preface
Ethiopia's land area of about 1.1 million km², the national statistical population of about 110 million, only 1/5 of the population lives in towns and cities, the current per capita cement consumption in Ethiopia is about 100 kg, according to its economic balance and rapid development trend, per capita demand there is still a certain room for growth. In recent years, Ethiopia's economic growth rate has been maintained at about 7.5%, in order to further stimulate the speed of economic development, the government has also issued a corresponding stimulus economic development plan, the future urbanization rate and the increase in road facilities have put forward higher requirements for cement consumption, cement demand gap will continue to expand. Raw fuels, as materials needed for the production of cement, raw materials, retarders, mixed materials, coal, etc. are all distributed in Ethiopia. Based on the author's many years of actual work experience in cement engineering in Ethiopia, the status of raw materials and materials for cement production in Ethiopia is described below.
01
Characteristics of raw fuels produced by cement
In terms of rock formations, the layers exposed in Ethiopia are precambrian to Quaternary strata, with the precambrian (Neoproterozoic), Mesozoic and Cenozoic strata being the most developed. The main rocks exposed in the formation are Precambrian basal metamorphic rocks (23%), Late Paleozoic-Mesozoic marine and continental sedimentary rocks (25%), Cenozoic basal acidic volcanic rocks (34%) and volcanic sedimentary rocks (18%).
Ethiopia is located in a highland area, with an average altitude of nearly 3000m, and its substances are abundant, and a variety of substances used in cement are basically preserved. Among the various raw materials required for cement production, the various raw materials for firing mature materials generally come from various types of sedimentary rocks, and the Ethiopian sedimentary rocks are mainly Mesozoic limestone, sandstone, clay, gypsum, etc.; Pumice, volcanic ash, and tuff, which are cement mixtures, are mostly volcanic sediments and are widely distributed in the territory.
At present, the cement standard implemented in Ethiopia is "CES28-Compusory Ethiopian Standard, First Edition 2013", which refers to the main components of cement clinker, mixed materials, retarder substance types, doping ratios, etc. in EN 197-1:2011, and there are no special requirements and regulations. The following is a separate introduction to crude cement fuels in conjunction with the status of rock formations in Ethiopia.
1.1 Characteristics of raw materials for cement production
In the production process of cement engineering, the demand for raw fuel is very large, so the clinker line of the cement plant site generally needs to be close to the limestone mine; Cement grinding, on the other hand, is generally close to the main sales area of cement products. Limestone, clay, sandstone, basalt, as well as retarder gypsum and mixed material pumice and volcanic ash, which are required for cement clinker production, have considerable resource distribution in Ethiopia.
According to the understanding of the raw materials used in the production of existing cement plants, limestone and sandstone are basically widely used in various plants, and the silicon aluminum raw materials are mainly clay and basalt, and some factories use volcanic ash as silicon aluminum raw materials. In clay and basalt, there are considerable iron components, and there is generally no need to add iron correction raw materials, and the raw materials are shown in Table 1.
Table 1 Chemical composition of raw materials used in cement plants in Ethiopia (%)
Figure 1 Stock information in Ethiopia
Limestone as the main calcareous raw materials required for cement production, accounting for about 80% of the raw material consumption, which mainly comes from the Mesozoic Late Cretaceous limestone mines, mostly marine sedimentary sources of limestone outcrops are larger, strong stratification, the composition between the layers according to the sedimentary sources have certain differences. Due to the influence of the formation of the rift valley belt in Ethiopia since the Cenozoic Era, the geological tectonic activity is intense, and there are often mixed with different sources of materials during the sedimentary period, such as flint nodule [see Figure 1 (a)], gravel, etc., these mixed components have a certain adverse impact on the grindability and burnability of raw materials, generally speaking, the content of flint silica in the limestone entering the factory is required to be within 4%, and the mix of high and low grade limestone should be done. In many limestone mines, a considerable amount of marl has been found nearby, which is also a good calcareous raw material, which helps to improve the grindability and burnability of raw materials.
As a siliceous correction raw material, the maturity of sandstone in Ethiopia is relatively high, the content of silica is mostly above 90%, and macroscopic visible is generally fine grained, or mixed with quartz gravel block cemented rock [see Figure 1(b)]. The existence of quartz sandstone with high roundness and maturity will have a certain adverse impact on the grindability, burnability and corresponding clinker quality of raw materials.
