[0016] Beneficial effects: 1. Synergistic effect of gradient crushing and multi-stage magnetic separation By coupling the variable diameter bar assembly (Φ80mm→Φ50mm gradient) with the helical angle of the guide channel (30°±5°), differentiated crushing of converter slag and construction waste is achieved, solving the problem of over-crushing (fine powder rate >10%) and under-crushing (coarse particle residue >15%) that exists in the traditional crushing process when hard metal slag and brittle construction waste are mixed. Improved crushing energy efficiency: Data from the examples show that crushing energy consumption is reduced to 18-20 kWh / t (compared to 28 kWh / t for traditional processes), while the micro powder generation rate is precisely controlled at 4.8%-5.2%, significantly reducing the metal loss rate in subsequent magnetic separation processes.

[0017] Metal enrichment enhancement: The synergistic effect of primary magnetic separation (1.1T strong magnetic separation + 0.4T weak magnetic separation) and secondary high-frequency pulse magnetic separation (composite waveform + 0.8T / cm gradient magnetic field) increases the iron recovery rate to 95%-97% (82% for traditional process), and the iron concentrate grade (TFe≥60%) meets the standard for direct reuse of steelmaking raw materials.

[0018] 2. Interface strengthening effect of waste co-processing Based on the optimization of the alkali activator formula (nano-kaolin modification + lithium-based bentonite synergistic activation) and high-frequency vibration compaction process (50Hz / 2mm), the contradiction between strength and permeability caused by the weak interfacial bonding between aggregate and cementitious materials in traditional solid waste-based permeable bricks is overcome: Breakthrough in mechanical properties: The compressive strength of the permeable bricks reaches 32-35MPa (25MPa for traditional processes), and the permeability coefficient is stable at 1.6×10^-2cm / s, achieving the high-strength permeable brick (Class II) index in JC / T945-2005 standard.

[0019] Maximizing solid waste content: Using ball mill waste (70%) and active micro powder (30%) as raw materials, 100% of natural aggregates and cement are replaced, and the comprehensive solid waste utilization rate reaches 98% (traditional process ≤75%).

[0020] 3. Closed-loop process and environmental benefits By integrating crushing temperature control (below 58℃) ​​with negative pressure adsorption (stone powder content ≤4.5%), a clean production system is constructed throughout the entire process. Dust / heat control: The double-layer water-cooled structure of the rod mill controls the crushing temperature below 58℃ (traditional process >80℃). Combined with the negative pressure adsorption module of the vibrating screen, the dust concentration in the working area is <5mg / m³ (national standard ≤10mg / m³).

[0021] Zero wastewater discharge: 100% recycling rate of cooling medium, alkali activator atomization spraying process reduces the amount of liquid additives by 60%, and no process wastewater is generated.

[0022] 4. Significant economic benefits Raw material cost savings: Direct recycling of iron concentrate reduces steelmaking raw material procurement costs by 35%, and permeable brick production costs are 40% lower than commercially available products.

[0023] Extended equipment life: High-chromium cast iron-rubber composite liners (ball mills) extend the replacement cycle of wear parts to 6,000 hours (traditional liners ≤ 4,000 hours). Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is the overall flowchart of the present invention; Figure 2 This is a schematic diagram of the multi-stage magnetic separation principle of the present invention. Detailed Implementation

[0025] The following is in conjunction with the appendix Figure 1-2 The specific embodiments of the present invention will be described in further detail below.

[0026] Example 1, by Figure 1-2 This invention provides a method for the resource utilization of converter slag and construction waste, step 1: pretreatment of the mixed materials. The converter slag and construction waste (mainly concrete blocks and brick fragments) are mixed at a mass ratio of 1:1. The moisture content of the mixture is controlled at 4.8% and the initial particle size is ≤100mm by using a drum dryer.

[0027] Step 2: Gradient Breaking Two-stage crushing is performed using a rod mill with variable diameter rod assemblies: Coarse crushing section: The bar diameter of the front section of the bar mill is Φ80mm, the bar spacing is 50mm, the spiral angle of the guide channel is 30°, and the material after crushing is D80=20mm (that is, 80% of the material passes through a 20mm screen). The energy consumption of the coarse crushing section is 15kWh / t.

