A sintering control process for improving lead and cadmium leaching rate of ceramic glaze

Abstract

This invention discloses a sintering control process for improving the lead and cadmium leaching rate of ceramic glazes, relating to the field of daily-use ceramic manufacturing technology. The process includes: determining the characteristic dielectric frequency of the glaze to be fired at its critical melting temperature, where the characteristic dielectric frequency corresponds to the frequency of synergistic polarization between lead or cadmium ions and glass network forging ions; heating the product to the critical melting temperature and holding it at that temperature, applying an intermittent pulsed electric field with a frequency equal to the characteristic dielectric frequency; after stopping the electric field, subjecting the glaze melt to temperature cycling treatment, applying periodic temperature fluctuations at a reference temperature; and after the last half-cycle of cooling, rapidly cooling through the glass transition temperature zone, followed by slow cooling to room temperature. This invention, through the synergistic control of characteristic frequency electric field writing, temperature cycling screening, and gradient rapid freezing, significantly reduces the lead and cadmium leaching rate of the glaze surface without changing the glaze formulation, increasing the maximum firing temperature, or extending the production cycle.

Description

Technical Field

[0001] This invention relates to the field of daily-use ceramics manufacturing technology, specifically to a sintering control process for improving the lead and cadmium leaching rate of ceramic glazes. Background Technology

[0002] When everyday ceramic tableware comes into contact with acidic foods, lead or cadmium ions contained in the glaze glass network are easily leached out by hydrogen ions and migrate into the food, posing a food safety hazard. According to GB 4806.4 "National Food Safety Standard for Ceramic Products", the lead leaching of flat ceramic tableware must not exceed 0.5 mg / L, and the cadmium leaching must not exceed 0.07 mg / L; for children's tableware, the cadmium leaching requirement is not to exceed 0.025 mg / L.

[0003] To reduce lead and cadmium leaching rates, the industry typically employs two technical approaches. The first approach involves developing lead- and cadmium-free glaze formulations, using oxides of elements such as barium, zinc, lithium, and boron to replace lead oxide as a flux, fundamentally eliminating the source of heavy metals. This approach faces numerous challenges in practice: the substitute components often cannot fully replicate the gloss, fluidity, and firing temperature range of lead glazes, leading to a decline in glaze quality; and the formulation development cycle is long, with high trial-and-error costs. The second approach involves increasing the firing temperature or extending the holding time to achieve full vitrification under thermodynamic equilibrium, forming a dense three-dimensional network of silicon-oxygen tetrahedra on the glaze surface, physically encapsulating heavy metal ions within the network's voids. However, this method comes at the cost of increased heat input, resulting in a significant increase in energy consumption; simultaneously, the increase in firing temperature is limited by the refractoriness of the body, and for medium- and low-temperature firing systems such as bone china and fine ceramics, the potential for temperature increases is very limited.

[0004] More importantly, neither of the above two approaches addresses the following core issue: without changing the glaze formula or increasing the maximum firing temperature, can the chemical state of heavy metal ions in the glaze glass network be actively altered to make it more resistant to acid corrosion?

[0005] Therefore, a sintering control process to improve the lead and cadmium leaching rate of ceramic glazes is proposed to address the above problems. Summary of the Invention

[0006] In order to overcome the above-mentioned defects of the prior art, the embodiments of the present invention provide a sintering control process for improving the lead and cadmium leaching rate of ceramic glaze, so as to solve the problem mentioned in the background art that the prior art cannot improve the lead and cadmium leaching rate by actively regulating the ion coordination state in the glaze melt during the sintering stage.

[0007] To achieve the above objectives, the present invention provides a sintering control process for improving the lead and cadmium leaching rate of ceramic glazes, comprising the following steps: S1: Determine the characteristic dielectric frequency of the ceramic glaze to be fired at the critical melting temperature. The characteristic dielectric frequency is the frequency corresponding to the cooperative polarization of lead ions or cadmium ions with glass network forging ions. S2: The product to be fired is heated to the critical melting temperature and held at that temperature. During the holding period, an intermittent pulsed electric field is applied to the glaze melt, the frequency of which is equal to the characteristic dielectric frequency. S3: After stopping the application of the intermittent pulsed electric field, the glaze melt is subjected to temperature cycling treatment, wherein the temperature cycling treatment is to apply periodic temperature fluctuations at a reference temperature. S4: After the last cooling half-cycle of the temperature cycling process is completed, the product is cooled through the glass transition temperature zone at a rapid cooling rate, and then cooled to room temperature at a slow cooling rate.

[0008] Furthermore, the critical melting temperature is when the viscosity of the ceramic glaze melt reaches 10. 4 The temperature corresponding to Pa·s.

[0009] Further, in step S1, the characteristic dielectric frequency is determined by heating the ceramic glaze sample to be fired to the critical melting temperature and holding it at that temperature; within a range of 0.1 Hz to 10 Hz... 5 The dielectric constant spectrum of the sample is measured within the Hz range; in the imaginary part of the dielectric constant spectrum, the characteristic peaks whose peak intensity changes with the lead or cadmium content of the glaze are identified as characteristic peaks generated by the co-polarization of lead or cadmium ions and glass network forging ions; the frequency corresponding to the characteristic peak is obtained as the characteristic dielectric frequency.

[0010] Furthermore, in S2, the field strength of the intermittent pulsed electric field is from 0.5 V / cm to 5 V / cm.

