Carbon dioxide-silicate process

K. E. L. Nicholas, The Carbon Dioxide-Silicate Process in Foundries, (British Cast Iron Research Association AIvcchurch), 1972. 

Carbon dioxide-silica process. Sand is mixed with sodium silicate (3.0 to 3.5% of sand volume), and the mixture is blown or hand-rammed into a core box or around a pattern. Carbon dioxide gas is passed through the compacted sand to harden the binder. The bonding strength eliminates the need for drying or baking the mold and metal can be poured into the mold immediately. Over-gassing should be avoided because it makes the mixture friable. 

Alternatives to fossil fuels, such as hydrogen, are explored in Box 6.2 and Section 14.3. Coal, which is mostly carbon, can be converted into fuels with a lower proportion of carbon. Its conversion into methane, CH4, for instance, would reduce C02 emissions per unit of energy. We can also work with nature by accelerating the uptake of carbon by the natural processes of the carbon cycle. For example, one proposed solution is to pump C02 exhaust deep into the ocean, where it would dissolve to form carbonic acid and bicarbonate ions. Carbon dioxide can also be removed from power plant exhaust gases by passing the exhaust through an aqueous slurry of calcium silicate to produce harmless solid products ... 

DeGuide A sugar extraction and purification process in which barium is recycled. Addition of barium hydroxide to molasses precipitates barium saccharate. A slurry of this is treated with carbon dioxide, forming barium carbonate and releasing the sugar. The barium carbonate is reconverted to the hydroxide by a two-stage process involving monobarium and tribarium silicates ... 

The next major raw material for which we discuss the derived chemicals is calcium carbonate, common limestone. It is the source of some carbon dioxide, but, more importantly, it is used to make lime (calcium oxide) and slaked lime (calcium hydroxide). Limestone, together with salt and ammonia, are the ingredients for the Solvay manufacture of sodium carbonate, soda ash. Soda ash is also mined directly from trona ore. The Solvay process manufactures calcium chloride as an important by-product. Soda ash in turn is combined with sand to produce sodium silicates to complete the chemicals in the top 50 that are derived from limestone. Since lime is the highest-ranking derivative of limestone in terms of total amount produced, we discuss it first. Refer to Fig. 2.1, Chapter 2, Section 1, for a diagram of limestone derivatives. 

For geologically short-term processes on the order of a few thousands of years, it is likely that the carbon dioxide carbonate system regulates oceanic pH. The long-term pH is controlled by an interplay of various near-equilibria involving carbonates and silicates. 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

In order for the flotation to be effective, the sludge must be recycled to increase the particle size of the calcium carbonate. This recycle also gives heavier flow in the settling basins, which results in faster settling rates. For the flotation to be effective, the magnesium hydroxide in the sludge must also be completely dissolved by the carbon dioxide and washed out. In the froth flotation process (used to separate the calcium carbonate from the clay, silt, or other water contaminants), an aqueous slurry of the sludge is first conditioned (mixed) with soda ash and sodium silicate to disperse the clay and... 

In the atmosphere CO2 is affected by processes that operate at different time scales, including interaction with the silicate cycle (see Chapter 2), dissolution in the oceans, and annual cycles of photosynthesis and respiration (see also Section 3). The relative effect of these processes is described below in the consideration of the whole carbon biogeochemical cycle and environmental aspects of biogeochemistry. Here, it is important to note that carbon dioxide is not reactive with other atmospheric species its MRT is 3 years (Figure 4). This value is largely determined by exchange with seawater (see Section 2). 

The structural molecules of the skeletons and shells of invertebrates, which function as physical defenses against predation, are important in marine environments, where they produce carbonate and silicate rocks. Deposition in anaerobic environments has also been the basis for the formation of the extensive deposits of gas and oil that now fuel modern industrial societies. Removal of carbon from the biosphere by organisms to produce carbonate rocks, coal, oil, and hydrocarbon gases has been responsible for the presence of oxygen in the atmosphere of the Earth. Reversal of this process by human consumption of fossil fuels has already produced a detectable increase in atmospheric carbon dioxide. 

The weathering of alkaline rocks is essentially a natural form of carbon dioxide capture and storage, but is a very slow process. Carbon dioxide can form stable carbonates through reaction with minerals that contain magnesium or calcium. A typical example would be the transformation of serpentine, a common silicate mineral, to magnesium carbonate, silica and water, i.e.. 

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