The phosphoric acid industry: equipment, materials, and corrosion

6.1 Erosion-corrosion

Erosion-corrosion (EC) is characterized in appearance by deep gullies, grooves, waves, rounded holes, and valleys, exhibiting a directional pattern usually related to the direction of the fluid flow. These EC effects impair the protective film present on passive alloys such as SS, causing breakdown of passivity. On active alloys such as CS, an active surface is maintained. In this way, the corrosion process is accelerated by mechanical removal of asperities, protuberances, and corrosion products such as oxides or phosphates, which otherwise might form a protective layer. A continuously renewed surface is generated, enhancing corrosion activity.

Corrosion, erosion, and abrasion are frequent problems in chemical and mineral processing plants, leading to failures of equipment operating under severe hydrodynamic conditions. Many metals and alloys, such as CS, SS, and Ni-based alloys, are susceptible to EC. All types of equipment that handle moving fluids, such as agitators, pumps, valves, nozzles, centrifuges, impellers, and ball mills, are likely to undergo EC. The interaction between the chemical and the mechanical factors, and their continuous effect on the metal surface, causes EC (Schorr et al., 2010a,b).

6.2 Erosion-corrosion measuring devices

Two testers for the study and measurement of EC were developed and built: the EC unit (ECU) and the EC tester (ECT). These instruments simulate the shear and impact forces acting on industrial equipment to evaluate the corrosive wear effects of the fluids on the equipment and their interaction. The specimen in the ECU consists of a cylinder rotated about a vertical axis while a jet of slurry impinges continuously onto its curved surface through an interchangeable nozzle. The ECT applies rotational sliding abrasion by the slurry on a static disk specimen. In both devices, a three-electrode system (working, reference, and counter electrodes) is used to evaluate the electrochemical corrosion component by the polarization resistance technique. The contribution of mechanical erosion to the synergic EC process is measured by weight loss on long-term exposures (Schorr, Weintraub & Andrasi, 1990).

Both devices were developed and built by the Corrosion and Materials Section of the IMI-TAMI Institute for Research and Development Ltd. Haifa, currently known as Israel Chemicals Ltd. (ICL), Israel. The electronic part of both testers was designed by Ch. Yarnitzky, Technion-Israel Institute of Technology, and built by Opal Co. Israel (Yarnitzky & Schorr, 1981). The instruments were applied in industrial WPA plants to monitor the influence of corrosion parameters during production and in pilot plants and in the laboratory to evaluate and select CRAs to assess the corrosivity of the ores and slurries handled and for analysis of EC failures.

The ECT was applied to develop a corrosion-resistant SS and to test it in WPA slurries. This patented alloy was called CED (Cogan, Engelberg, the inventors’ names; D for Deshanim, Hebrew for fertilizers; US Patent 4740353 A) (Cogan, 1987). After proving its suitability for the WPA, this SS was successfully used for the fabrication of pumps and valves for the industrial plants of Fertilizers and Chemicals Ltd., Haifa, Israel (Cogan, 1988).

Cogan and Schorr wrote and published (1987–1988) a bilingual (Hebrew and English) periodical bulletin entitled Corrosion Reporter dealing with corrosion problems at ICL industrial plants.

6.3 Erosion-corrosion in wet-process phosphoric acid plants

EC occurs frequently in plants operating with siliceous rocks, which contain hard and dense forms of silica, such as quartz, and other inert components that are not degraded during acidulation. Under these conditions, the erosive solids, suspended in the reaction slurry and driven past the SS surface or impinging on it, act synergistically and lead to rapid failure. Some countries (Colombia, Pakistan, and Venezuela; Table 1) own PRs rich in hard SiO₂, which damages agitators and pumps made of rubber-lined CS, and thus, they employ austenitic SS.

The EC tester was taken into an industrial plant and connected directly to the reactor containing the agitated slurry (Figure 3). It applies rotational sliding abrasion on a static specimen; corrosion is measured by the polarization resistance technique and by weight loss to assess the contribution of mechanical erosion (Schorr, 2007; Schorr et al., 2010a,b).

Figure 3:

WPA plant EC measurement in the slurry supply mode.