[0011] Furthermore, in S2, each on / off cycle of the intermittent pulsed electric field consists of an on-time period and an off-time period. The duration of the on-time period is 100 to 500 times the period corresponding to the characteristic dielectric frequency, and the duration of the off-time period is 3 to 8 times the on-time period.

[0012] Furthermore, in S3, the temperature fluctuation range of the temperature cycle processing is 5°C to 10°C, the duration of each temperature fluctuation cycle is 30 seconds to 120 seconds, and the number of cycles is 5 to 20.

[0013] Furthermore, in S3, the reference temperature for the temperature cycling process is the critical melting temperature, or a temperature 0°C to 5°C below the critical melting temperature.

[0014] Furthermore, in S4, the rapid cooling rate is not less than 150°C / h, and the slow cooling rate is 50°C / h to 80°C / h.

[0015] Furthermore, in S4, the glass transition temperature zone is a temperature range from a temperature 50°C higher than the glass transition temperature of the ceramic glaze to a temperature 20°C lower than the glass transition temperature.

[0016] Furthermore, the holding time at the critical melting temperature in S2 is 1 / 2 to 2 / 3 of the holding time of the ceramic glaze at the highest firing temperature.

[0017] The technical effects and advantages of this invention are as follows: Compared with the prior art, the present invention has the following technical effects and advantages: First, the present invention sets up an "active ion state design" strategy to reduce the dissolution rate. By utilizing the kinetic path dependence of the glaze melt during the cooling process, lead ions or cadmium ions are locked in a strong coordination binding state with the glass network forming ions. This binding state has intrinsic acid corrosion resistance, thereby eliminating the dependence on increasing the firing temperature to achieve thermodynamic vitrification.

[0018] Secondly, this invention requires no changes to the glaze formula, no increase in the maximum firing temperature, and no extension of the production cycle, and can be fully implemented on existing medium- and low-temperature daily-use ceramic production lines. The critical melting temperature is 15°C to 30°C lower than the traditional maximum firing temperature, and the holding time is shortened to 1 / 2 to 2 / 3 of the traditional process. Only an electric field application device and a temperature circulation control program need to be added to the cooling section of a conventional tunnel kiln or roller kiln. The project is highly feasible and economical.

[0019] Third, this invention achieves active regulation of ion coordination states through a tandem synergistic mechanism of "electric field writing-temperature cycling screening". Intermittent pulsed electric fields resonate at characteristic dielectric frequencies to write strongly coordinated seeds. Subsequent temperature cycling utilizes the relaxation time differences between strong and weakly bound ions to spontaneously screen and enrich the seeds. Comparative data show that electric field writing alone reduces lead dissolution rate by only 23.5%, and temperature cycling alone reduces it by only 14.7%, while the combined use of both reduces it by 73.5%, verifying the synergistic effect of this tandem mechanism.

[0020] Fourth, this invention employs a gradient cooling strategy. Above the glass transition temperature, a rapid cooling rate of at least 150°C / h freezes the selected ion coordination state, while in the low-temperature region, a slow cooling rate of 50°C / h to 80°C / h releases thermal stress. This significantly reduces the dissolution rate while ensuring the glaze's thermal shock resistance meets requirements. Comparative examples show that while simple full-range rapid cooling does reduce the dissolution rate to some extent, it results in microcracks in the glaze and fails to meet thermal shock resistance standards, further confirming the rationality and indivisibility of this gradient cooling design. Attached Figure Description

[0021] Figure 1 This is a schematic flowchart of the sintering control process according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the dielectric constant spectrum of the lead-containing glaze in Example 1 at the critical melting temperature; Figure 3 This is a schematic diagram of the temperature-time curve during the temperature cycling process in Example 1; Figure 4 This is a schematic diagram of the temperature-time curve during the gradient cooling stage in Example 1. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0023] The general experimental conditions are as follows: 1. Lead-containing glaze formulation and raw material specifications Unless otherwise specified, Examples 1-7 and Comparative Examples 1-4 all use the same lead-containing medium-temperature daily-use ceramic glaze formula. The purity and specifications of each raw material are as follows: Quartz powder, chemical formula SiO2, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd., passed through a 200-mesh sieve; Alumina, chemical formula Al2O3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd., passed through a 300-mesh sieve; Red lead, chemical formula Pb3O4, chemically pure, with a content of not less than 97.0% based on PbO, manufactured by Xilong Scientific Co., Ltd., and passed through a 200-mesh sieve. Potassium carbonate, chemical formula K2CO3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd. Sodium carbonate, chemical formula Na2CO3, analytical grade, content not less than 99.8%, Sinopharm Chemical Reagent Co., Ltd. Calcium carbonate, chemical formula CaCO3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd. Magnesium oxide, chemical formula MgO, analytical grade, content not less than 98.5%, Sinopharm Chemical Reagent Co., Ltd. Titanium dioxide, chemical formula TiO2, chemically pure, content not less than 98.0%, Sinopharm Chemical Reagent Co., Ltd., passed through a 300-mesh sieve.

[0024] The chemical composition of the lead-containing glaze is shown in Table 1.

