Circulation of heavy metal thallium in co-processing cement kilns

The suspension-preheater flash-furnace (SF) process for cement clinkering introduces a calcining flash furnace operating as an integral portion of a suspension preheater, rotary kiln, and clinker cooler system. Preheated raw meal from the suspension preheater is calcined in the flash furnace to 80 to 90 percent decarbonation. The capacity of the SF rotary kiln is increased thereby, since its function is limited to sintering rather than sintering and calcining. The flash furnace incorporates vortex flow to accomplish the mixing of fuel, raw meal, and gases. Effective distribution within the flash furnace limits temperature gradients and eliminates localized elevated temperatures that might otherwise lead to coating and material build-up. Hot combustion gases are recovered from the clinker cooler for use in the flash furnace. An orifice in the kiln gas exit duct serves to balance the parallel gas flows through the kiln and secondary air duct. Operating pressures and temperatures are described. The smaller size of the rotary kiln used in the SF process leads to extended refractory life as demonstrated by operating experience. The SF process is adaptable to the control of internal alkali, sulfur, and chlorine cycles through a gas bypass and through a unique material withdrawal arrangement from the flash furnace. Existing plants may be modified to incorporate the SF process. Nearly 20 commercial scale plants are operating or are being constructed.

The raw meal quality control of cement is an important link in cement production. The pass rate of raw meal quality control has a crucial impact on clinker calcination. Cement production is a complex large-scale system control process, the production process has randomness, hysteresis and uncertainty., the quality control of raw meal has a certain degree of control difficulties. Most cement production enterprises adopt manual control to control. Intelligent control can make up for the defects caused by manual control, and the intelligent control system can take effective detection means and control methods according to the situation in the field. This paper will elaborate on the process and control difficulties of raw meal production from the perspective of intelligent control. First, classify intelligent devices that detect raw meal. Then, fuzzy identification of work condition. Third, control by trend control and deviation control. Finally, the intelligent control results are analyzed and verified.

In the process of clinker calcination, the target value of the raw meal decomposition rate (RMDR) is different in the easy and difficult calcination stages because the boundary conditions of raw meal (i.e., raw meal flow, raw meal ingredients, and particle size) change frequently, where RMDR cannot be guaranteed to be within its desirable range. To solve this problem, an intelligent setting control method is proposed in this paper for a clinker calcination process. The proposed approach is realized by on‐line adjustment of the setpoints of control loops in line with the changes of raw meal boundary conditions. This method consists of five modules, namely an RMDR target value setting model, a control loop pre‐setting model, a feedback compensation model based on the fuzzy rules, a feedforward compensation model based on the fuzzy rules, and a soft measurement model for RMDR. Successful application to the clinker calcination process of the Jiuganghongda Cement Plant in China has been made, where the efficiency of the proposed method has been validated by the results of the practical application.

Fate of thallium during precalciner cement production and the atmospheric emissions

Recycling ash into the first stage of cyclone pre-heater of cement kiln

The present work focuses on the possibility of recycling the cement kiln bypass dust(CKBp) in cement industry through its replacement to the raw meal (clay, limestone) and/orclinker. Replacement of CKBp dust by washing processes techniques were applied on reusecement kiln bypass dust for El Arish Cement Company (ACC) in the central Sinai forsolving the problem of high Cl contents to be conformable with the quality control factors.This study proved that replacement process of raw meal by washed CKBp dust did not affectthe quality of the raw mix therefore replacement limit 3% for raw meal is considered. Theprocess of replacement on the other hand will save raw meal and reduced the cost. Using ofwashed CKBp dust to replace raw meal will consume every day about 156 to 160 tons (about78% CKBp dust). Using of 5% washed CKBp to replace the clay lead to consume about 114t/d from CKBp dust. Thus, the remove of about 52% from the total quantity CKBp dust wasachieved. The produced clinker per day was about 6000 t. If we replaced 3% clinker bywashed CKBp dust about 180 t clinker is saved every day. This consumed about 81% perday from CKBp dust this results for one production line.