(1) WPA industrial reactor. (2) Centrifugal slurry pump. (3) Slurry test vessel. (4) EC tester. (5) Electrochemical measuring instrument.

A trial was conducted at a WPA plant to determine the corrosivity of the reaction system under different regimes as regards clay addition. The EC rates were measured with the ECT. A decrease in the alumina content leads to an increase in corrosion rate of the SS and Ni-based alloy tested; mainly UNS S31600 and N08028 exhibits corrosion resistance in both regimes, i.e. with and without clay addition (Figure 4). As the SiO₂ content of the rock (0.95%) is high and Al₂O₃ content (0.04%) is low, Al₂O₃ from the added clay has a dominant effect on the filter acid gypsum crystal properties. When the addition of clay was interrupted, the plant went into a pasty slurry and consequent filter plugging, returning to normal operation only when clay addition was resumed.

Figure 4:

Influence of clay addition in WPA production on EC rate.

Addition of an alumina-silicate clay to an igneous rock reduces the corrosion rate of SS. UNS S31500, N08904, N08028, and S31803. The corrosion measurements were carried out with the ECT immersed in the reaction slurry at a PA pilot plant. These alloys are commonly used for fabrication of chemical equipment in PA plants (Schorr, 1998a,b).

A series of laboratory EC tests were performed in a reaction slurry, taken from a PA plant, simulating PA production. The test conditions and results are presented in Table 4. An increase in the content of free F (tests 1 to 12) indicates the absence of clay, which, if present, would have complexed F. Therefore, the EC rates of all alloys tested increase, as regards their different corrosion resistances. High levels also lead to high corrosion rates. They affect the passive state of UNS S31600 (Figure 4) and UNS N08904 (Table 4) and cause breakdown of passivity, as indicated by low E_cor values in the active region and high corrosion rates. Alloys N08028 and N06030 are the least affected, exhibiting low corrosion rates.

The corrosion behavior of various austenitic SS is expressed by their electrode potential and the potential shift toward more active values. Two austenitic SS and a Ni-based alloy were tested under different conditions: static, rotation, impingement, and rotation with simultaneous impingement. Alloy S31600 changed its electrode potential from the noble region (0.27 V) to the active region (-0.13 V) and underwent breakdown of passivity with high corrosion rates (up to 8.9 mm/year), particularly when subjected to rotation plus impingement. Alloy N10276 maintains its passive state with low corrosion rates (Table 5).

The oxidizing power of an acid solution such as filter acid, which influences the corrosion behavior of the active-passive alloys, is expressed by the oxidation-reduction potential E_orp value of filter acid, 0.48 V, which is lowered to 0.14 V by the reducing character of HF and HCl (test 12). Under these conditions, active corrosion with high corrosion rates is recorded on UNS N08904 (Schorr, 1993a,b).

7.1 Influence of impurities

PR is the principal source of dissolved and suspended impurities in WPA. Other impurities such as Cl^- may be introduced in process water, particularly brackish water. Sometimes, contaminated H₂SO₄ obtained from the hydrometallurgy industry introduces additional impurities. The impurities impart undesired color and turbidity to the WPA and increase the corrosiveness of PA. Cl^- and F^- are particularly corrosive, and other impurities that affect corrosion are SiO₂, Al₂O₃, alkali metal salts, SO₄^2-, S^2-, organic matter, and oxidizing agents. The corrosivity of a specific impurity depends on its chemical nature, the concentration of its active species, and its interaction with other acid constituents and with the surface of the specific metal (Schorr, 1981).

In addition to affecting corrosion, some impurities change the density and viscosity of the acid and form acid sludges and sediments. In contact with metallic surfaces, these sediments may influence the corrosion behavior of the metal by forming deposits that promote localized corrosion.

7.2 Effect of operating conditions

WPA plants operate under severe conditions that include elevated temperatures and rapid heat transfer, high acid concentration, agitation and circulation of liquids containing erosive suspended solids, aeration, formation of foam, and volatilization of corrosive acidic vapor that condenses on cooler metallic surfaces. Destructive corrosion results from combinations of these factors in which their combined action is greater than the sum of their separate actions.