[0025] Table 1 Chemical composition of lead-containing glaze (mass percentage) Accurately weigh each raw material according to the proportions in Table 1, with a weighing accuracy of ±0.01g. Add deionized water and sodium carboxymethyl cellulose as suspending agents, and wet mill in a ball mill at 300 r / min for 4 hours. The resistivity of the deionized water is not less than 18 MΩ·cm. The amount of sodium carboxymethyl cellulose added is 0.3% of the dry weight of the glaze, and its purity is chemically pure, sourced from Sinopharm Chemical Reagent Co., Ltd. The ball milling media are zirconia balls, 10mm in diameter, with a purity of not less than 95%, and the mass ratio of material, balls, and water is 1:2:0.8. The ball-milled glaze slurry is passed through a 325-mesh sieve, and the glaze slurry density is adjusted to 1.6 g / cm³. 3 Measured with a hydrometer, for future reference.

[0026] 2. Determination of key parameters The maximum firing temperature of this lead-containing glaze is 1120℃, and the standard holding time is 30 minutes. The glaze obtained after holding at the maximum firing temperature for 30 minutes has passed quality inspection and is deemed qualified in terms of gloss, smoothness, and transparency.

[0027] Using a high-temperature rotational viscometer (Orton RSV-1600, manufactured by Orton Ceramic Foundation, USA, equipped with a Brookfield DV-II+ viscosity measurement system, rotor model SC4-27, rotational speed range 0.1-250 r / min), the glaze powder sample was heated in air at a rate of 5℃ / min. Temperature was measured using a type S thermocouple (platinum-rhodium 10-platinum, accuracy ±1℃). The measured melt viscosity of the glaze reached 10. 4 The critical melting temperature corresponding to Pa·s is 1095℃.

[0028] Using a differential scanning calorimeter (model Netzsch STA 449 F3 Jupiter, manufactured by Netzsch GmbH, Germany), 20.0 mg ± 2.0 mg of glaze powder was placed in an alumina crucible and heated from room temperature to 1200 °C at a heating rate of 10 °C / min under a nitrogen atmosphere (nitrogen purity 99.999%, flow rate 50 mL / min). The DSC heat flow curve was recorded, and the temperature corresponding to the midpoint of the heat flow curve step was taken. The glass transition temperature of the glaze was measured to be 520 °C.

[0029] 3. Sample Preparation The glazed substrate is a standard daily-use ceramic unglazed disc produced in the same batch, with a diameter of 200mm±2mm and a thickness of 4mm±0.2mm. The water absorption rate of the unglazed disc is 18%±2%, which is determined according to GB / T 3299 "Determination of Water Absorption Rate of Daily-use Ceramics".

[0030] The glazing method is immersion glazing: the unglazed disc is immersed in the glaze slurry for 3 seconds, then lifted out at a uniform speed and drained at room temperature. The glaze thickness is 0.3mm to 0.5mm, calculated by dividing the difference in mass of the unglazed disc before and after glazing by the theoretical density of the glaze, 2.5 g / cm³. 3 Calculate the glaze area. After glazing, let the unglazed body air dry at room temperature for 24 hours, and then dry it in a forced-air drying oven at 105℃±2℃ for 2 hours until constant weight (the difference between two consecutive weighings should not exceed 0.1g), and set aside for later use.

[0031] 4. Sintering equipment and temperature calibration All sintering tests were conducted in the same programmable temperature controlled high-temperature electric furnace (model Nabertherm LH 30 / 14, manufactured by Nabertherm GmbH, Germany). This furnace has a rated power of 15 kW, a maximum temperature of 1400℃, and furnace chamber dimensions of 300mm × 300mm × 300mm. Temperature was measured using S-type thermocouples, and the temperature profile was set and recorded via a Eurotherm 3504 programmable controller, with a temperature control accuracy of ±1℃.

[0032] To ensure the accuracy and uniformity of furnace temperature during the temperature cycling process, furnace temperature calibration was performed before the formal test: a calibrated standard S-type thermocouple was placed at the product placement location, and the temperature distribution within a 200mm × 200mm area in the center of the furnace was measured under a constant temperature of 1095℃. It was confirmed that the maximum deviation between the temperature at each measuring point within this area and the temperature at the center point at the same time did not exceed ±3℃. The temperature cycling program was controlled based on the feedback signal from the thermocouple measuring the product placement area within the furnace.

[0033] 5. Electric field application device The electric field application device consists of a function signal generator, a high-voltage amplifier, and platinum electrode plates. The function signal generator is a Tektronix AFG3102C, manufactured by Tektronix Corporation, USA. It is a dual-channel amplifier with a frequency range of 1μHz to 100MHz and a frequency resolution of 1μHz. The high-voltage amplifier is a Trek 10 / 10B-HS, manufactured by Trek Industries, USA. It has an output voltage range of 0 to ±10kV, a bandwidth of DC to 19.5kHz, and an output current range of 0 to ±10mA. The platinum electrode plates are 99.99% pure, measuring 250mm × 250mm × 1mm, and are placed parallel to each other with a spacing of 220mm.

[0034] The electrode plates are fixed to the two side walls of the furnace chamber by high-temperature resistant insulating supports made of 99% alumina ceramic. The plane of the electrode plates is parallel to the vertical plane. The product to be fired is placed in the center of the bottom surface of the furnace chamber, with the plane of the product's glaze parallel to the plane of the electrode plates. The horizontal distance between the electrode plates and the glaze surface of the product is 10mm. The electrode plates are made of platinum wire with a diameter of 0.5mm, which is led out through an insulated and sealed lead-out hole on the furnace wall and electrically connected to a function signal generator and a high-voltage amplifier outside the furnace.