[0028] Fine crushing section: The diameter of the rods in the rear section of the rod mill gradually changes to Φ50mm, the spacing between the rods decreases to 43mm, the length of the compression zone accounts for 45%, the fine material after crushing has a d80 of 2.8mm, the proportion of micro powder (<0.15mm) is 5.2%, and the crushing energy efficiency ratio E = 20kWh / t (satisfying the formula: E = 15×(D80 / d80)^(0.5) + 0.3R, where R = 5.2).

[0029] Step 3: Primary Magnetic Separation High-intensity magnetic separation: The crushed material is fed into the front section of the primary magnetic separator with a magnetic field strength of 1.1T and an airflow velocity of 9m / s to separate iron concentrate (TFe=92%).

[0030] Weak magnetic separation: The remaining material enters the later stage with a magnetic field strength of 0.4T to recover iron-containing tailings (TFe=35%), while non-magnetic materials are transferred to screening and grading.

[0031] Step 4: Screening and Grading Use a double-layer vibrating screen (10mm screen aperture in the upper layer, 3mm screen aperture in the lower layer): Graded crushed stone (>10mm): accounting for 28%, used for roadbed filling.

[0032] Manufactured sand (3-10mm): accounting for 52%, with stone powder content controlled at 4.5% through negative pressure adsorption, conforming to GB / T14684 standard.

[0033] Active micro powder (<3mm): accounting for 20%, used in the preparation of permeable bricks.

[0034] Step 5: Secondary magnetic separation enhancement Iron-containing tailings are fed into a ball mill (high-chromium cast iron-rubber composite liner, 31% filling rate) and ground to D50=0.1mm. Then, they are separated by a high-frequency pulsed magnetic field (frequency 75Hz, square wave and sawtooth wave composite waveform, duty cycle 50%, magnetic field gradient 0.8T / cm) to obtain iron concentrate (TFe=60%), and the recovery rate is increased to 95%.

[0035] Step 6: Co-preparation of permeable bricks using waste materials The ball mill waste and activated micro powder were mixed at a mass ratio of 3:7, and an alkaline activator (6% addition) was simultaneously atomized and sprayed: Alkali activator formulation: nano-kaolin modified slag powder (48%), sulfoaluminate cement clinker (28%), lithium-based bentonite (12%), polycarboxylate superplasticizer (1.5%), silica fume (10.5%), with each component having a D50 ≤ 8μm.

[0036] The mixture is compacted by high-frequency vibration (50Hz, 2mm amplitude) and pressed into permeable brick blanks. Curing conditions: Phase 1: Curing at a constant temperature of 50℃ for 24 hours.

[0037] Second stage: Increase the temperature to 80℃ to accelerate curing for 12 hours.

[0038] Finished product performance: compressive strength 32MPa, permeability coefficient 1.6×10^-2cm / s, meeting JC / T945-2005 standard.

[0039] Example 2 Step adjustment and parameter optimization Gradient crushing: The proportion of gradually changing length of the fine crushed segment variable diameter bar is increased to 50%, the micro powder generation rate is reduced to 4.8%, and the crushing energy efficiency ratio E=18kWh / t.

[0040] Secondary magnetic separation: Adjust the duty cycle of the high-frequency pulsed magnetic field to 80% and the magnetic field action time to 0.2s, and increase the TFe content of the iron concentrate to 70%.

[0041] Permeable brick curing: Extend the first stage of curing to 30 hours, and increase the compressive strength to 35MPa.

