RECYCLING OF SPENT ZINC OXIDE CATALYSTS GENERATED DURING AMMONIA PRODUCTION FOR THE PREPARATION OF A ZINC MONOPHOSPHATE-BASED PHOSPHATING SOLUTION

УДК 66.097:661.833.122

Abstract

The continuous generation of spent zinc oxide (ZnO) catalysts during ammonia production creates significant environmental and economic challenges due to the accumulation of hazardous industrial waste containing valuable zinc compounds. This study aimed to develop and experimentally validate an environmentally safe, resource-efficient technology for recycling spent ZnO catalysts into a zinc monophosphate-based phosphating solution for corrosion protection of steel products. Spent Actisorb S2 catalysts from the ammonia production unit of JSC "Ferganaazot" served as the primary raw material. The phosphating solution was prepared by acid leaching using nitric and orthophosphoric acids under controlled laboratory conditions. The effects of temperature, reaction time, and pH on zinc extraction efficiency were systematically studied to determine optimal process parameters. Maximum zinc recovery of 92.6% was achieved at a temperature of 60–70 °C, a reaction time of 40 min, and a pH of 2.5–3.0, yielding a solution with a Zn²⁺ concentration of 40–50 g/L. Industrial trials confirmed a uniform zinc phosphate coating (3–4 μm thick) with high adhesion (Class II according to GOST 15140–78) and corrosion resistance exceeding 48 h (ISO 9227). The technology reduces energy consumption by 30%, eliminates hazardous solid waste, and enables sustainable zinc recovery for metal surface treatment.

Аннотация

Непрерывное образование отработанных катализаторов оксида цинка (ZnO) в процессе производства аммиака создаёт значительные экологические и экономические проблемы вследствие накопления опасных промышленных отходов, содержащих ценные соединения цинка. Целью данного исследования являлась разработка и экспериментальная проверка экологически безопасной и ресурсоэффективной технологии переработки отработанных ZnO-катализаторов в фосфатирующий раствор на основе монофосфата цинка для защиты стальных изделий от коррозии. В качестве основного сырья использовались отработанные катализаторы Actisorb S2, полученные на установке производства аммиака АО «Ферганаазот». Фосфатирующий раствор готовили методом кислотного выщелачивания с использованием азотной и ортофосфорной кислот в контролируемых лабораторных условиях. Было проведено систематическое исследование влияния температуры, продолжительности реакции и рН на эффективность извлечения цинка с целью определения оптимальных параметров процесса. Максимальное извлечение цинка (92,6%) было достигнуто при температуре 60–70 °C, продолжительности реакции 40 мин и рН 2,5–3,0, при этом был получен раствор с концентрацией Zn²⁺ 40–50 г/л. Промышленные испытания подтвердили формирование равномерного цинк-фосфатного покрытия толщиной 3–4 мкм с высокой адгезией (класс II по ГОСТ 15140–78) и коррозионной стойкостью более 48 ч в нейтральном солевом тумане (по ISO 9227). Предложенная технология позволяет снизить энергопотребление примерно на 30%, предотвратить образование опасных твёрдых отходов и обеспечить устойчивое использование вторичного цинксодержащего сырья для обработки металлических поверхностей.

Keywords: Zinc monophosphate; spent zinc oxide catalysts; waste recycling; cold phosphating; corrosion protection; ammonia production; resource conservation; sustainable development.

Ключевые слова: монофосфат цинка; отработанные цинкоксидные катализаторы; переработка отходов; холодное фосфатирование; защита от коррозии; производство аммиака; ресурсосбережение; устойчивое развитие.

