Furnace acid

There are numerous variations of the wet process, but all involve an initial step in which the ore is solubilized in sulfuric acid, or, in a few special instances, in some other acid. Because of this requirement for sulfuric acid, it is obvious that sulfur is a raw material of considerable importance to the fertilizer industry. The acid—rock reaction results in formation of phosphoric acid and the precipitation of calcium sulfate. The second principal step in the wet processes is filtration to separate the phosphoric acid from the precipitated calcium sulfate. Wet-process phosphoric acid (WPA) is much less pure than electric furnace acid, but for most fertilizer production the impurities, such as iron, aluminum, and magnesium, are not objectionable and actually contribute to improved physical condition of the finished fertilizer (35). Impurities also furnish some micronutrient fertilizer elements. 

Electric force microscopy, 3 332 Electric furnace acid, 11 121 Electric furnace ferromanganese production, 15 552-553 Electric furnace process, for zircon, 26 627-628... 

By far the most important derivative of sulfuric acid is phosphoric acid. It has been unknowingly used as fertilizer for hundreds of years. The wet process method of manufacture was important until 1920, when furnace acid began increasing in popularity. The wet process, however, has made a comeback because of plant design improvements 60% of phosphoric acid was made by this method in 1954, 88% in 1974, and over 90% currently. The furnace process is used only to make concentrated acid (75-85%) and pure product. It is very expensive because of the 2000 °C temperature required. In the furnace process phosphate rock is heated with sand and coke to give elemental phosphorus, which is then oxidized and hydrated to phosphoric acid. A simplified chemical reaction is ... 

Currently only one furnace acid plant remains in operation in the United States to supply elemental phosphorus and a few uses that cannot be satisfied by purified wet acid produced from wet process acid. 

Prior to the late 1940s, diammonium phosphate (NH4)2HP04 was considered to be too unstable for use as a fertilizer. However, at about that time, exploratory work by TVA indicated otherwise. The first demonstration-scale diammonium phosphate made by TVA for fertilizer use was a monocrystal product made by ammoniation of pure electric-furnace acid in a vacuum crystallizer.13 Demonstration programs showed this material to be stable and to be an excellent high-analysis fertilizer. Small amounts of similar product were produced by others using electric-furnace acid and by-product ammonia. However, general... 

With the system that employed these raw materials, numerous advantages of fluid mixtures were established, but a need to improve the economics of the procedure was obvious. Pure electric-furnace acid is much more expensive than wet-process acid, so ways were sought to use the cheaper but impure wet-process acids. Direct ammoniation of wet-process acid causes precipitation of numerous impurities as an intolerable, gelatinous sludge. Also, plant-food solubilities are undesirably limited in the orthophosphate system, so the concentration (grade) of the product made with electric-furnace orthophosphoric acid was limited to 8-24-0. A series of developments by TVA led the way to solving both of these problems. 

The process was carried out in a ceramic retort (inside a furnace) and bird-beak condenser (outside the furnace). Acid composition was adjusted by adding or evaporating water. 

Phosphoric acid is directly used in the treatment of metals. For applications in the food sector, furnace acid is generally used. Phosphates for washing powders are manufactured from both furnace acid and purified wet-process acid. 

Initially, polyphosphoric acid was used which was produced by the combustion of white phosphorus (see Section 1.5.1.2.1), solutions with 11% N and 37% P2O5 being obtained. These furnace acids are currently too expensive for fertilizers, due to increased energy costs, and the polyphosphoric acids now used are manufactured from wet-process acid. 

The first step in analysing plastics for metals content in polymers by ICP-AES technique is that they must be prepared in solutions that are suitable for nebulization. There are four general methods applicable for sample preparation for metal analysis by ICP-AES and they are solvent dissolution of some plastics dry ashing using a muffle furnace acid digestion using a microwave oven and oxygen bomb combustion. 

Derivation (1) Action of sulfuric acid on pulverized phosphate rock (2) action of hydrochloric acid on phosphate rock, with extraction by tributylphosp-hate (3) by heating phosphate rock, coke, and silica in an electric furnace, burning the elemental phosphorus produced, and then hydrating the phosphoric oxide (furnace acid). 

Three main routes are employed for the commercial production of phosphoric acid. The dry process, also called the combustion process yields a furnace acid. It is obtained via the combustion of yellow phosphorus in air followed by hydration of phosphorus pentoxide product with water. The other two processes operate by acidulation of phosphate rock with strong acids, and are referred to as wet processes. The Dorr process uses sulfuric acid, and the Haifa process uses hydrochloric acid. The wet processes are discussed separately later. 

Furnace phosphoric acid is pure enough for most uses as obtained directly from the process. Food-grade applications require removal of traces of arseni-ous oxide (Table 10.6). Arsenic is present to the extent of 50-180 ppm (as AS2O3 equivalent) in the feed phosphorus because of the similarity of its chemical properties to those of phosphorus (same group in the periodic table). This ends up in the product acid on oxidation and hydration [1]. It may be removed by the addition of the requisite amount of sodium sulfide or hydrogen sulfide to the furnace acid, which precipitates it as the arsenic III and V sulfides (e.g., Eq. 10.20). 