[0035] The pulse waveform output from the signal generator is amplified by a high-voltage amplifier and then applied to both ends of the electrode plates. A digital oscilloscope (model Tektronix TDS 2024C, bandwidth 200MHz, sampling rate 2GS / s) is connected to both ends of the electrode plates via a high-voltage probe to monitor the actual applied voltage waveform and amplitude in real time. The electric field strength is calculated by dividing the effective value of the applied voltage by the distance between the electrode plates.

[0036] 6. Testing Methods Lead leaching was tested according to GB 31604.34, "National Food Safety Standard - Determination of Lead and Migration in Food Contact Materials and Articles". The immersion solution was a 4% aqueous acetic acid solution, prepared by diluting glacial acetic acid (analytical grade, purity not less than 99.5%, Sinopharm Chemical Reagent Co., Ltd.) with deionized water to a volume fraction of 4%. The immersion temperature was 22℃±2℃, and the immersion time was 24 hours±0.5 hours. The immersion container was a borosilicate glass beaker (soaked in 10% nitric acid solution for 24 hours before use, and rinsed with deionized water until the pH of the washing solution was neutral). The volume of the immersion solution was calculated as 1.5 mL per square centimeter of the product surface area.

[0037] After the soaking is completed, a flame atomic absorption spectrometer (model PerkinElmer AAnalyst 400, manufactured by PerkinElmer, USA, with a lead hollow cathode lamp wavelength of 283.3 nm, a spectral bandwidth of 0.7 nm, and an acetylene-air flame) is used to measure the lead concentration in the soaking solution. The standard curve is established using a lead standard solution (national standard substance, concentration 1000 μg / mL, number GBW(E)080129), and is serially diluted with 4% acetic acid to 0.1, 0.5, 1.0, 2.0, 5.0 mg / L before use, with a correlation coefficient not less than 0.999. Five parallel samples are tested for each group, and each sample is a complete product. The result is the arithmetic mean of the test values of the five samples. The method detection limit is 0.01 mg / L, and the quantification limit is 0.03 mg / L.

[0038] The test for cadmium dissolution amount is carried out in accordance with GB 31604.24 "National Food Safety Standard - Determination and Migration of Cadmium in Food Contact Materials and Articles". The test conditions are the same as above, and a cadmium hollow cathode lamp with a wavelength of 228.8 nm is used.

[0039] The glaze glossiness is measured using a 60° glossiness meter (model BYK micro-TRI-gloss, manufactured by BYK-Gardner, Germany). Five evenly distributed points are selected and measured at different positions on the glaze surface of each sample, and the arithmetic mean is taken.

[0040] The Vickers hardness is measured using a Vickers hardness tester (model Future-Tech FV-700, manufactured by Future-Tech, Japan). The load is 200 gf, and the holding time is 15 seconds. Five indentations are measured at different positions on the glaze surface of each sample, and the arithmetic mean is taken.

[0041] The thermal shock resistance is carried out in accordance with GB / T 32978 "Method for Determination of Thermal Shock Resistance of Domestic Ceramic Ware": The sample is kept in a forced-air drying oven at 180°C ± 5°C for 30 minutes, taken out and quickly put into still tap water at 20°C ± 2°C, and visually inspected for cracks, peeling or discoloration on the glaze surface. Five samples are tested for each group, and the hot and cold cycles are repeated five times, recording the number of cycles with cracks and the number of samples. If no cracks appear in the five samples after five cycles, it is judged as qualified; otherwise, it is judged as unqualified.

[0042] Figure 1 The complete process of the sintering control process of the present invention is shown, including four steps: S1 characteristic frequency measurement, S2 electric field writing, S3 temperature cycle screening, and S4 gradient rapid freezing.

[0043] Example 1

[0044] The lead-containing glaze is treated using the complete process of the present invention.

[0045] Step S1: Determine the characteristic dielectric frequency.

[0046] Approximately 5g of glaze powder was placed into a platinum crucible (99.95% purity, 10mL volume) and placed in the sample chamber of a high-temperature dielectric spectrometer (Novocontrol Concept 80, manufactured by Novocontrol Technologies, Germany, frequency range 3μHz to 40MHz, temperature range room temperature to 1600℃). The temperature was increased to 1095℃ at a rate of 5℃ / min and held for 30 minutes. Under isothermal conditions at 1095℃, the glaze powder was analyzed at a frequency range of 0.1Hz to 1000MHz. 5 The frequency is scanned sequentially within the Hz frequency range, with 50 frequency points per order of magnitude, and a sinusoidal alternating electric field with an amplitude of 0.5 V / cm is applied.

[0047] Simultaneously, a control glaze sample was prepared by removing PbO and replacing it with an equimolar amount of CaO. The mass percentage of CaO in the control formulation was 9.58%, and the mass percentage of PbO was 0%. The dielectric loss spectrum of the control glaze was measured under the same conditions. The difference spectrum was obtained by subtracting the dielectric loss spectra of the full-component glaze and the control glaze frequency-by-frequency. The difference spectrum showed a peak value at 2.5 × 10⁻⁶. 3 A characteristic loss peak is observed at Hz (see...) Figure 2 The dielectric constant spectrum diagram shown indicates that the dielectric constant is determined to be a mixture of lead ions and Al. 3+ Ti 4+ The characteristic peak generated by the coordinated polarization of ions in the network-forming body. The frequency corresponding to this characteristic peak is 2.5 × 10⁻⁶. 3 Hz is used as the characteristic dielectric frequency.

