The kraft process is a process for creating wood pulp out of wood, for use in paper production. Unlike many other chemical engineering processes, the kraft process is not named after its inventor, but instead derived from the German word kraft, meaning “strong.”
This name was chosen by the inventor of the process himself, Carl Ferdinand Dahl, who intended to market the superior strength of the paper created from this process.
A resident of Danzig, Kingdom of Prussia (present-day Germany), Dahl invented the process in 1879, and had himself awarded a U.S. patent for the invention on April 15, 1884. His invention was first put into action when a pulp mill in Sweden first began using it in 1890.
The kraft process has undergone significant improvements throughout the century, especially since the invention of the recovery boiler during the early 1930s by G.H. Tomlinson. The innovation helped it surpass the sulfite process, another pulp-making process, in usage and catapulted it to the widespread popularity that it enjoys today.
The kraft process begins with presteaming common wood chips. This involves collecting wood chips that are 12–25 millimeters (0.47–0.98 inches) in length and 2–10 mm (0.079–0.39 in) in width, and wetting them before heating with steam. This causes cavities within the wood chips to be filled with both air and moisture.
After this, the wood chips are impregnated with white and weak black liquor by heating up to 100 °C (212 °F). During this process, liquor penetrates the capillary structure of the wood chips, and saturates them homogeneously throughout.
White liquor, so-named because of its white opaque color, is a strongly alkaline, aqueous solution of sodium sulfide (Na2S), sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), sodium thiosulfate (Na2S2O3), sodium chloride (NaCl), calcium carbonate (CaCO3) and water. However, only the first two (and to a lesser extent, the third) compounds actually contribute to the breakage of extractives–cellulose fiber bonds; the other components of white liquor are considered to be chemically inert.
Black liquor, on the other hand, is simply the residue created from the consumption of white liquor during the previous batches of the kraft process. Black liquor is thus a mixture of woodchip residues in white liquor. Aside from being used as a digesting agent during the early stages of the kraft process, black liquor is also combusted in the recovery burner in order to recover useful compounds from the black liquor and generate extra power for the pulp mill.
The rationale for recycling spent white liquor is pretty simple: economy. Not all of the active components of white liquor are spent up during digestion, and disposing them right after just one use is fiscally imprudent and environmentally irresponsible as well. Black liquor is, as its name suggests, a viscous, aqueous, black liquid that turns water to dark caramel upon contamination, and is very toxic to aquatic life. About 7 tons of black liquor is produced for every 1 ton of pulp manufactured under the kraft process. Recycling black liquor (i.e. spent-up white liquor) greatly reduces the amount of it that goes into our ecosystem.
During digestion, the wood chip–liquor mixture is placed into a highly pressurized vat for several hours at temperatures ranging from 170 to 176 °C (338 to 349 °F). The liquor mixture act to digest the pulpwood into paper pulp by removing lignin (a complex chemical compound found in the wood’s secondary cell wall), hemicellulose (a polymer also found in the cell wall) and other extractives. This is done in order the pulpwood cellulose fibers that are used as ingredient in making paper. Reactions between nucleophilic bisulfide (HS-) or sulfide (S2-) and the woodchip components underpin this step of the kraft process.
Digestion produces a solid pulp known as a “brown stock.” This product is then collected and washed to rid it off the inorganic compounds that came from liquor impregnation. Atmospheric pressure is reduced in the containers in order to let steam arise from the brown stock, and cool them down. Efficiently designed pulp mills recycle this steam to turbines in order to generate electrical power.
Afterwards, the pulp is passed through sieves in order to remove dirt and other unwanted contaminants; and then washed again for several times in order to produce a final product that is clean pulp. Finally, the pulp is bleached to give it paper’s familiar white color. Several chemicals may be added after this process in order to improve the quality of the pulp.
The kraft process produces a lot of by-products, the most notable of them being crude sulfate turpentine and tall oil soap. Both of which can be used as ingredients of a wide range of retail and industrial products, including lubricants, soaps, solvents, inks, binders and many more. Effluent produced by kraft-process pulp mills are extremely detrimental to the environment and should be recycled whenever possible.
Gold is one of the most highly valued metals today, as it has been since the dawn of human civilization. Its rich yellowish color evokes the very idea that we almost always relate to it: wealth. Gold had been used for several millennia as currency in the past, with the United States being the last country to use the gold standard in 1932. Despite the takeover of fiat currency in virtually every jurisdiction, gold remains of high economic value.
