Dirt bypasses in the cement industry

First: Introduction:

The dry method was introduced in the cement industry in the Arab Republic of Egypt at the beginning of the last century, and the wet method was the prevailing one for this industry. The dry method produces dust from various sources in the production line, the amount of which is approximately three times the dust produced by the wet method. Part of it is recycled in the same industry, and remains left behindThe dirt of the bypasseswith no benefit from it until now.

Second: Composition of the dirt of the side paths:

The primary raw materials extracted from cement manufacturing quarries - which are clay and limestone - contain alkali salts: chlorides and sulfates. Sulfur is also present in raw fuel, which is oxidized to sulfur trioxide gas during the burning process. Due to the abundance of oxygen, the fuel also includes alkalis.

The elements (potassium, sodium, sulfur, and chlorine) are called rotating elements. These elements volatilize in the calciner during the production of “Clinker” at temperatures ranging between 700 and 1100°C, then they condense in the less hot areas, are absorbed into the raw material stream, their concentration increases, and then volatilize again and occur again. The phenomenon is that an internal circle is formed, resulting in solid materials that impede the continuous flow of raw materials inside the furnace, and causes a complete blockage of the raw materials flow. Which requires stopping production equipment for cleaning until it is restarted again.

Figure No. 1: Dirt of cement side passes (bypass) in the “cement clinker” production line.

The rotary phase reactions produce sulfate salts, alkali chlorides (sodium and potassium), and calcium sulfate. The proportions of these salts vary according to their content in the raw materials. The percentage of salts decreases in raw material quarries in rainy countries; Due to its easy dissolution, it increases in countries with dry climates.

A side passage hole was opened before entering the rotary kiln as shown in Figure No. (1); To withdraw part of the hot gas stream, including the dust it contains, to the outside, reduce volatile elements from the furnace line, and prevent blockage of the blast furnace inlet.

Third: How much dust:

The accumulation of corridor dirt amounts to about 5% of the total daily productivity of each production line, meaning more than one hundred tons/day for each production line. The majority of this dirt is currently disposed of by burial and is not of use. Which makes it a burden on the environment and affects the national economy.

Fourth: Properties of dust:

Cement bypass dust is: “a soft white substance similar in softness to cement. Its chemical composition is variable and depends on the proportions of volatile elements in the raw materials and fuel and the operating conditions of the bypass suction system.”

In addition to the salts mentioned as a result of the rotating stage, free lime resulting from the calcining of limestone comes out with the stream of dust drawn from the side passage. Double salts of dicalcium silicate and carbonate may also be present and some cement returned from the incompletely fired kiln.

Chlorides in the dirt of the bypasses range between 2 and about 9%, and sulphates may reach about 12%, and consequently the proportions of sodium, potassium, and calcium change.

The percentage of loss by burning at 1000 m varies depending on the degree of lime carbonation, the length of time and the method of storage, and the percentage of free lime in newly collected active dust may exceed 20%.

Lime turns into inactive calcium carbonate when exposed to air during storage. Likewise, the contents of the returned cement, represented by the elements silica, alumina, iron oxide, and magnesia, vary in chemical analyses.

Fifth: Utilizing the dirt of the side paths:

Methods of utilizing the dirt of side lanes depend on the time of collection and storage. This can be summarized in two basic points:

• Active dust collected from the bottom of cyclones immediately after pulling, and its quicklime content is high.
• Dust that has been exposed to the air loses the property of active lime; To convert it into calcium carbonate.

A. Active dust:

The presence of free lime in active soils can be exploited as follows:

1. In the manufacture of sand bricks using an autoclave (under pressure and heat).
2. As fertilizers to increase soil alkalinity, treat organic waste as compost, and treat sewage sludge.
3. in wastewater purification; Because of its ability to adsorb chromium(III), lignin, and heavy metals, and to assist in the precipitation of phosphate (coagulator).
4. In the absorption of sulfur trioxide (desulfurization) and carbon dioxide in special units; To get rid of these gases.
5. In the work of whiteness.
6. In the rubber industry instead of zinc oxide.

B. Stored dust:

Properly stored dirt, in which free lime has been converted to calcium carbonate, can be used as follows:

1. As a stabilizer for soil and sand dunes.
2. As a filler for flowable fill materials.
3. As a filler for fiber-reinforced concrete composites or glass/polyester composites.
4. As a filler for sidewalks and roads.

C. Salts in active dust:

Alkali chloride salts can activate pozzolana and accelerate the degradation of silicates, in addition to the free lime needed to activate pozzolana. This principle can be applied in vast areas with great environmental benefits. Due to the possibility of obtaining pozzolanic materials from many wastes, such as the products of burning coal in power generation plants, burning organic garbage, burning sludge not mixed with heavy elements from industrial waste, and burning agricultural waste in biofuel generation units, in addition to the possibility of obtaining high-quality pozzolana from burned clay, and its availability in the waste of some well-known industries such as the ferrosilicon industry, that is, the dirt of the bypasses can be exploited to produce environmentally friendly building materials. And economical.

