| الملخص | ABSTRACT
Due to the availability of cheap and good quality limestone and the abilities to produce PCC using the carbonation process route, this study was done with a bench scale experiments. High purity limestone of Wadi-Ghadaf calcined to produce quick lime which is converted to calcium hydroxide by slaking, then high purity powder 99.5 % of CaCO3 is precipitated with 95% brightness and a particle size range 6-12µ via injection of CO2 in slaked lime slurry. The optimum conditions were optimized in each production stage. Technological route of 3,500 ton/year PCC production was proposed with the required equipments, estimations of investment and production costs were obtained in preliminary economical feasibility study, indicated a yearly profit 524,000,000 ID beside other economical benefits that the project supplied.
Keys: PCC, Limestone, Paper Industry.
INTRODUCTION
Industrial CaCO3 is produced by two ways; by extracting and grinding the natural ore GCC and by chemical precipitation PCC. Synthetic PCCs with a very fine and controlled particle size offer a range of technical effects beyond the capability of GCC fillers and other more expensive and sophisticated additives used to improve physical characteristics of finished products. PCC is considered as one of the most versatile chemicals used in the modern paint/coatings, rubber, plastics, glass, textiles, ink and paper making markets. Various chemical routes, however, have been followed to precipitate the CaCO3 (Teir et al., 2005) but the most frequently used methods where based on the double decomposition of Na2CO3 with either Ca(OH)2 or CaCl2. The first one called the Lime Soda Process, the second method is the Solvay process. PCC was also produced from the reaction of Ca(OH)2 with bicarbonate solution which is generated from water softening process. However, the most widely process used of carbonation was carried out with CO2 gas injection in an aqueous suspension of Ca(OH)2 "milk of lime", due to the cheap raw materials used (Bleakley, et al., 1994)
MATERIALS AND METHODS
• Materials: Glass-grade Limestone deposits of Wadi-Ghadaf, which have a reserve amounting about 38,450,000 ton. The quality of limestone is depends on its purity especially the presence of colored impurities such as Fe2O3 (the main factor), MgO, and insoluble maters. The chemical composition as well as XRD pattern of Wadi-Ghadaf limestone sample which was received from that supplied to glass industry is shown in table (1) and fig. (1).
Carbon dioxide: CO2 gas used is a compressed gas.
• Methods : The process which is performed in this work is comprised of three stages:
Calcinations of Limestone: Crushed limestone with particle size (4-10, 2-4, and 1-2) cm and a ground sample of (75µ) is burned in a muffle furnace, to decompose limestone into calcium oxide (quicklime) and carbon dioxide:
CaCO3 (s) →CaO(s) +CO2 (g) ΔH =165.54 kJ/mol. …….. (1)
Samples of limestone were calcined at different temperatures (800, 900, 1000, 1100, and 1200) °C for variable residence time (30, 60, 90 and 120) minutes and a rate of heating of about 10°C/min. At the end of the calcination, the weight of the cooled quicklime determined immediately to calculate the loss in weight for each test.
Slaking of lime: Quicklime, produced from the calcination was slaked with excess of water in different solid (15, 20 and 25) wt%. The slurry was stirred vigorously for variable time periods (15, 30 and 60) min. Reaction can be represented as follows:
CaO(s) + H2O(l) → Ca(OH)2(s) ∆H =-65.47 kJ/mol. …….. (2)
The slurry (milk of lime) was screened through (45µ) sieve to remove impurities originated from the limestone. Samples were drawn from that slurry for chemical and XRD analysis to determine its contents.
Carbonation of slaked lime: The milk of lime was carbonated by bubbling carbon dioxide gas into the slurry to precipitate CaCO3 according to the hydrothermal reaction:
Ca(OH)2(s) +CO2(g) →CaCO3(s) + H2O(l) ΔH=-112.48 kJ/mol. …… (3)
The precipitation of calcium carbonate, however, was studied at various reaction time (15, 30, and 60) min., carbonation temperature (10, 35, 50, and 90) °C, solid percentages (10, 15, and 20) wt % and different CO2 flow rate (708, 424.8, 283.2, 212.4, and 141.6 lit/h). The purity of PCC products were tested by chemical analyses and X-ray for the process optimization.
DISCUSSION AND CONCLUSION
Calcination of Limestone:
Effect of Calcination Temperature: According to eq.(1), theoretical Wt% loss for the dissociation of limestone into CaO and CO2 gas is 44%. It was found from experiments that the higher the calcination temperature the higher the loss in weight which is mean that more conversion occur, but it can be noticed that after 1000 oC, there is no significant increase in the weight loss. Therefore, a temperature of 1000 oC can be considered as an optimum temperature for limestone calcination.