Life cycle assessment (LCA) was carried out to quantify and analyze the environmental impact and benefit caused by the utilization of coal gangue as alternative raw material and fuel in cement clinker production. The optimal dosage of coal gangue was determined by comparing among different mixing amount scenarios and Portland cement clinker (clinker without adding any waste) considering the phases of coal gangue disposal, transportation, and raw meal grinding and clinker calcination. The results showed that: 1) After adding coal gangue to the raw meal, almost all the considered environmental impacts of cement clinker including human toxicity potential, photochemical smog potential, especially abiotic depletion potential decreased significantly. However, global warming potential and acidification potential increased slightly in comparison with Portland cement clinker. 2) Compared with the Portland cement clinker, the single environmental indicator reduced after adding coal gangue and the indicator decreased gradually with the dosage increasing.

A new process has been developed for processing chlorine-bypass-dust into new materials that can be sold or used in the cement kiln. Bypass dust is the dust that is rejected from a kiln system in order to lower the chlorine or alkali content of the produced clinker. Many cement plants have routinely landfilled this dust over many years. The strategic concept of "zero waste" is the basis for this system, but other smaller reasons also exist for its development: · The landfill disposal costs are high in some areas. · Bypass dust, which is typically wasted, has a unit cost of 20-30% of the cost per ton of producing clinker - depending on the cement making process type - and this cost can be recuperated. · For environmental reasons it may be beneficial to reduce the landfill area within the overall plant site area. · An increase in alternative fuels may cause an increase in bypass dust that needs to be extracted, thereby, causing higher disposal specific production costs. · The sales price of salt runs at approximately 350 USD/t which is potentially a new income stream for the plant. The dust reduction system is made up of the following sections: 1. Washing of bypass dust with water; 2. Filtration to obtain a salt solution (brine); 3. Chemical treatment (purification and conditioning of the brine); 4. Evaporation of this clean-neutralized brine to obtain the alkali-salts. The products of the system are: 1. Raw meal, low in alkalis and chlorine, that can be directly used as kiln feed. 2. High purity salts that can be used for food, de-icing roads, or making fertilizer. 3. Heavy metals sludge that can be treated separately to obtain valuable metals. A pilot plant has been installed with positive results and now the first industrial-sized plant is under construction.

Thermodynamic calculation on the smelting slag of direct recycling of electric arc furnace stainless steelmaking dust was presented. An induction furnace was used to simulate electric arc furnace smelting to recover the metals from the dust. The elements of iron, chromium and nickel in the ingot and the components of metal oxides in the slag were analyzed. The thermodynamic model for FeO-Cr2O3-MgO-SiO2 slag was set up and the active concentrations of substances in the slag at 1 550 °C were determined by thermodynamic calculation according to the experimental data. The results show that the apparent equilibrium constant and quantitative distribution of chromium between slag and steel are unstable and affected by the mass ratios of pellets to start iron and metal reducing agent to the pellets. In order to get satisfactory chromium recovery from the direct recycling of electric arc furnace stainless steelmaking dust, it is important to ensure the mass ratio of pellets to the steel below 0.20 and the mass ratio of metal reducing agent to pellets over 0.18 in practical smelting runs.

In recent years, the variability in the composition of cement raw materials has increasingly impacted the quality of cement products. However, there has been relatively little research on the homogenization effects of equipment in the cement production process. Existing studies mainly focus on the primary functions of equipment, such as the grinding efficiency of ball mills, the thermal decomposition in cyclone preheaters, and the thermal decomposition in rotary kilns. This study selected four typical pieces of equipment with significant homogenization functions for an in-depth investigation: ball mills, pneumatic homogenizing silos, cyclone preheaters, and rotary kilns. To assess the homogenization efficacy of each apparatus, scaled-down models of these devices were constructed and subjected to simulated experiments. To improve experimental efficiency and realistically simulate actual production conditions in a laboratory setting, this study used the uniformity of the electrical capacitance of mixed powders instead of compositional uniformity to analyze homogenization effects. The test material in the experiment consisted of a mixture of raw meal from a cement factory with a high dielectric constant and Fe3O4 powder. The parallel plate capacitance method was employed to ascertain the capacitance value of the mixed powder prior to and subsequent to treatment by each equipment model. The fluctuation of the input and output curves was analyzed, and the standard deviation (S), coefficient of variation (R), and homogenization multiplier (H) were calculated in order to evaluate the homogenization effect of each equipment model on the raw meal. The findings of the study indicated that the pneumatic homogenizer exhibited an exemplary homogenization effect, followed by the ball mill. For the ball mill, a higher proportion of small balls in the gradation can significantly enhance the homogenization effect without considering the grinding efficiency. The five-stage cyclone preheater also has a better homogenization effect, while the rotary kiln has a less significant homogenization effect on raw meal. Finally, the raw meal processed by each equipment model was used for clinker calcination and the preparation of cement mortar samples. After curing for three days, the compressive and flexural strengths of the samples were tested, thereby indirectly verifying the homogenization effect of each equipment model on the raw meal. This study helps to understand the homogenization process of raw materials by equipment in cement production and provides certain reference and data support for equipment selection, operation optimization, and quality control in the cement production process.