[0048] S2 step: Electric field writing.

[0049] The glazed unglazed disc is placed between two platinum electrode plates in a high-temperature electric furnace, with the disc centered on the plates and the glaze surface parallel to the plates. The temperature is increased to 1095℃ at a rate of 5℃ / min and held for 15 minutes. During this holding period, an intermittent pulse signal is generated using a function signal generator: pulse frequency 2.5 × 10⁻⁶. 3 The pulse group envelope is a square wave with a frequency of Hz. The power-on period is 0.1 seconds, and the power-off period is 0.5 seconds. The pulse signal is amplified by a high-voltage amplifier to achieve an electric field strength of 2 V / cm between the electrode plates, i.e., an effective voltage of 44V. The waveform parameters are confirmed to meet the settings by monitoring with an oscilloscope. The signal is applied continuously for 10 minutes and then stopped.

[0050] Step S3: Temperature Cyclic Screening.

[0051] After the applied electric field is stopped, the temperature reference is adjusted to 1095℃ via the furnace controller and locked to remain constant throughout the temperature cycling process. The temperature cycling program is then initiated: the temperature fluctuates sinusoidally between 1091℃ and 1099℃, with a fluctuation range of 8℃. Each fluctuation cycle lasts 60 seconds, consisting of a 30-second heating half-cycle and a 30-second cooling half-cycle, for a total of 10 cycles. At the end of the last cooling half-cycle, the temperature has precisely returned to the reference temperature of 1095℃ (see [reference]). Figure 3 The diagram shows the temperature-time curves for the temperature cycling process.

[0052] Step S4: Gradient quenching.

[0053] The rapid cooling program is activated the instant the temperature drops to 1095℃. The furnace door is opened to its maximum opening and the fan is started at its maximum speed, causing the furnace temperature to decrease from 1095℃ at an average rate of 180℃ / h. The cooling rate is monitored and adjusted in real time via S-type thermocouples inside the furnace: if the actual cooling rate is lower than 175℃ / h, the fan speed is increased appropriately; if it is higher than 185℃ / h, the furnace door opening is reduced appropriately.

[0054] When the temperature indicated by the thermocouple inside the furnace drops to 500°C, close the furnace door and fan, and switch the cooling program (see [link]). Figure 4 The temperature-time curve of the gradient cooling stage is shown in the diagram. The product is slowly cooled to room temperature at an average rate of 60°C / h. The product is removed when the furnace temperature drops below 50°C.

[0055] The lead leaching amount was measured to be 0.18 mg / L. The glaze gloss was 91 GU, the Vickers hardness was 5.8 GPa, and the thermal shock resistance was qualified.

[0056] Example 2

[0057] The only difference between this embodiment and Embodiment 1 is that the number of temperature cycles in step S3 is adjusted to 5. All other steps, parameters, raw materials, and equipment are exactly the same as in Embodiment 1.

[0058] The lead leaching amount was measured to be 0.31 mg / L. The glaze gloss was 92 GU, the Vickers hardness was 5.7 GPa, and the thermal shock resistance was satisfactory.

[0059] Example 3

[0060] The only difference between this embodiment and Embodiment 1 is that the number of temperature cycles in step S3 is adjusted to 20. All other steps, parameters, raw materials, and equipment are exactly the same as in Embodiment 1.

[0061] The lead leaching amount was measured to be 0.12 mg / L. The glaze gloss was 90 GU, the Vickers hardness was 5.9 GPa, and the thermal shock resistance was satisfactory.

[0062] Example 4

[0063] The only difference between this embodiment and Embodiment 1 is that the temperature fluctuation range in step S3 is adjusted to 5°C, that is, the temperature fluctuates sinusoidally between 1093°C and 1097°C. All other steps, parameters, raw materials, and equipment are exactly the same as in Embodiment 1.

[0064] The lead leaching amount was measured to be 0.26 mg / L. The glaze gloss was 91 GU, the Vickers hardness was 5.8 GPa, and the thermal shock resistance was satisfactory.

[0065] Example 5

[0066] The only difference between this embodiment and Embodiment 1 is that the temperature fluctuation range in step S3 is adjusted to 10°C, that is, the temperature fluctuates sinusoidally between 1089°C and 1099°C. All other steps, parameters, raw materials, and equipment are exactly the same as in Embodiment 1.

[0067] The lead leaching amount was measured to be 0.15 mg / L. The glaze gloss was 90 GU, the Vickers hardness was 5.7 GPa, and the thermal shock resistance was satisfactory.

[0068] Example 6

[0069] The only difference between this embodiment and embodiment 1 is that the rapid cooling rate in step S4 is adjusted to 150℃ / h, which is achieved by adjusting the furnace door opening and fan speed. The actual cooling rate is controlled within the range of 145℃ / h to 155℃ / h. All other steps, parameters, raw materials, and equipment are exactly the same as in embodiment 1.

[0070] The lead leaching amount was measured to be 0.22 mg / L. The glaze gloss was 92 GU, the Vickers hardness was 5.7 GPa, and the thermal shock resistance was satisfactory.

[0071] Example 7

[0072] The only difference between this embodiment and Embodiment 1 is that the reference temperature in step S3 is adjusted to 1090℃, which is 5℃ below the critical melting temperature. The temperature is kept constant at 1090℃ throughout the entire temperature cycling process, and the temperature fluctuates sinusoidally between 1086℃ and 1094℃. All other steps, parameters, raw materials, and equipment are exactly the same as in Embodiment 1.