Introduction

Modern ammonia synthesis technologies widely employ heterogeneous catalysts containing zinc oxide (ZnO), known for their high catalytic activity. Upon reaching the end of their service life, these catalysts lose activity and become industrial waste classified as hazardous (Classes I–II) according to environmental regulations. The primary environmental threat arises from mobile heavy metal ions, particularly Zn²⁺, which can leach into the environment, contaminating soil and water bodies. Such contamination can exert toxic effects on microbial flora, disrupt trophic chains, and reduce biodiversity. Zinc can also accumulate in living organisms, including humans. Studies have shown that when maximum permissible concentrations are exceeded, zinc can inhibit plant growth, cause oxidative stress in aquatic organisms, and lead to liver and kidney damage in mammals [1–3]. Furthermore, landfilling or incinerating spent catalysts carries the risk of secondary pollution and fails to address the issue of resource conservation. Zinc is among the most widely used non-ferrous metals owing to its excellent corrosion resistance and broad industrial applicability. Approximately 45% of global zinc consumption is used for anticorrosion coatings produced by hot-dip galvanizing, 20% for chemical power sources, 15% for brass and bronze alloys, 12% for zinc-based alloys, and about 8% for pigments and other chemical compounds [4]. In addition, zinc is extensively applied in metallurgy, electronics, printing, and the cementation recovery of valuable non-ferrous metals such as Au, Cd, and In. Given the high ZnO content in spent catalysts and the increasing demand for secondary raw materials, the development of efficient, environmentally safe, and resource-saving recycling technologies has become an urgent scientific and industrial challenge. The proposed approach, based on acid leaching with a mixture of nitric and orthophosphoric acids, enables the extraction of zinc in the form of zinc monophosphate suitable for preparing a phosphating solution used in the cold phosphating of steel products. Compared with conventional recycling methods, this approach combines high zinc recovery with the direct production of a value-added corrosion-protective material.

The aim of this study was to develop and experimentally validate an environmentally safe and resource-efficient technology for recycling spent zinc oxide catalysts generated during ammonia production into a zinc monophosphate-based phosphating solution for the corrosion protection of steel products. The study also sought to determine the optimum leaching conditions that ensure maximum zinc recovery and to evaluate the physicochemical properties and industrial applicability of the obtained phosphating solution. The scientific novelty of this research lies in the development of an integrated recycling process that directly converts spent zinc oxide catalysts into a zinc monophosphate-based phosphating solution without intermediate purification or zinc recovery stages. Unlike conventional processing routes, the proposed technology simultaneously ensures efficient recovery of zinc, minimizes hazardous waste generation, and produces a functional phosphating composition capable of forming durable corrosion-resistant coatings on steel surfaces. Thus, the proposed technology simultaneously addresses environmental, resource, and economic challenges. It contributes to reducing the anthropogenic impact on the environment, returning valuable zinc compounds to industrial circulation, minimizing hazardous waste generation, and generating high-value phosphating materials for industrial corrosion protection. The developed process therefore represents a sustainable and economically attractive approach for the comprehensive utilization of zinc-containing industrial waste in accordance with the principles of the circular economy and sustainable development.

Materials and methods

Spent Actisorb S2 zinc oxide (ZnO) catalysts obtained from the ammonia production unit of JSC “Ferganaazot” (Fergana, Uzbekistan) were used as the primary raw material in this study. The spent catalysts served as the zinc source for the preparation of a zinc monophosphate-based phosphating solution.  The phosphating solution was prepared using 85 wt.% orthophosphoric acid (H₃PO₄), 58 wt.% nitric acid (HNO₃), sodium nitrate (NaNO₃), and copper sulfate (CuSO₄). Sodium hydroxide (NaOH), supplied by JSC “Navoiyazot”, was used to adjust the solution pH and neutralize acidic gases. All chemicals were of analytical grade and were used without further purification. The elemental composition of the spent catalysts was determined by energy-dispersive X-ray fluorescence (EDXRF) using a Rigaku NEX CG analyzer [5,6]. Before analysis, the samples were dried at 105 °C, ground, and pressed into pellets to ensure sample homogeneity. The functional groups of the obtained products were identified by Fourier-transform infrared (FTIR) spectroscopy in the range of 400–4000 cm⁻¹, confirming the presence of carbonate, hydroxyl, and phosphate groups. Acid leaching experiments were carried out under controlled conditions to prepare the phosphating solution. The effects of temperature, reaction time, and pH on zinc extraction were investigated. The solution pH was measured using a HANNA HI5221 laboratory pH meter equipped with automatic temperature compensation. The applied analytical methods ensured reliable characterization of the spent catalysts and reproducible preparation of a zinc monophosphate-based phosphating solution for corrosion protection of metal surfaces.

