Processes

Pulp and Paper Production - The Kraft Process Overview

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.

Pulp and Paper MillThe kraft process has undergone significant improvements throughout the century, especially since the invention of the recovery boiler during the early 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.

Industrial Gold Extraction Process Overview

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. 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.

Gold Bullion

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.

Ostwald Process Overview - Industrial Ammonia Production

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.

Nitric Acid

The Ostwald Process

The Ostwald process is the procedure for making nitric acid.

It was patented in 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 but 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.

Stage One

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.

Stage Two

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

Ostwald Process

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.

Industrial Aluminium Smelting - Hall Heroult Process Overivew

Aluminum is the most widely used nonferrous metal in the world today.

As of, world aluminum production was at 44,100 kilotons (about 41 percent of which was smelted in China alone).

Its popularity is unsurprising actually considering that the metal (both in its pure and alloyed forms) is renowned for its resilience despite its light weight, its high conductivity and its ability to withstand corrosion. And of course, we should not discount that it is, after all, the most abundant metal in the Earth’s crust.

In spite of this, aluminum production does present one main challenge: finding it in pure, elemental form is extremely and impractically difficult—this fact owing much to aluminum’s very high chemical reactivity. That is why, as with most metals, it is first mined in ore form before undergoing purification.

The Bayer process is the first of which, and is concerned with extracting aluminum oxide from bauxite ore. Aluminum oxide, also known as alumina, produced from this process is further refined into pure aluminum using the Hall–Héroult process.

 

 Charles Martin Hall

The Hall–Héroult process starts with the dissolution of alumina in molten cryolite (Na3AlF6; sodium hexafluoride) vat, which is also known as a “cell.” A small amount of aluminum fluoride is added into the alumina–cryolite mixture to reduce the melting point of cryolite from 1,012 °C (1,854 °F) to about less than 1,000 °C (1,830 °F). Take note though that alumina has a melting point of more than 2,000 °C (3,630 °F).

Thus only a small amount of alumina actually dissolves in the cryolite vat, which is then passed with an electric current.

The current itself is used to heat up the cells, and the electrolysis resulting therefrom leads to deposition of pure aluminum at the cathode as precipitate, while the anode, on the other hand, serves to produce carbon dioxide. Liquid elemental aluminum is siphon-transferred either in batches or by continuous flow to casts where they solidify into ingots or final-cast products.

 Paul Heroult

The electrolysis of the alumina–cryolite mixture is mainly driven by the amount of current passed through the vat. Voltage, on the other hand, is a no brainer: electrolysis can be performed efficiently in electric potentials of five Volts to as low as three Volts. The voltage requirement is further reduced as the carbon dioxide anode is oxidized. This means that anodes would have to be constantly replaced in order to increase electrical efficiency and thus minimize associated power costs.

There are two methods that can be employed in manufacturing anodes: the Söderberg and the prebake processes.

  • The Söderberg uses the continuous addition of pitch to the top of the anode. The pitch is readily baked by the excess heat created by the electrolysis of the alumina–cryolite mixture into carbon form, which is used to react with the mixture.
  • The prebake process involves, as its name suggests, the prebaking of pitch in large gas ovens prior to dipping into the mixture—this is as opposed to baking them onsite as with the previous method. The latter process is preferred because it is slightly more efficient and produces less greenhouse emmissions.

The rate of production being proportional to the amount of current, industrial smelting cells usually consume hundreds of thousands of Amperes at any given point in time during operation.

Meanwhile, this enormous amount of current can create a significantly strong magnetic field within the vats—one that is large enough to cause alumina–cryolite mixtures to swirl (and thus aid the further dissolution of alumina in cryolite) on their own even without mechanical assistance. This is a phenomenon often exploited by cell engineers to minimize cost and maximize efficiency.

Power is supplied by in-site transformers which convert grid-sourced alternating current to direct current. The large demand of aluminum smelters for power naturally makes sites where cheap and constant supply of power is readily available an undeniably popular option.

To be precise, hydroelectric power plants are the power source of choice in the developed world, where aluminum smelting factories are usually built a few kilometers from these plants in order to minimize transmission costs as well as the possibility of intermittent power outages due to transmission-side problems.

The Hall–Héroult process in itself was a very large improvement from previous methods, which involved heating bauxite with pure sodium or potassium inside a vacuum. These methods were far more complicated and resource intensive.

The high prices of sodium and potassium then contributed all the more to the historical prices of aluminum, so much so to the extent that aluminum during the early 19th century were far more costlier than gold, silver or platinum. (Interestingly, Napoleon III of France has been fabled to have kept his aluminum silverware only for use of his most important guests.)

With the advance of smelting technology, aluminum prices did go down, but it was only through the high efficiency of the being the Hall–Héroult process that we enjoy cheap aluminum prices today.

Various alternatives have been explored to replace the Hall–Héroult, many of them seeking to minimize the carbon footprint that it produces. There have been no convincing contenders as of yet, however, and the Hall–Héroult process remains exclusively to be the method for aluminum production.

Overview of Industrial Zinc Production Process

Since ancient times, zinc ores were converted into zinc compounds having healing properties. Metallic zinc was first produced in India in the 14th century and then in China in the 17th century.