[0073] The lead leaching amount was measured to be 0.20 mg / L. The glaze gloss was 91 GU, the Vickers hardness was 5.8 GPa, and the thermal shock resistance was satisfactory.

[0074] Example 8

[0075] This embodiment uses cadmium-containing medium-temperature daily-use ceramic glaze to verify the applicability of the process of the present invention. The purity and specifications of each raw material are as follows: Quartz powder, chemical formula SiO2, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd., passed through a 200-mesh sieve; Alumina, chemical formula Al2O3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd., passed through a 300-mesh sieve; Cadmium oxide, chemical formula CdO, chemically pure, content not less than 99.0%, Shanghai Maclean Biochemical Technology Co., Ltd., passed through a 200-mesh sieve; Potassium carbonate, chemical formula K2CO3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd. Sodium carbonate, chemical formula Na2CO3, analytical grade, content not less than 99.8%, Sinopharm Chemical Reagent Co., Ltd. Calcium carbonate, chemical formula CaCO3, analytical grade, content not less than 99.0%, Sinopharm Chemical Reagent Co., Ltd. Boric acid, chemical formula H3BO3, analytical grade, content not less than 99.5%, Sinopharm Chemical Reagent Co., Ltd., calculated as B2O3.

[0076] The chemical composition of the cadmium-containing glaze is shown in Table 2.

[0077] Table 2 Chemical composition of cadmium-containing glazes (mass percentage) The maximum firing temperature of this cadmium-containing glaze is 1080℃, with a standard holding time of 25 minutes. Using a high-temperature rotational viscometer (model and testing conditions as before), with a heating rate of 5℃ / min, the melt viscosity of the glaze was measured to reach 10. 4 The critical melting temperature corresponding to Pa·s is 1050℃. Using DSC (model and test conditions as before), 20.0mg±2.0mg of glaze powder was taken, and the glass transition temperature of the glaze was measured to be 495℃.

[0078] Step S1: Take approximately 5g of glaze powder and place it in a platinum crucible. Determine the characteristic dielectric frequency using the same comparative method as in Example 1. Prepare a control glaze sample with CdO removed and replaced by an equimolar amount of CaO. The mass percentage of CaO in the control formulation is 10.04%, and the mass percentage of CdO is 0%. Measure the dielectric loss spectrum under the same conditions. In the difference spectrum obtained by subtracting the two spectra frequency-wise, the difference is within 4.8 × 10⁻⁶. 2 A characteristic loss peak was observed at Hz, which was determined to be caused by cadmium ions reacting with Al. 3+ Characteristic peaks generated by the coordinated polarization of ions in the network-forming body. The characteristic dielectric frequency was obtained as 4.8 × 10⁻⁶. 2 Hz.

[0079] Step S2: Place the glazed unglazed disc between platinum electrode plates in the electric furnace, and heat it to 1050℃ at a rate of 5℃ / min, holding it at that temperature for 12 minutes. During the holding period, apply an intermittent pulsed electric field with a frequency of 4.8 × 10⁻⁶. 2 Hz, field strength 3 V / cm, i.e., effective voltage 66V, power-on period 0.5 seconds, power-off period 2.0 seconds, total application time 8 minutes.

[0080] Step S3: After the electric field stops, the reference temperature is locked at 1050℃ and kept constant throughout the entire temperature cycling process. The temperature fluctuates sinusoidally between 1047℃ and 1053℃ with a fluctuation range of 6℃. Each fluctuation cycle lasts 50 seconds, with a heating half-cycle of 25 seconds and a cooling half-cycle of 25 seconds, for a total of 8 cycles.

[0081] Step S4: After the last half-cycle of cooling is completed and the temperature drops to 1050℃, rapidly cool at 170℃ / h, with the actual cooling rate controlled within the range of 165℃ / h to 175℃ / h. When the furnace temperature drops to 475℃, switch to slow cooling at 55℃ / h until the temperature is below 50℃ before removing the furnace.

[0082] The cadmium leaching amount of this cadmium-containing glaze under the traditional slow cooling process is 0.15 mg / L. After using the process of this embodiment, the cadmium leaching amount is 0.04 mg / L. The glaze surface gloss is 90 GU, the Vickers hardness is 5.9 GPa, and the thermal shock resistance is qualified.

[0083] Comparative Example 1 It adopts a traditional slow cooling process.

[0084] The glazed unglazed discs were placed in a high-temperature electric furnace and heated from room temperature to 1120°C at a rate of 5°C / min, and held at 1120°C for 30 minutes. After the holding period, the furnace was turned off, and the furnace door remained closed, allowing the products to cool naturally. The actual cooling rate inside the furnace was monitored and recorded by S-type thermocouples inside the furnace: the average cooling rate was 80°C / h in the range of 1120°C to 500°C, and the average cooling rate was 40°C / h in the range of 500°C to room temperature. The furnace door was opened and the products were removed when the furnace temperature dropped below 50°C.

[0085] The lead leaching amount was measured to be 0.68 mg / L. The glaze gloss was 92 GU, the Vickers hardness was 5.8 GPa, and the thermal shock resistance was satisfactory.

[0086] Comparative Example 2 It adopts the traditional rapid cooling process.