7.3 The interaction of HF with SiO₂, Al₂O₃, and MgO

During the production of WPA, HF or F^- (free fluoride ion) is highly corrosive, but complexed fluoride compounds, e.g. H₂SiF₆, are non-corrosive toward the austenitic SS and Ni-based alloys currently used in WPA service. Consequently, it is a common practice in the WPA industry to add silica- and alumina-containing clays or acid-soluble silicates to reduce the corrosion effects (Schorr, 1996). In igneous phosphate ores containing Mg⁺² compounds, similar reactions with HF occur, resulting in their dissolution during the reaction stage. The free fluoride content is reduced, leading to a significant decrease in corrosion (Schorr, 1993a,b).

Becker (1989) states that, according to his personal experience, acceptable corrosion rates are obtained when F/SiO₂ (% weight ratio) in the phosphate ore is held below the threshold value of 1.4. A more accurate and complete calculation of the complexing ratio is based on the stoichiometry of the complex-forming reaction between F^- and equivalent SiO₂, Al₂O₃, and MgO:

The chemical equivalent ratio is

An equivalent ratio, F/(SiO₂+Al₂O₃+MgO)>1, indicates the presence of non-complexed fluoride, and consequently, enhanced corrosivity during production of WPA. When the equivalent ratio is <1, the ore contains enough SiO₂, Al₂O₃, and MgO to ensure the formation of non-corrosive fluoride complexes.

7.4 Electrochemical corrosion behavior

The electrochemical behavior of several metallic materials in different PA solutions and pure PA is graphically illustrated by current density-electrode potential plots, with the following electrochemical parameters: open-circuit potential (E_opc), critical current density (i_crit), primary passive potential (E_pp), passivation current density (i_p), passivation potential (E_p), and transpassive potential (E_t) (ASTM G3 and G5) (Figure 5).

Figure 5:

Anodic potentiodynamic polarization plots in H₃PO₄ at 20°C, showing typical corrosion behavior.

Active corrosion behavior of CS is shown in plot 1; plot 2 traverses three regions of corrosion behavior: active, passive, and transpassive, the last indicated by the appearance of a yellow color in the colorless PA solution, due to the presence of CrO₄^2-, generated in the oxidative dissolution of SS containing Cr, corroborated by chemical analysis. Plot 3 starts with passivity; transpassivity appears at E_t at about 1.00 V, but a total passive state is maintained in 100% PA (plot 4). The electrochemical behavior of SS in PA is related to acid concentration and temperature. The presence of halides ions in PA accelerates the corrosion, with the increase in rate being dependent upon the halide chemical nature and content (Alon, Schorr, & Yahalom, 1975). Plot 5 of Ta in 85% PA demonstrates no corrosion activity due to the protective Ta₂O₅ film.

A typical behavior of a corrosion-resistant SS is expressed by the shape and the electrochemical parameters of the schematic anodic polarization plot, which summarizes the corrosion-activating and corrosion-inhibiting factors present in WPA (Figure 6). Silica, alumina, and magnesia decrease the value of i_CC (critical current density) and inhibit the corrosion activity (arrows to the left). F^-, Cl^-, H₂SO₄, and mechanical factors increase corrosion activity (arrows to the right) (Schorr, 1998a,b).

Figure 6:

Typical anodic polarization plot showing the effect of the chemical and mechanical factors prevailing in the production of WPA.

8.1 Fertilizers

WPA is the most important intermediate in the fertilizer industry because it is a major constituent of triple superphosphate, ammonium phosphate, and mixed NPK fertilizers. Industrialized countries produce PK and NPK fertilizers for domestic utilization and for export; they are also supplied as slow-release fertilizers and as special fertilizer for “fertigation” (fertilization combined with irrigation) and for foliar application. Fertilizers may be acidic, neutral, or basic; their pH and hygroscopicity affect their corrosiveness in the presence of moisture (Agarwal, 2002).

Ground PR is directly dispersed in agricultural fields in tropical regions to neutralize the soil natural acidity.

8.2 Metallurgy

Chemical conversion coatings are applied on steel and aluminum surfaces for protection against corrosion. Phosphating solutions, containing PA with special active additives, are employed for protection of steel vehicles, office furniture, aircraft, merchant and military ships, and machinery. The phosphate coating ensures the adhesion and performance of posterior painting (Sanchez, Montanes, Garcia-Anton & Guenbour, 2011; Santana, Pepe, Jimenez-Pique, Pellice & Ceré, 2013).