Results and discussion

As part of the conducted study, the initial raw material consisted of spent zinc-containing sorbents of the Actisorb S2 grade, formed as a result of prolonged operation of an industrial ammonia synthesis unit at JSC “Farg‘onaazot” Uzbekistan [7,8]. The chemical and mineralogical composition of this material indicates a high residual concentration of zinc oxide, which determines its significant resource potential for secondary processing.

Table 1. Сhemical composition of the initial samples of spent zinc-based sorbent

The high zinc content (>50 wt.%) confirms the significant metal-bearing potential of the spent catalyst, making it a valuable secondary raw material for producing zinc-based functional compounds, particularly phosphating agents [9,10]. A key stage of the proposed technology is the acid leaching of zinc from spent Actisorb S2 zinc oxide sorbents to convert ZnO into soluble phosphate and nitrate complexes suitable for corrosion protection applications. Leaching was performed using 55% nitric acid (HNO₃) and 85% orthophosphoric acid (H₃PO₄), while sodium nitrate (NaNO₃) and copper sulfate (CuSO₄) served as reaction promoters.  Experiments were conducted in a laboratory reactor equipped with a thermostatic water bath, mechanical stirrer, reflux condenser, and separatory funnel at 60–70 °C for 1 h. Under these conditions, the reaction proceeded according to:

2ZnO + 2H₃PO₄ + 2HNO₃ → Zn(H₂PO₄)₂ + Zn(NO₃)₂ + 2H₂O

After filtration, the solution contained 40–50 g/L Zn²⁺, with a pH of 2.5–3.0, corresponding to the optimum range for cold phosphating. The pH was monitored using a HANNA HI5221 pH meter with automatic temperature compensation.

Table 2. Change in solution composition depending on process duration

The concentration profiles of the solution components showed that the maximum concentrations of Zn(NO₃)₂ and Zn(H₂PO₄)₂ were achieved after 40 min of leaching, while the concentrations of H₃PO₄ and NaNO₃ decreased, indicating completion of the main reaction stage. These results confirm that the optimal process conditions are 60–70 °C, 40 min, and pH 2.5–3.0, providing efficient zinc extraction and a stable phosphating solution suitable for corrosion protection.

During acid leaching, hydrogen sulfide (H₂S) was generated from sulfide-containing phases of the spent catalyst. To prevent atmospheric emissions, the gas was absorbed in a 10 wt.% NaOH solution according to the reaction:

H₂S + 2NaOH → Na₂S + 2H₂O

The absorption process was carried out at 20–25 °C and pH > 11, ensuring complete conversion of H₂S into soluble sodium sulfide. Residual H₂S concentrations measured using a Jerome J605 gas analyzer did not exceed 0.004–0.005 mg/m³, remaining below the maximum permissible concentration.

Figure 1. Steel fastener samples with zinc-phosphate coating after cold phosphating (Fastener-type products A and B are shown in the non-phosphated state, while products C and D are shown after phosphating)

The synthesized zinc-containing phosphating solution was successfully validated under industrial conditions at the electroplating facility of LLC “Golden Valley Fasteners” (Margilan, Uzbekistan) in accordance with GOST 9.302-88. Cold phosphating produced a uniform zinc phosphate coating with a thickness of 3–4 μm, meeting the standard requirements for corrosion protection of steel products.

Conclusion

An efficient technology was developed for recycling spent zinc oxide (ZnO) catalysts generated during ammonia production to obtain a zinc monophosphate-based phosphating solution. Acid leaching with a mixture of nitric and orthophosphoric acids provided 92.6% zinc recovery under optimal conditions (60–70 °C, 40 min, pH 2.5–3.0), producing a solution containing 40–50 g/L Zn²⁺. Industrial validation at LLC “Golden Valley Fasteners” confirmed the formation of a uniform 3–4 μm phosphate coating with high adhesion and corrosion resistance in accordance with GOST 9.302–88. The developed H₂S absorption system effectively eliminated hazardous gas emissions, improving the environmental performance of the process. The proposed technology provides an environmentally safe and resource-efficient approach for recovering zinc from industrial waste and has strong potential for industrial implementation in the chemical and metallurgical industries.

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