[0042] Comparison of experimental data: Beneficial effects: 1. Synergistic effect of gradient crushing and multi-stage magnetic separation By coupling the variable diameter bar assembly (Φ80mm→Φ50mm gradient) with the helical angle of the guide channel (30°±5°), differentiated crushing of converter slag and construction waste is achieved, solving the problem of over-crushing (fine powder rate >10%) and under-crushing (coarse particle residue >15%) that exists in the traditional crushing process when hard metal slag and brittle construction waste are mixed. Improved crushing energy efficiency: Data from the examples show that crushing energy consumption is reduced to 18-20 kWh / t (compared to 28 kWh / t for traditional processes), while the micro powder generation rate is precisely controlled at 4.8%-5.2%, significantly reducing the metal loss rate in subsequent magnetic separation processes.

[0043] Metal enrichment enhancement: The synergistic effect of primary magnetic separation (1.1T strong magnetic separation + 0.4T weak magnetic separation) and secondary high-frequency pulse magnetic separation (composite waveform + 0.8T / cm gradient magnetic field) increases the iron recovery rate to 95%-97% (82% for traditional process), and the iron concentrate grade (TFe≥60%) meets the standard for direct reuse of steelmaking raw materials.

[0044] 2. Interface strengthening effect of waste co-processing Based on the optimization of the alkali activator formula (nano-kaolin modification + lithium-based bentonite synergistic activation) and high-frequency vibration compaction process (50Hz / 2mm), the contradiction between strength and permeability caused by the weak interfacial bonding between aggregate and cementitious materials in traditional solid waste-based permeable bricks is overcome: Breakthrough in mechanical properties: The compressive strength of the permeable bricks reaches 32-35MPa (25MPa for traditional processes), and the permeability coefficient is stable at 1.6×10^-2cm / s, achieving the high-strength permeable brick (Class II) index in JC / T945-2005 standard.

[0045] Maximizing solid waste content: Using ball mill waste (70%) and active micro powder (30%) as raw materials, 100% of natural aggregates and cement are replaced, and the comprehensive solid waste utilization rate reaches 98% (traditional process ≤75%).

[0046] 3. Closed-loop process and environmental benefits By integrating crushing temperature control (below 58℃) ​​with negative pressure adsorption (stone powder content ≤4.5%), a clean production system is constructed throughout the entire process. Dust / heat control: The double-layer water-cooled structure of the rod mill controls the crushing temperature below 58℃ (traditional process >80℃). Combined with the negative pressure adsorption module of the vibrating screen, the dust concentration in the working area is <5mg / m³ (national standard ≤10mg / m³).

[0047] Zero wastewater discharge: 100% recycling rate of cooling medium, alkali activator atomization spraying process reduces the amount of liquid additives by 60%, and no process wastewater is generated.

[0048] 4. Significant economic benefits Raw material cost savings: Direct recycling of iron concentrate reduces steelmaking raw material procurement costs by 35%, and permeable brick production costs are 40% lower than commercially available products.

[0049] Extended equipment life: High-chromium cast iron-rubber composite liners (ball mills) extend the replacement cycle of wear parts to 6,000 hours (traditional liners ≤ 4,000 hours).