Losses of phosphoric acid mists or phosphorus pentoxide fumes through a corrosion-induced rupture of ductwork or a vessel, or from a process-related parameter, represent the most commonly experienced emission control problems with furnace acid plants. When this occurs, a white plume, or ghost, of aerosol droplets or particles remains visible downwind long after the steam component has dissipated. Corrosion problems are now better understood so that more exposed components are constructed of stainless steel. If high temperatures are also involved, the equipment is lined with graphite [28]. 

Process parameters of importance for mist and fume control include measures to minimize the moisture content of the combustion air fed to the phosphorus burners. A high moisture content in this gas stream tends to increase mist formation. Solid phosphorus pentoxide has been found to be very difficult to dissolve in either water or phosphoric acid. This prompts temperatures in the hydrator to be maintained high enough so that absorption takes place from the vapor phase. Further studies have shown that uptake of phosphorus pent-oxide vapor in 70% phosphorus acid is only about 60% and climbs with increasing acid concentrations up to about 88% phosphoric acid. Operating with two fiber beds operated in series, the first bed as an agglomerator, and the second as a collector, can also efficiently control mists and fumes from furnace acid plants [30]. Mass containment efficiencies of better than 99.9% were reported for a median aerosol particle diameter of 1.1 to 1.6 fim. 

Sources of data include U.N. Statistical Yearbooks [3], and Chemical and Engineering News [18]. Includes both furnace acid and wet process products. 

End uses of phosphoric acid are highly dependent on the process used to produce it. About 875,000 tonnes (as P2O5) of furnace phosphoric acid was produced in the U.S. in 1984, about 7% of the total U.S. production [70]. Nearly half of this furnace acid is still dedicated to the production of sodium, potassium, and calcium phosphates for use in detergents and commands an 80% or so price premium over fertilizer grade, wet process acid. A further 5% is destined as a food-grade product, which is mostly used as salts in foods, bakery goods, and soft drinks. Some of this grade is also used as a supplement in animal feeds. These are relatively all small-scale uses. 

What quantity of elemental phosphorus and water would theoretically be required using the furnace acid process to produce 1 tonne of 100% equivalent phosphoric acid ... 

At that particular period, Victor Chemical Works was making a transition from manufacturing phosphoric acid from phosphorus produced by the blast furnace to phosphorus produced by the electric furnace. The blast furnace phosphoric acid was less pure. It contained the above elements as impurities. The unsatisfactory material was made from the more pure electric furnace acid. So they solved the problem by adding the impurities to the "pure electric furnace acid. After that the addition of some of the impurities became part of the process. 

Type F Furnace-welded Open hearth, electric furnace, acid-oxygen-steam, or basic oxygen. Continuous furnace butt weld. Not required... 

The first step in the production of furnace acid is to produce elemental phosphorus in an electric furnace (Figure 11.32). Phosphate nodules or olher lump... 

The furnace acid and its salts account for about 85% of the use of white P. Heavy inorganic chemicals such as PCI3, P4O10 and P4S10 account for about another 12%, and all other industrial phosphorus compounds the remainder. The annual world production of white phosphorus declined considerably in the 1990s, but is still around 0.5 x 10 tons. 

Orthophosphoric acid, H3PO4, lies at the start of the main commercial route to all manufactured phosphorus compounds (Figure 12.1). Furnace acid is used mostly for food phosphate production while both furnace acid and purified wet process acid are used for detergent phosphate manufacture. The acid itself has many direct applications, which are discnssed below. 

Vanadium Vmadiiun ore mining, by-product found in flues fi om burning oil in furnaces, acid phmt cat[Pg.311]

The fertilizer industry continues to improve its products to give higher and higher yields of nutrient to plants. Triple Super Phosphate fertilizer is made by first treating phosphate ore with sulfuric acid to make a crude form of phosphoric acid that is often referred to as green acid, because it is colored green and not because it has not been cured or ripened. The green acid is also called wet acid, to differentiate it from phosphoric acid made from elemental phosphorus. Phosphoric acid produced from elemental phosphorus is the purest of acids and it is called furnace acid. 

If the objective is to prepare the safest mineral fiber that it is theoretically possible to manufacture, no trace of a toxic substance can be tolerated There was never any doubt in the author s mind that only food grade raw material could be used in this process, although the question of using purified wet phosphoric acid, derived from fertilizers, was raised from time to time. In my judgement, the best commercially available purified wet phosphoric acid is inferior to furnace acid and, although it probably could be used, it would be done with some loss of a safety margin that has been paramount throughout the history of this project. Safety is the hallmark of phosphate fibers. When safety is not an issue, some other fibers have superior properties. 

There are phosphoric acids derived from two different kinds of processes. One phosphoric acid is called furnace acid and is prepared from elemental phosphorus. The second source is referred to as wet phosphoric acid, or green acid because it contains enough impurities to be green before it is purified. Wet acid is purified by some kind of solvent extraction process. In general furnace acid is purer than purified wet phosphoric acid, but both can be made very pure. 

Wet acid is made by allowing phosphate rock to react with sulfuric acid to form calcium sulfate, gypsum, and crude phosphoric acid. Phosphoric acid is extracted selectively into an organic solvent, from which it is stripped to yield a good quality of phosphoric acid. Furnace acid or purified wet acid can be purchased for similar prices. For many uses they are of approximately equal value. I remain biased toward furnace acid when purity is an issue. 

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