ABSTRACTThe formation, reduction and emission behaviours of chlorobenzenes (CBzs) and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were studied in a typical dry cement kiln in China. The operating conditions were scrutinized at various positions by monitoring the concentrations of CO, NOx, HF and HCl in the flue gas. Furthermore, the concentration, gas/particle partition and congener distribution of CBzs and PCDD/Fs were analyzed at the outlet of cyclone pre-heater, suspension pre-heater, raw mill and stack. The main PCDD/F formation zone was proven to be the first stage of pre-heater and the congener distribution of PCDD/Fs was significantly affected by the recycling ash. Specially, the concentration of CBzs continually increased with the flowing of flue gas. The final emissions of PCDD/Fs and CBzs in the flue gas were 0.016 ng I-TEQ Nm–3 and 26 µg Nm–3, respectively. Moreover, the reduction efficiency of I-TEQ via bag filter was up to 81% and 86% for gas and particle phase PCDD/Fs, respectively. Most importantly, the mass balances indicated that raw meal was the dominant input and the rotary kiln incinerating waste was clearly a weighed PCDD/F and CBz sink. Furthermore, HCBzs correlated well with the total I-TEQ during the whole process of cement production.

Refractories in Cement-making

A hybrid intelligent control strategy for raw meal supply process has been proposed and successfully applied to an annual clinker production capacity of 0.73 million ton of Jiuganghongda Cement Plant in China. The raw meal supply process is a time-varying, long time delay and large inertia system. In addition, a large fluctuation of the raw meal flow affects the stability of the raw meal silo weight. There is often a phenomenon that silo weight is close to zero or overweight, resulting in discontinuity of the raw meal decomposition process in calciner and the clinker calcination process in rotary kiln. Therefore, it is difficult to control efficiently the process by conventional control methods. In this paper, a hybrid intelligent control method that incorporates PI control and fuzzy control with rule-based reasoning feedforward control is presented. Condition identification module in switching rules identifies current conditions according to the input variables. Industrial application results show that the hybrid intelligent control method has high accuracy and robustness.

Most of the research of building materials companies at present focuses on recycling waste to minimize the cost of their products and for the safe disposal of these wastes. In this work, the by-pass cement kiln dust (BCKD), the dust that is discarded from the kiln system was used for the preparation of roof tiles. It is partially used to substitute clay in percentages from 5% up to 25%. to prepare roof tiles abiding by ASTM C1167 for two objectives, the safe disposal of waste and the production of low-cost roof tiles. The raw materials were assessed by particle size analysis, XRD, and XRF. The mud was poured into moulds with dimensions of 150 * 30 * 25 mm3 and pressed under a pressure of 10 MPa then dried. The dried samples were fired to temperatures 1000, 1100, and 1150 °C with a soaking time of one hour for each temperature. The linear firing shrinkage, water absorption, bulk density, and breaking strength were measured to determine the properties of the fired specimens. The initial rate of absorption and runoff properties were also determined for the optimal samples. The results showed that samples containing 8% and 12% of BCKD waste that fired at 1100 °C and 1150 °C had cold water absorption of 14% and a saturation coefficient of 0.85, both of which were below the maximum values of the standard limits. Furthermore, the measured breaking strength of about 2700 N was much higher than the necessary minimum value. The results showed that samples met the requirements of ASTM 1167 for clay-type roof tiles containing 8% BCKD waste and fired to 1100 °C. For economic and environmental benefits, 1100 °C firing temperature is recommended than 1150 °C. The initial rate of absorption was found to be 2.8 kg.m−2 and the runoff properties were found to be 0.02 mg.l−1, 7.53, 110.5 mg.l−1 for total dissolved solids, pH and total solids respectively.

Simulation of heavy metals behaviour during Co-processing of fly ash from municipal solid waste incineration with cement raw meal in a rotary kiln