PA solutions are employed for removal of rust from corroded surfaces, the black phosphate coating improving their corrosion resistance; PA solutions are utilized for cleaning and sanitation of equipment and machinery in industrial plants, e.g. dairy plants; for electropolishing of SS, aluminum, and copper alloys; and as a flux component in soldering. Fuel cells for electricity generation operate with PA as a liquid electrolyte.

8.3 Chemicals

PA is used for many chemical devices and operations: to remove mineral deposits, to clean hard water stains, to etch solution nitride in microfabrication, in hydroponics to lower the pH of nutrient solution, as an etching agent for semiconductors, in cosmetic and skin care products to adjust the pH, for drinking water treatment, for preparation of synthetic rubber, for leather tanning, and as additive for varnishes, pigments, and paints. It is also used as an additive in the manufacture of fire bricks, for fire-retarding agents, and ceramic colors and as a catalyst in polypropylene polymerization and in enhancing the setting action of synthetic resins. Laundry detergents contain soluble phosphates. Phosphonates and organophosphorus compounds from PA are used as corrosion and scale inhibitors for water treatment and in the desalination industry.

8.4 Food and beverages

Pure food-grade PA (Food, 2012; Li, Xiang S, Zeng, Wang & Wang, 2012) is an additive to food sauces, mayonnaise, and fruit juices, and it is used to acidulate cola-type beverages. Calcium phosphate salts are added to baked goods. They are also used in white, soft cheeses to avoid water segregation (Giulini, 2015), in sugar and edible oil refining, and for bacteria control in food processing. Sodium and potassium phosphate salts serve as food preservatives.

8.5 Medicine

PA combined with zinc powder forms zinc phosphate, which is used as a dental cement. In orthodontics, it is applied to clean and roughen the teeth before inserting brackets and other dental devices. Eliminating plaques and whitening the teeth is done with PA derivatives. Orthopedic metallic implants are covered with calcium phosphate to promote their integration with near osseous tissue. Phosphatic cements have applications in bone system surgery.

9.1 Problems in wet-process phosphoric acid production

Despite the great variety of rocks (Table 1) and the different PA processes and plant procedures, WPA production problems (schematically displayed in Figure 7) are basically similar (Schorr & Lin, 1997). These problems are recorded in accordance with the three WPA stages: reaction, filtration, and concentration.

Figure 7:

Problems in WPA production stages.

The central problem appears in the huge reactor, fitted with SS agitators, pumps, and ducts. The corrosion intensity depends on the P₂O₅/F ratio of the PR feed to the reactor. Two additional problems are the emission of fluoride pollutant volatile gases, mist, and fumes through the reactor chimneys and the form and size of the gypsum crystals, which affect filterability in the filtration stage, plugging the filters with scales and sludge. In the concentration stage, particularly when producing food-grade acid (85%–95% H₃PO₄), heavy metals (Cu, Cd, Cr, Pb, Hg, and V) contaminate and impart an undesirable tint. Great efforts are invested to partially remove or eliminate these contaminates by analytical, chemical, and electrochemical procedures. Many improved mining methods, ore beneficiary technologies, and production processes have been developed, patented, and implemented in several countries in order to obtain products with higher quality and purity (Blasco-Tamarit, García-García, Ferrándiz, Antón & Guenbour, 2011; El Kayar & Perrot, 2014).

9.2 Mineral modifiers: Phosphoric acid, quality and corrosion reduction

Industrial minerals and chemical are widely applied in many industries as additives to improve process efficiency and product quality. The concept of mineral modifiers involves alteration of the mineral and chemical composition of PR into a composition more suitable for processing and production of concentrated acid and derived fertilizers.