This fact is particularly evidenced by the usage of newly mined gold: 50 percent goes into jewelry, 40 percent into financial investments, and 10 percent to industrial usage (such as in electronics, dentistry, and aeronautics). Curiously enough, the applications of gold are so far reaching that it is even used as ingredient in some high-culture cuisines.
As with most precious metals, gold occurs very rarely in nature. This is because of gold’s high density which causes it to sink among other elements found in the Earth crust. Thus, almost all of the gold believed to be in Earth can be found only in the planet’s core.
This is also why practically all of the gold mined from the earth comes from gold-containing meteorites that have crashed in an earlier geological period.
Because of its rarity and high value, it is absolutely essential that gold extraction processes be as close to 100-percent efficient as they can be.
Gold extraction is the process of recovering elemental gold from gold ores.
Gold typically occurs already in metal form (i.e. it is not chemically bonded to other elements as a compound) as sizable nuggets, which can be as huge as coinage or as little as fine grains of sand. In fact, gold may even occur in microscopic amounts while embedded in rocks. Because of this much of the recovery process is actually focused on two things: increasing concentration and increasing purity from contaminants, which is referred to as “refining.”
Gravity concentration is perhaps the oldest method for increasing the concentration of naturally occurring gold. This is traditionally done by using metal pans to displace lighter materials by effect of centrifugal force, thus leaving the heavier gold nuggets in the middle of the pan. This method of gold concentration still remains in use in many small-scale mining today.
The same principle is used today in industrial settings with the use of sluices. These devices are flatbeds that are lined with troughs that act as trapping mechanisms. By passing a pulp of ore and water, gold is allowed to settle in the troughs, while lighter materials that are generally found with gold (such as silica) simply flow through the sluice and eventually get disposed as effluent.
Efficiency is achieved by maintaining a consistent speed in the flow of pulp that is slow enough to allow the gold to settle in the troughs but fast enough to not let the contaminants, referred to as “gangue,” to do so.
In cases where distinct and visible gold particles still fail to appear despite undergoing gravity concentration (i.e. in instances where original gold concentration is very low), froth flotation may be employed. Froth flotation works by selectively segregating materials in terms of their hydrophobicity (i.e. their property to repel or be repelled by water molecules). Froth flotation is generally used when a high concentration of sulfide minerals are found in the ores.
During this process, surfactants and wetting agents are added into the ore to increase the difference in hydrophobicity.
Froth flotation is usually directly followed by cyanidation.
However, in cases where cyanidation is seen as too environmentally taxing, or where the ore is naturally resistant to the process, roasting or wet-pressure oxidation may be applied before cyanidation. Roasting or wet-pressure oxidation works to remove sulfides that may have associated with gold that may prevent gold from being dissolved during cyanidation.
The following is the reaction during this process: Au2S+3 O2→2 Au2O+2 SO2
Leaching involves dissolving gold with cyanide for later precipitation, which is essential in order to ensure that even microscopic amounts of gold can still be recovered. Cyanidation, also known as the “cyanide process” or the “MacArthur–Forrest process,” is the industry standard for leeching.
It uses the following reaction: 4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH
There are several methods for precipitating gold from the cyanide solution. The most economical of which is the carbon-in-pulp process, which involves passing the leeched pulp through several tanks of activated carbon. The carbon acts as a trap for Na[Au(CN)2], which is then removed from the carbon using high temperatures and pH. Afterwards, the resulting solution is passed through electroextraction (or electrowinning) cells.
As part of the refining stage, this procedure uses electrolysis to allow gold to deposit in the cathode area.
Nitric acid (HNO3), also known as “spirit of niter,” is a very strong acid.
Between 75-80% of industrial nitric acid is used as a raw material in the production of fertilizer.
It is also used in niche industries such as rocket fuel, woodworking (where it is used to artificially age wood) and cleaning stainless steel. It remains an important chemical, however, in the laboratory as it is used as an analytical reagent.
The Ostwald Process
The Ostwald process is the procedure for making nitric acid.
It was patented in 1902 by Wilhelm Ostwald, a Nobel Prize–winning German chemist.