[0050] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for the resource-based treatment of converter slag and construction waste, characterized in that, include: A gradient crushing unit includes a rod mill with a variable diameter rod assembly. The diameter of the variable diameter rod assembly gradually changes from Φ80mm to Φ50mm along the material travel direction. The inner wall of the rod mill cylinder is provided with a guide groove with a helical angle of 25°-35°. The length of the gradual section of the variable diameter rod assembly accounts for 40%-50% of the total length of the rod mill. The spacing between adjacent rods decreases by 10%-15% along the material direction. The surface of the rod body is provided with a helical groove with a depth of 0.5-1mm. The temperature control module is integrated into the double-layer water-cooled jacket structure of the rod mill. The flow rate of the cooling medium in the jacket is 2.5±0.2m³ / h, and the crushing temperature is controlled at 55-60℃. The multi-stage magnetic separation unit includes a primary magnetic separator and a secondary magnetic separator. The primary magnetic separator has a front-end magnetic field strength of 1.0-1.2T and a rear-end magnetic field strength of 0.3-0.5T. The secondary magnetic separator uses a high-frequency pulsed magnetic field with a frequency of 75±5Hz. The waveform of the high-frequency pulsed magnetic field of the secondary magnetic separator is a composite waveform of square wave and sawtooth wave, with a duty cycle adjustment range of 20%-80% and a magnetic field gradient of 0.5-1.0T / cm. The waste co-processing unit is equipped with a dynamic mixing ratio micro-powder-waste feeder and an alkali activator atomizing spray device, with a mixing ratio adjustment accuracy of ±2%. The method includes the following steps: (a) Mix converter slag and construction waste at a mass ratio of 1:1 ± 0.1, and control the moisture content of the mixture to ≤5%; (b) Gradient crushing stage: Two-stage crushing is carried out in the rod mill. The coarse crushing stage outputs a particle size of 15-25mm, and the fine crushing stage is crushed to a particle size D90≤3mm through the compression zone of the variable diameter rod, and the proportion of micro powder with a particle size <0.15mm is ≤6%; (c) Primary magnetic separation stage: The crushed material is subjected to strong magnetic separation with a magnetic field strength of 1.0T to separate iron concentrate; the remaining material is subjected to weak magnetic separation with a magnetic field strength of 0.4T to extract iron-containing tailings. The magnetic separation airflow velocity is 8-10m / s. (d) Screening and grading stage: Non-magnetic materials are graded into graded crushed stone >10mm, manufactured sand 3-10mm and activated micro powder <3mm by double-layer vibrating screen. The stone powder content of the manufactured sand is controlled to ≤5% by negative pressure adsorption. (e) Secondary magnetic separation enhancement stage: Iron-containing tailings are fed into a ball mill equipped with high-chromium cast iron-rubber composite liners. The ball mill media filling rate is 30%±2%. After grinding, the iron concentrate is separated by a 75Hz high-frequency pulse magnetic field to obtain iron concentrate with TFe≥60%. (f) Waste co-preparation stage: The ball mill waste and active micro powder are mixed at a mass ratio of 3:7, and 6%±0.5% of alkali activator is sprayed simultaneously by atomization. The alkali activator includes nano-kaolin modified slag micro powder, sulfoaluminate cement clinker, lithium-based bentonite, polycarboxylate superplasticizer and silica fume. The mixture is then compacted by high frequency to form a permeable brick body. After curing at 50±2℃ for 24 hours, the temperature is increased to 80℃ to accelerate curing for 12 hours.

2. The method for resource-based treatment of converter slag and construction waste according to claim 1, characterized in that: The gradient crushing in step (a) adopts a two-stage crushing mode, with the coarse crushing stage crushing to 10-30mm and the fine crushing stage crushing to <5mm with a micro powder generation rate ≤8%.

3. The method for resource-based treatment of converter slag and construction waste according to claim 1, characterized in that: The alkaline activator in step (f) consists of the following components: Nano-kaolin modified slag powder: 45%-50%; Sulfoaluminate cement clinker: 25%-30%; Lithium-based bentonite: 10%-15%; Polycarboxylate superplasticizer: 1%-2%; The remainder is silica fume, and the particle size of each component is D50≤10μm.

4. The method for resource-based treatment of converter slag and construction waste according to claim 1, characterized in that: The crushing energy efficiency ratio (kWh / t) of the fine fragments in step (b) satisfies the following relationship: Wherein, D80 is the 80% sieve particle size (mm) of the coarse crushed product, d80 is the 80% sieve particle size (mm) of the fine crushed product, R is the micro powder generation rate (%), and the E value is controlled within the range of 18-22 kWh / t.

5. The method for resource-based treatment of converter slag and construction waste according to claim 1, characterized in that: The magnetic field strength and frequency of the high-frequency pulsed magnetic field in step (e) satisfy the following: Where H0=1.2T, f=75Hz, and the magnetic field duration t is matched with the material flow rate to be 0.1-0.3s.

6. The method for resource-based treatment of converter slag and construction waste according to claim 1, characterized in that: The permeable bricks prepared in step (e) have a compressive strength ≥30MPa and a permeability coefficient ≥1.5×10⁻⁶. -2 cm / s, and the curing temperature is maintained at 50±5℃ for 48 hours.

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