Clays are fine-sized hydrous alumino-silicates that develop plasticity when mixed with water. The most common clay minerals are classified on the bases of structure into the kaolinite, montmorillonite, and illite groups, which find applications in the PA industry. Kaolinite is a relatively homogeneous mineral with fine-sized particles, but it contains some free quartz and has high water content. Its reactivity reaches a value of 55%–60%. Amorphous alumino-silicates, such as natural perlite, are uniform, homogeneous, and clean minerals; they do not contain free quartz and have low water content. Perlite reacts rapidly with the acids in the PA reactor and has a high reactivity value, 90%, and sometimes, even higher.

Tailor-made clay, e.g. perlite, has been prepared by Prof. I. J. Lin, Mineral Engineering Research Centre, Technion, Israel Institute of Technology, by manipulating the SiO₂/Al₂O₃ ratio to suit the requirement of the particular PR. As a result of its high reactivity, it was applied to inhibit corrosion and increase filterability (Lin & Schorr, 1996; Schorr, 1996; Schorr & Lin, 1996).

The corrosivity of PR during the production of PA depends on three main factors: the chloride content, the interaction between HF formed in the reaction slurry and the SiO₂, Al₂O₃, and MgO in the rock, and the operating conditions. Consequently, it is common practice in the PA industry to add clays or acid-soluble silicas to reduce the corrosion effects. Table 1 shows that sedimentary rocks rich in silica and alumina, e.g. Florida PR exhibit appropriate content of SiO₂ and Al₂O₃, indicating a low degree of corrosivity. Igneous rock with a reduced fluoride content and sufficient silica and magnesia to complex it have low corrosivity.

Various investigators have proposed to employ distinct organic compounds as corrosion inhibitors to protect steel in PA solutions (Li, Deng & Fu 2011; Oguzie et al., 2012; Boudalia et al., 2013b; Zarrouk et al., 2013; Ousslim et al., 2014).

9.3 Phosphogypsum waste disposal

During the production of WPA, gypsum (CaSO₄·2H₂O) is formed as a by-product contaminated with remaining amount of PA [see Equation (1)]. Phosphogypsum (PG) is radioactive due to the presence of materially occurring uranium and thorium. Marine-deposited PR has a higher level of radioactivity than igneous phosphate deposits because uranium is present in seawater. Industrial WPA plants dispose it as a waste on land covered with impermeable rubber sheets to avoid infiltration into the soil.

The US Environmental Protection Agency has banned most applications of PG with radium-226 concentrations greater than 10 pCi/g. Central Florida has a large quantity of PG deposits, particularly in the Bone Valley region. There are about 1 billion tons of PG stacked in 25 stacks in Florida; about 30 million new tons are generated each year (Florida Industrial and Phosphate Research Institute, 2015).

9.4 Global phosphoric acid industry

The global market for PA and its phosphate derivates is expanding worldwide; this trend is expected to continue in the next years, thus producing innovative products. World fertilizer demand (a central sector of this industry) in 2010 has been characterized, based in the recovery in traditional markets and a sustained level of consumption in emerging markets. Over the next 5 years, global capacity and production of PA would increase further. The following data show the world supply of PA, expressed as P₂O₅:

According to the Food and Agriculture Organization (2011) of the United Nations, the world PR supply was expected to increase by 3.2% per annum between 2011 and 2015.

Morocco is the largest producer and exporter of PR-based products. This industry is managed by the Office Cherifien des Phosphates, which deals with both mined ore and the production of its derivates to maintain its pole position in the PA industry (Morocco, 2008).

The production in the Middle East, Africa, and Latin America is growing at an outstanding rate because of a strong demand of PA and its phosphate products. It includes Argentina, Brazil, Algeria, Jordan, Egypt, Iran, Qatar, Israel, and Saudi Arabia. North America and European markets are predicted to generate a strong demand for these products. China and India have been the largest growth markets due to their rising domestic requirements for food supply.

Latin American countries (Mexico, Venezuela, Colombia, Brazil, and Peru) are very active in this industry, searching for PR reserves, extracting PR from mines, converting them into ores, constructing and operating PA plants, and producing phosphate-based products in particular food-grade PA.

Innophos Fosfatados de Mexico operates plants in the Petroleos Mexicanos (PEMEX), Industrial Petrochemical Complex “Pajaritos” at Coatzacoalcos, Veracruz (Schorr, 1996). It has expanded its facilities to increase food-grade PA and phosphate salts capacities.