Effect of Calcination Time: From experimental work it is clear that the retention time of 30 minutes has the lowest weight loss%. This is occurred because the core of the limestone pebbles remains calcium carbonate while the outside converted to calcium oxide. So, about 98.2% of the theoretical loss Wt% due to the CO2 emissions occurred during the first 60 minutes of calcination. XRD pattern of CaO produced at optimum calcination is shown in fig. (2).
Slaking of lime:
Effect of Limestone particle size: Limestone with a range of 4-10 cm resulted in a higher weight loss during calcination at 1000 oC. This is may be due to the re-hydration occurred with the reduction in particle size. Also limestone with 4-10cm particle size suitable for the next stage of slaking, producing more homogeneous slaked lime. Vertical shaft kiln seem to be suitable with this particle size.
Effect of Solid (wt.) %: As it can be extracted from the experiments, the Ca(OH)2% increased as solid% increases from 10 to 15% and slightly increase from 15 to 20% but it is decreasing beyond 20%solid. However, the Ca(OH)2% shows a sharp decreasing when lime solid % increased from 20 to 25%. Taking into consideration the economical factors, 20% of solid lime seems to be reasonable. When the amount of slaking water increase, the rate of hydration retarded and mutes the heat evolved in the diluted mixture which is retarded the slaking and incomplete hydration occurs. Also adding insufficient water resulted in incomplete hydration of the lime (Boynton, 1980).
Effect of Slaking Time: From the results of the experiments, it can be conclude that 15 minutes of slaking time is an optimal time for slaking and beyond it almost no hydration occurred. A conversion of Ca(OH)2 of about 90.7% was obtained at 15 minutes. Hence, different periods of time were applied to detect the best slaking time. The hydrator typically provides an average residence time of 10 to 15 min.
Carbonation of Slaked Lime:
Effect of Carbonation Time: It can be easily notice that carbonation time has significant effect on the purity of the produced PCC. Hence, at 60 minutes remarkable PCC productivity of 99.73% was produced.
Effect of Carbonation Temperature: As it can be observed, the best PCC productivity was 99.73%, and it can obtained under a range of temperatures (30-40) oC, according to the results in table (2) the best temperature for carbonation producing PCC with homogeneous dominant particle size of about (12 µ) is 35 oC. While carbonation at 10 oC produced PCC with more fine particles of about (6 µ) which is more preferable. The XRD of the slaked lime under the optimum conditions is shown in fig.(3).
Effect of Slaked Lime Solid wt %: As it can be noticed, and as hydrated lime solid% increases beyond 15%, PCC percentage decreased.
Effect of CO2 gas Flow Rate: As it can be noticed, every increase in CO2 gas flow rate, reflected by an increase in PCC%, a considerable high productivity of PPC 99.27% obtained using a flow rate of CO2 gas of about 425 (lit/hr)/kg.
The optimum conditions of PCC production can briefed as; a flow rate of 425 (lit/hr) of CO¬¬2 gas per kg of calcium hydroxide, carbonation temperature 10 oC, carbonation time 60 minutes and a solid percentage of about 15%. XRD of the product is shown in fig.(4) with a brightness of (95%) .
PROJECT FINANCIAL PARAMETERS
Preliminary economical feasibility study indicate, at a 3500 ton/year production capacity of PCC, and with a 1,214,000,000 ID investment cost, the total production coast is of about 1,031,162,000 ID, with yearly profits of 439,000,000 ID, and a recapturing period of about (2) years.
REFERENCES:
1- Bleakley S., Johns R., Precipitated Calcium Carbonate, U. S. Pat. 5,342,600. 1994.
2- Boynton S. Robert, 1980. Chemistry and Technology of Lime and Limestone, 2nd Ed., Wiley-Interscience Publication.
3-Teir S., Eloneva S., Zevenhoven R, 2005. Production of precipitated calcium carbonate from calcium silicates and carbon dioxide, Energy Conversion and Management, Vol. 46, pp. 2954-2979.
Table (1): Raw Limestone of Wadi Ghadaf Composition
Composition CaO L.O.I Fe2O3 MgO SO3 AL2O3 P2O5 Cl
% 54.78 43.12 0.089 0.15 0.07 0.13 0.04 0.06
Table (2): PCC product particle size and structure at deferent carbonation temperatures.
Carbonation Temperature oC 10 35 50 90
Dominant Particle Structure sub rounded sub rounded sub rounded, sub hedral sub rounded,sub hedral
Dominant Particle Size (µ) 6 and12 12 12 and 24 12 and 24
Fig.1: XRD of Wadi Ghadaf Limestone. Fig. 2: XRD of quick lime produced at optimum conditions.
Fig. 3: | en_US |