Curiously enough, invention of the Ostwald process is usually credited by academic historians to Charles Frédéric Kuhlmann, who devised the reactions used in the Ostwald process.
Also, the time of invention itself is not believed to be 1902 but 1908 instead. It is said that the then-increasing demands for ammonia and the subsequent completion of the Haber–Bosch process for creating led Ostwald to refine and commercialize the process.
Alternatively, other historians claim that the six-year duration was simply the time it took between the filing of the patent and its granting to Ostwald.
At any rate, both the Haber–Bosch and Ostwald process would prove indispensable to Germany’s war effort as it entered World War I and domestic demand for fertilizers (an ammonia-based product) and explosives (of which nitric acid is a primary component) surged.
The Ostwald process works by converting ammonia into nitric acids in two stages.
The first stage involves oxidizing oxygen by heating at a temperature range of 780 to 950 °C (1436 to 1742 °F) and pressure of 1.4 MPa (14 atm).
This reaction is done in the presence of a 10-percent rhodium platinum gauze catalyst. The reaction is highly exothermic (ΔH = −950 kJ/mol), and produces nitric oxide as follows: 4 NH2 + 5 O2 → 4 NO + 6 H2O
The heat produced from the reaction further oxidizes the nitric oxide into nitrogen dioxide. 2 NO + O2 → 2 NO2
As both reactions are exothermic—the second oxidation reaction has a slightly lower heat of reaction of −117 kJ/mol, but it is still heat-producing nonetheless—it is normal for operating temperatures to eventually exceed the recommended 780–950 °C (1436–1742 °F) range.
One must take in mind that both oxidation reactions presented above are reversible, and shifting the equilibrium in favor of the products side can be achieved by cooling. This is done by passing the gasses through a heat exchanger.
By the second stage, the nitrogen dioxide produced from oxidation is readily soluble in water. Nitrogen dioxide is immediately converted into nitric acid as follows: 3 NO2+ H2O → 2 NHO3+NO
As you can see, nitric oxide is again produced, this time during the production of nitric acid. This nitric acid is collected and recycled for reoxidation using the same reaction.
In cases where the second stage is conducted in air (as opposed to nitrogen dioxide being absorbed by water), the following reaction prevails: NO2+ O2+2 H2O → 4 NHO3
The above flow chart is a very simplified and generalized view of the ostwald process – in reality there is a large amount of heat recovery and other streams involved.
Overall yield of the Ostwald process is pegged at 93 to 98 percent.
This rate is significantly reduced in the event that the following unfavorable side reaction occurs: 4 NH3+ 6 NO → 5 N2+6 H2O
In this reaction, no nitric acid is produced.
To make things worse, ammonia is reconverted into atmospheric nitrogen.
This is due to the gas mixture (ammonia and nitric oxide) interacting in the presence of the platinum-based gauze catalyst. The catalyst adsorbs ammonia and causes it to react with atmospheric oxygen. This causes ammonia to lose its hydrogen atoms (dehydrogenization) and to dimerizate into atmospheric nitrogen.
Product-selectivity in favor of nitric oxide can be promoted by diluting the amount of nitrogen atoms with respect to the amount of oxygen atoms. This in turn forces much of the catalyst-adsorbed nitrogen atoms to react with ambient oxygen atoms instead of dimerizating with other nitrogen atoms.
In other words, increasing the concentration of atmospheric oxygen produces higher nitric oxide (and thus nitric acid) yields.
Likewise, another unfavorable by-product, nitrous oxide (N2O), may be produced by a side reaction between catalyst-adsorbed dehydrogenated ammonia (i.e. bare nitrogen atoms) and nitric oxide: 2 NH3+ 2 O2 → N2O+3 H2O
The production of nitrous oxide can be prevented by simply increasing the temperature of the catalyst in order to accelerate the desorption velocity of nitric oxide, thus allowing it to become reoxidized in the air using the second reaction from the first stage, instead of reacting with other catalyst-adsorbed nitrogen atoms.
Either unfavorable side reactions can be averted by minimizing the time of contact between the gases involved.
Increasing the concentration of the nitric acid product can be done through distillation—that is passing the nitric acid vapor over concentrated sulfuric acid. As the sulfuric acid is aqueous and is more acidic than nitric acid, the former acts as a dehydrating agent for the later.
The end result is a highly concentrated nitric acid solution.