The silicon and aluminum raw materials of the Ethiopian cement plant are mainly clay, basalt, and a small amount of volcanic ash. Cenozoic magmatic rocks are mainly volcanic exposed basalt, and a large number of basalt eruptions form the Central Plateau of Ethiopia, composed of alkaline laban basalt and fused tuff. The basalt distributed in Ethiopia is larger in volume, and the sediment clay produced by weathering has a high iron content, and generally does not need to add iron correction raw materials. However, due to the short geological history of the formation of the clay layer on the surface of Ethiopia, the sedimentary thickness of the clay is generally not large, and the amount of clay required for the production line with a daily output of 5,000 tons of clinker is about 500,000 tons.
Clay is soil for local agricultural production (multi-inguila) and is a valuable land resource for ethiopia, a highland country, so there is a certain sustainability in the use of clay as a raw material for silicon aluminum cement. In this regard, certain shale resources can be selected as an alternative or supplement. In view of the shortage of silicon aluminum raw materials, there are cement plants that use volcanic ash as silicon aluminum raw materials, but their high temperature reactivity is poor, and the quality of clinker involved in firing is generally poor.
1.2 Characteristics of cement production mixtures
At present, Ethiopia's industrial base is weak, and the corresponding industrial waste residues such as slag, fly ash and other materials are less, and the mixed materials that can be used at present are mostly natural materials. Due to the intense magmatic activity since the Cenozoic period, the pumice, volcanic ash, tuff, etc. produced by it have a relatively large distribution in Ethiopia, which can be mixed as a mixture of clinker and ground into cement. Ethiopia has a rainy season of more than 4 months, pumice [Figure 1(c)] has a high porosity and strong water retention, and its high moisture needs to be considered when utilizing it, and the impact of its high moisture on mill yield and possible effects on cement performance. The unity of the mixed material determines that the cement varieties are mainly volcanic ash cement and ordinary Portland cement.
1.3 Characteristics of cement production fuels
Ethiopian coal mines are mainly endowed in Precambrian schist and Late Miocene-Pliocene lake sediments, and the formation period of coal is relatively short, about 13 to 15 million years, mostly distributed in the central plateau and the west side of the rift valley belt, mainly lake-Xiang sedimentation, and the foliar characterization is obvious [see Figure 1(d)]. The coal mines that have been found are mainly lignite, with high ash content, low fixed carbon, high average moisture content and volatile content (see Table 2).
Table 2 Results of the analysis of the fuel industry used in the ethiopian cement plant
Ethiopian cement plants generally use South African coal as a firing fuel, but in recent years the national government has promulgated relevant regulations on the use of domestic coal resources, that is, domestic cement plants must use a certain amount of domestic lignite to stimulate the use of domestic lignite, thereby reducing dependence on foreign information and reducing foreign exchange expenditures. Individual cement plants have tried to use petroleum coke in South Sudan or the Middle East as fuel to reduce production costs.
According to the author's comprehensive analysis of multi-layer coal in a coal mine, the thermogravimetric analysis of the combustion performance of this typical lignite shows that the combustion start temperature of this lignite is 327 °C, the temperature is low, and its combustion end temperature is 491 °C, so it can be burned completely in a short temperature range. However, its volatile content is higher, and the volatiles can be precipitated and burned at lower temperatures. The storage and grinding process of such coal need to be controlled in a certain temperature range, which has a certain safety risk of use, and this feature needs to be considered in the relevant process design.
Figure 2 Thermogravimetric analysis of local lignite in Ethiopia
According to the above characteristics, local lignite can not be used alone as a firing fuel, in the high quality requirements of the firing kiln head fuel is generally used with high calorific value imported coal, low calorific value coal domestic lignite is generally used in the tail of the sintered kiln, and need to control its calorific value within a certain range, see Table 2.
02
epilogue
(1) Ethiopia's cement raw materials are mainly limestone, sandstone and clay, etc., and do not need to add iron correction raw materials separately. Due to the short geological formation period, limestone, sandstone, etc. are formed in sedimentary rocks since the Mesozoic Era, of which the maturity of sandstone is high, which has a certain impact on raw material grinding and firing. Clay has a high iron content and generally does not need to add iron correction raw materials.
(2) The source of mixed materials is relatively single, and the cement varieties are ordinary Portland cement and volcanic ash cement. Mixed materials are mainly natural materials - pumice, volcanic ash, tuff, limestone, etc., and the unity of mixed materials determines that its cement varieties are mainly volcanic ash cement and ordinary Portland cement.
(3) The local fuel is mainly lignite, and its thermal performance is difficult to use as cement firing fuel alone, and it is also necessary to import high calorific value fuel mixture.
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