[0087] The glazed unglazed discs were placed in a high-temperature electric furnace and heated from room temperature to 1120°C at a rate of 5°C / min, and held at 1120°C for 30 minutes. After holding, the furnace door was fully opened, and the furnace cooling fan was activated for forced ventilation cooling of the furnace chamber. The actual cooling rate inside the furnace was monitored and recorded by the S-type thermocouple inside the furnace: the average cooling rate was 200°C / h in the range of 1120°C to 500°C. Below 500°C, the fan was turned off, the furnace door remained open, and the products cooled naturally with the furnace. The products were removed when the furnace temperature dropped below 50°C.

[0088] The lead leaching amount was measured to be 0.41 mg / L. The glaze gloss was 90 GU, and the Vickers hardness was 5.9 GPa. In the thermal shock resistance test, two out of five samples showed microcracks in the glaze surface during the third cycle and were therefore deemed unqualified.

[0089] Comparative Example 3 The only difference between this comparative example and Example 1 is that step S3 does not involve temperature cycling, while step S4 uses conventional slow cooling.

[0090] The glazed unglazed disc was placed in a high-temperature electric furnace and heated to 1095°C at a rate of 5°C / min, and held at that temperature for 15 minutes. During the holding period, an intermittent pulsed electric field was applied, with the electric field parameters being exactly the same as in Example 1. After the electric field stopped, no temperature cycling was performed, the furnace door remained closed, and the product was allowed to cool naturally to below 50°C before being removed. The cooling rate was the same as in Comparative Example 1.

[0091] The lead leaching amount was measured to be 0.52 mg / L. The glaze gloss was 91 GU, the Vickers hardness was 5.7 GPa, and the thermal shock resistance was satisfactory.

[0092] Comparative Example 4 The only difference between this comparative example and Example 1 is that no electric field is applied in step S2.

[0093] The glazed unglazed disc was placed in a high-temperature electric furnace and heated to 1095°C at a rate of 5°C / min, and held at that temperature for 15 minutes. No electric field was applied during the holding period. After the holding period, temperature cycling and gradient cooling were performed, with the temperature cycling and cooling parameters being exactly the same as in Example 1.

[0094] The lead leaching amount was measured to be 0.58 mg / L. The glaze gloss was 91 GU, the Vickers hardness was 5.8 GPa, and the thermal shock resistance was satisfactory.

[0095] The experimental results are as follows: The key process parameters of each embodiment and comparative example are summarized in Tables 3 and 4, and the lead and cadmium leaching amount and glaze performance are summarized in Table 5.

[0096] Table 3 Summary of sintering and electric field parameters for the examples and comparative examples Table 4 Summary of Temperature Cycling and Cooling Parameters for Examples and Comparative Examples Table 5 Summary of Lead and Cadmium Dissolution Amounts and Glaze Properties for Examples and Comparative Examples Note: In Table 4, the rapid cooling rate of Comparative Example 1 is the average cooling rate of slow cooling in the furnace, the rapid cooling rate of Comparative Example 2 is the average cooling rate of forced rapid cooling, and the rapid cooling rate of Comparative Example 3 is the average cooling rate of slow cooling in the furnace.

[0097] In Table 5, for "reduction ratio compared with traditional process", for Examples 1 - 7, it is calculated based on the lead dissolution amount of 0.68 mg / L in Comparative Example 1; for Example 8, it is calculated based on the cadmium dissolution amount of 0.15 mg / L of this cadmium-containing glaze in the traditional slow cooling process. In the "thermal shock resistance" column, if all 5 samples pass 5 cycles without cracks, it is judged as qualified, otherwise it is judged as unqualified.

[0098] The analysis of experimental results is as follows:[[]]END]] The following conclusions can be drawn from the data in Table 5:[[]]END]] First, the comparison between Example 1 and Comparative Example 1 with the traditional slow cooling process shows that the lead dissolution amount decreases from 0.68 mg / L to 0.18 mg / L, a decrease of 73.5%, and the glossiness, Vickers hardness and thermal shock resistance of the glaze are not deteriorated. This confirms that the synergistic effect of the four-step process of the present invention can significantly improve the lead dissolution rate under the constraints of not changing the glaze formula, not increasing the maximum firing temperature, and not extending the production cycle. The maximum firing temperature of Example 1 is 1095 °C, which is lower than 1120 °C of Comparative Example 1, and the holding time of 15 minutes is shorter than 30 minutes of Comparative Example 1.

[0099] Second, Comparative Example 2 uses a simple rapid cooling process. Although the lead dissolution amount decreases to 0.41 mg / L, the thermal shock resistance is unqualified. Example 1 achieves a lower dissolution amount of 0.18 mg / L while maintaining qualified thermal shock resistance, verifying the rationality and inseparability of the gradient cooling strategy of the present invention of "rapidly cooling above the glass transition temperature zone to freeze the ion state and slowly cooling in the low temperature zone to release thermal stress".

[0100] Third, Comparative Example 3, which only involved electric field writing without temperature cycling and gradient cooling, showed a lead leaching amount of 0.52 mg / L; Comparative Example 4, which only involved temperature cycling and gradient cooling without electric field writing, showed a lead leaching amount of 0.58 mg / L. The improvement rates for both were only 23.5% and 14.7%, respectively, both significantly lower than the 73.5% improvement in Example 1. This strongly demonstrates a synergistic effect between the two steps of S2 electric field writing and S3 temperature cycling screening. Electric field writing provides strong coordination seeds for temperature cycling, while temperature cycling screens and amplifies these seeds. Both steps are indispensable and must be performed in combination to achieve a significant effect.