D. Calcium sulphate in dust:

This property is exploited in the activation of granulated iron slag; To obtain a good weld material, studies have been done on adding earth to hot molten slag; For immediate disposal of volatile alkali salts.

H. Dust in the glass industry:

The studies succeeded in finding areas for manufacturing glass in its various forms from the dirt of the side lanes. It was possible to manufacture pressed glass, fiberglass, and sponge products.

Sixth: Dust in its condition in the Portland cement industry:

Studies have shown the possibility of adding different percentages of dust that vary depending on its composition to the kiln feed (about 1%), or replacing part of the clay (maximum 5%), or replacing limestone with dust (up to about 4%). Other research has also addressed this topic. When cement production was carried out by the wet method, this method represented one of the immediate solutions in reusing the dust, but it is used with limitations in the method. Dry.

Seventh: Dust in its original condition as a replacement for Portland cement in building materials:

Many studies have been conducted in this field and have concluded that the permissible percentages of chlorides and sulfates in cement mortar, concrete, clay bricks, ceramics, tiles, artificial stones, and blocks must be adhered to in accordance with the Egyptian specifications for these industries.

Eighth: Treatments proposed to get rid of dust salts:

Several researches have been conducted that serve the problem of salt disposal, but their applications are limited. These attempts can be summarized as follows:

A. Burn treatment:

The idea of treating side lane dust by burning depends on the volatilization of alkali salts at temperatures between 700 and 1000°C. To obtain an alkali-free product rich in free lime, burnt dirt contains calcium sulphate, and is successfully used in the activation of granulated iron slag.

In the 1990s, the Helwan Cement Factory entered into application with this theory, and established a dust treatment unit by burning with the German company Lurgi. The burning process was also proposed in a calciner in a fluidized bed reactor or in a shaft kiln or a precalciner. Tests also showed the possibility of re-burning the dust. To obtain low quality cement clinker or colored cements when some suitable oxides are added.

B. Water washing treatment:

Extensive tests were carried out to get rid of salts from side passage dust, the most famous of which was presented by the Center for Metals Research and Development in the 1990s. However, these experiments did not address how to get rid of washing water rich in these salts, and they did not find any resonance. It is worth noting that one of the famous European companies has finally succeeded in designing a unit for treating these solutions and extracting high-purity salts with economic benefits. Large.

Ninth: Current dealing methods for transporting dust, suggestions and solutions:

The dust is transported from the cyclone collection areas by workers, and is placed in - sometimes - closed cars for transport to open storage areas located in an intermediate area between the factories.

Storage areas do not have barriers or barriers to air currents, which facilitates the transfer of dust to neighboring places, and causes extensive environmental damage. In the 1990s, the National Research Center presented a solution to convert fine dust into granules through a granulation unit that is easy to transport and store.

Tenth: The use of alternative fuels from municipal waste and their impact on the dirt of bypasses:

Given the start of using municipal waste as an alternative fuel in the Arab countries, it is expected that the percentage of dirt in sideways will increase from the current rate of chlorides it contains, and this will result in an increase in the environmental, economic, and industrial burden. Rainy Western countries that did not face the problem of dirt paths began; Because the raw materials are free of volatile elements, taking into account this phenomenon and trying to find appropriate solutions.

Eleven: New technology for bypass systems:

In 2009 AD, it was announced that new systems for side dust gases were introduced in the Heidelbergcement cement factory in Germany to solve this problem. The gases were returned to the clinker cooler, the SOX gases were treated with hydrated lime, and dust feeding materials were added. To facilitate handling and recycling in the production line.

Twelve: Proposals to contain the problem of side corridor dust:

A- Conduct a quick inventory of the actual quantity of this waste.

B- Conduct an urgent study on how to absorb this waste in the various industries mentioned in this report.

C- Beginning the implementation of processing units and offering them to factories; To speed up and facilitate reuse.

D- Making connections with local and global technologies; To contribute to developing quick solutions to this problem, and to coordinate among interested parties. To ensure the desired success is achieved.

E- Establishing a site in the Arab Cement Union and the Academy of Scientific Research jointly with universities and research centers interested in this topic; To inventory and publish all published research and patents; To stimulate applications and innovations, and allocate research and application budgets to the competent authorities. To finance solutions.

And - Issuing a guide for the reuse and recycling of driveway dirt to guide factories, while holding awareness workshops on the content of the booklet.

G- Providing a summary of the most important points related to cement soil to the Arab Cement Union; To be presented to the League of Arab States within the framework of the Clean Production Project in the Arab Cement Industry.

Note:

The dirt from the bypasses in the cement industry has been included among industrial waste in the Egyptian waste code. To determine the limits of the use of waste in building materials.