Ten million tons of zinc is now produced annually around the world; 80 per cent of the mines exist underground, 12 per cent are a combination of underground and open pits and the remaining 8 per cent are solely open pits.

They contribute 64, 21 and 15 per cent to the overall zinc production volumes respectively. In all cases, the mined ore contains copper, iron, lead, silver and other mineral impurities, and the zinc content is only around 10 to 15 per cent.

The ore is crushed and ground at the mining site to increase its zinc content to 55 per cent. It forms a concentrate, which is the raw material used in industrial zinc production processes.

Extraction of Metallic Zinc from the Concentrate:

The zinc concentrate contains up to 30 per cent of sulfur with traces of lead, iron and silver. There are two main processes to recover metallic zinc from this concentrate, as described below.

1. Hydrometallurgical or Electrolytic Process:

  • The zinc concentrate is roasted or sintered at 900 degrees Celsius in the presence of air to get rid of the sulfur content. Zinc sulfide converts to zinc oxide, while sulfur and oxygen react to create sulfur dioxide gas; sulfuric acid is the main byproduct of the commercial zinc production process.
  • The impure zinc oxide is called calcine; it is powdered and mixed with diluted sulfuric acid through a process called leaching to separate zinc from iron. Zinc oxide is quite active and contains dissolved metallic zinc. Iron precipitates while lead and silver do not dissolve; hence these substances can be easily removed.
  • Cadmium, cobalt, copper and nickel dissolve in the solution, so further purification of the dissolved solution is carried out by adding zinc powder. Zinc is a reactive substance; it oxidizes and dissolves while the other impurities lie below zinc in the electrochemical series and hence convert back to metallic form; they are separated through precipitation.
  • The purified zinc sulfate solution is electrolyzed between aluminum cathodes and lead alloy anodes with a difference of 3.5 Volts. The electric current passing through the electrolytic solution for about 48 hours causes pure zinc to get deposited on the cathode.
  • The metallic zinc can be easily stripped off from the aluminum cathodes. Once it dries, it is melted and cast into high grade (HG) or special high grade (SHG) zinc ingots of 99.95 and 99.99 per cent purity respectively.

2. Pyrometallurgical or Imperial Smelting Process:

This is another process used in 10 per cent of the zinc production cases. It produces both metallic zinc and lead in a 2:1 ratio.

  • The concentrates are roasted on a slow-moving grate in a special imperial melting furnace. Air is blown over the grate to burn the sulfur present in the concentrate and produce zinc oxide and lead oxide.
  • The presence of carbon in the furnace reduces these oxides to metallic zinc and lead respectively.

Imperial smelting is a high energy-consuming thermal process, making it too expensive commercially; its use is restricted to Japan, China, Poland and India. Also, the zinc produced through this process contains some amounts of copper, tin, lead, cadmium and iron; it has a lower purity than the final product of the electrolytic process. The electrolytic process of producing zinc is the most common and is used 90 per cent of the time.

Bayer Process - Infographic Overview

The Bayer Process was developed by Karl Bayer, an Austrian Chemist, in 1887. This allowed the cheap production of alumina, and along with the Hall-Heroult process reduced the cost of aluminium by 80%.

The process itself is based around extracting aluminium hydrate from bauxite using caustic soda. The aluminium hydrate (commonly referred to as hydrate within the industry) is then precipitation out where it is filtered, and calcined to remove the free and crystallized water and form aluminium oxide (alumina).

Source

Alumina refineries are generally split into 4 areas:

  1. Digestion - This is the first area where bauxite is milled to an appropriate size, mixed with recycled caustic soda, and heated to 150 degrees to extract as much hydrate as possible. The slurry is also cooled to increase its supersaturation and improve the downstream processes.
  2. Clarification - This area is used to remove the solid iron remaining in the slurry, to leave a relatively pure solution of caustic and hydrate. The iron product commonly called red mud due to its colour is then washed to recover as much caustic soda as possible before it is disposed of in large red mud ponds.
  3. Precipitation - The solution is cooled down to encourage crystalization within many large tanks. This can take over 24 hours to maximize yield. The slurry is then classified based on particle size and filtered, where the coarsest product is sent to calcination, and the finer particles are recycled back to precipitation as crystal seed to improve yield.
  4. Calcination - The hydrate product is heated to approximately ~1000 degrees C to remove the free and crystallized water and form alumina. This is then sent to smelters to produce alumina.

All the other areas within the refinery are used to improve yield and reduce energy consumption:

  • Evaporation - due to the open nature of most refineries significant quantities of water are added to the system diluting out the caustic soda. This is removed by vaporizing the filtrate, where the condensate is recycled back to the boilers to be re-used as steam to heat the digestion stream.
  • Security filtration - used to ensure the stream to precipitation is free of solids, particularly iron, to protect product quality.

Alumina refineries are very complex in their nature, and a huge amount of potential still exists for improvement. It is very energy intensive, and uses a lot of caustic soda which significantly increases costs. Due to the significant amount of equipment required to undergo this process the use of chemical engineers is numerous to improve individual equipment performance, plan equipment downtime, and identify synergy between systems.