[0101] Fourth, Examples 2 and 3 show that as the number of temperature cycles increased from 5 to 20, the lead leaching amount further decreased from 0.31 mg / L to 0.12 mg / L, and the reduction rate increased from 54.4% to 82.4%, confirming that temperature cycle screening has a cumulative effect. Those skilled in the art can select an appropriate number of cycles according to the target leaching rate requirements.

[0102] Fifth, Examples 4 and 5 verified that the effect was effective within a temperature fluctuation range of 5°C to 10°C. Example 6 verified that the effect remained significant even under a rapid cooling rate of 150°C / h. Example 7 verified the feasibility of the process when the reference temperature was 5°C below the critical melting temperature.

[0103] Sixth, in Example 8, the process was applied to a cadmium-containing glaze system, and the cadmium leaching amount decreased from 0.15 mg / L in the traditional process to 0.04 mg / L, a reduction of 73.3%, indicating that the process of the present invention has universal applicability to different heavy metal ions and different glaze systems.

[0104] Based on the above results and analysis, the sintering control process of the present invention, under the constraints of not changing the glaze formula, not increasing the maximum firing temperature, and not extending the production cycle, reduces the lead and cadmium leaching rate by more than 50%, up to a maximum of 82.4%, through the spatiotemporal coordinated regulation of the external electric field and thermal regime, while maintaining the basic properties of the glaze surface such as gloss, hardness, and thermal shock resistance without deterioration.

[0105] The three key input parameters required for the process—critical melting temperature, characteristic dielectric frequency, and glass transition temperature—can all be accurately obtained using conventional testing methods such as high-temperature viscometers, high-temperature dielectric spectrometers, and differential scanning calorimetry. Clear selection ranges and criteria are provided for the parameters in each step. The raw material specifications, instrument models, operational details, temperature control methods, electric field application methods, and cooling rate adjustment methods in all embodiments are fully disclosed. Those skilled in the art can make corresponding adjustments and accurately reproduce the process based on the specific glaze characteristics, demonstrating significant technological advancement and industrial applicability.

[0106] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A sintering control process for improving the lead and cadmium leaching rate of ceramic glazes, characterized in that, Includes the following steps: S1: Determine the characteristic dielectric frequency of the ceramic glaze to be fired at the critical melting temperature. This characteristic dielectric frequency is the frequency corresponding to the cooperative polarization of lead ions or cadmium ions with glass network forging ions. S2: The product to be fired is heated to the critical melting temperature and held at that temperature. During the holding period, an intermittent pulsed electric field is applied to the glaze melt, the frequency of which is equal to the characteristic dielectric frequency. S3: After stopping the application of the intermittent pulsed electric field, the glaze melt is subjected to temperature cycling treatment, wherein the temperature cycling treatment is to apply periodic temperature fluctuations at a reference temperature. S4: After the last cooling half-cycle of the temperature cycling process is completed, the product is cooled through the glass transition temperature zone at a rapid cooling rate, and then cooled to room temperature at a slow cooling rate.

2. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1, characterized in that, In S2, each on / off cycle of the intermittent pulse electric field consists of an on-time period and an off-time period. The duration of the on-time period is 100 to 500 times the period corresponding to the characteristic dielectric frequency, and the duration of the off-time period is 3 to 8 times the on-time period.

3. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1, characterized in that, In S2, the field strength of the intermittent pulsed electric field is from 0.5 V / cm to 5 V / cm.

4. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1, characterized in that, In S3, the temperature fluctuation range of the temperature cycle processing is 5°C to 10°C, and the duration of each temperature fluctuation cycle is 30 seconds to 120 seconds.

5. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 4, characterized in that, In step S3, the temperature cycling process is repeated 5 to 20 times.

6. The sintering control process according to claim 1, characterized in that, In S3, the reference temperature for the temperature cycling process is the critical melting temperature, or a temperature 0°C to 5°C below the critical melting temperature.

7. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1, characterized in that, In step S4, the rapid cooling rate is not less than 150°C / h, and the slow cooling rate is 50°C / h to 80°C / h.

8. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 7, characterized in that, In step S4, the glass transition temperature zone is a temperature range from 50°C above the glass transition temperature of the ceramic glaze to 20°C below the glass transition temperature.

9. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1, characterized in that, In step S1, determining the characteristic dielectric frequency includes: The ceramic glaze sample to be fired is heated to the critical melting temperature and held at that temperature. From 0.1 Hz to 10 5 The dielectric constant spectrum of the sample was measured within the Hz range; In the imaginary part of the dielectric constant spectrum, the characteristic peaks whose peak intensity changes with the lead or cadmium content of the glaze are identified as characteristic peaks generated by the synergistic polarization of lead or cadmium ions and glass network forging ions. The frequency corresponding to the characteristic peak is obtained as the characteristic dielectric frequency.

10. The sintering control process for improving the lead and cadmium leaching rate of ceramic glazes according to claim 1 or 9, characterized in that, The critical melting temperature is when the viscosity of the ceramic glaze melt reaches 10. 4 The temperature corresponding to Pa·s; The holding time at the critical melting temperature in S2 is 1 / 2 to 2 / 3 of the holding time of the ceramic glaze at the highest firing temperature.

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