As a key product widely used in food nutritional fortifiers, feed additives, and pharmaceutical iron supplements, ferrous gluconate faces environmental issues such as "large consumption of acids and alkalis, concentrated wastewater discharge, and high energy consumption" in traditional production processes. The core goal of environmentally friendly production processes is to reduce the environmental load of the production process through green raw materials, clean reactions, resource recycling, and low-carbon energy consumption, while ensuring product purity (usually required to be ≥98%) and yield (≥85%). This aligns with the "dual carbon" goals and the development trend of green chemical engineering. Its exploration can be carried out around the following directions.

I. Environmental Pain Points of Traditional Production Processes and Directions for Green Improvement

The traditional processes of ferrous gluconate mainly include the "direct complexation of gluconic acid" and the "gluconate displacement method", both of which have significant environmental shortcomings, providing improvement targets for the exploration of green processes:

"Direct Complexation of Gluconic Acid"

Using gluconic acid and iron powder as raw materials, the reaction proceeds at 80-90°C to generate ferrous gluconate. Excess iron powder (10%-15% excess) must be added to ensure the complete reaction of gluconic acid. After the reaction, the remaining iron powder needs to be separated by filtration. If the resulting filter residue (containing unreacted iron powder and a small amount of iron oxides) is directly discarded, it not only causes waste of iron resources but may also produce iron-containing wastewater due to oxidation. At the same time, the reaction requires adjusting the pH to 3.5-4.0 (to avoid hydrolysis of ferrous ions). Traditionally, dilute sulfuric acid is used for adjustment, which easily introduces sulfate impurities. Subsequent purification requires multiple water washings, generating wastewater with high chemical oxygen demand (COD) and high salt content (COD is usually ≥800mg/L, salt content ≥5%).

"Gluconate Displacement Method"

First, glucose is oxidized with nitric acid to produce gluconic acid, which then reacts with sodium carbonate to prepare sodium gluconate. Finally, a displacement reaction with ferrous sulfate is carried out to generate ferrous gluconate. This process has the problem of "two-step reaction and long process". The nitric acid oxidation process produces nitrogen oxide (NOₓ) waste gas (mainly NO and NO₂), leading to air pollution. The sodium sulfate by-product generated in the displacement reaction needs to be separated by crystallization. The crystallization mother liquor contains a large amount of sodium ions and unreacted gluconate, and direct discharge will cause water pollution and raw material waste.

Based on this, green processes need to specifically address the three major pain points of "resource waste, waste gas discharge, and high salt and high COD in wastewater". The core improvement directions are: selecting low-pollution alternatives for raw materials, reducing harmful by-products in the reaction process, recycling wastewater/slag, and transforming energy consumption to low-carbon energy.

II. Core Exploration Paths of Environmentally Friendly Production Processes

(1) Green Raw Materials: Replacement and Recycling of Low-Pollution Raw Materials

Raw material selection is the starting point of green processes. By replacing high-pollution raw materials and reusing waste, the environmental load is reduced from the source.

Green Preparation of Gluconic Acid to Replace the Nitric Acid Oxidation Method

The traditional preparation of gluconic acid by oxidizing glucose with nitric acid is the main source of NOₓ waste gas. It can be replaced by the "bio-oxidation method" or "electrochemical oxidation method":

Bio-oxidation method: Using microorganisms or enzyme preparations such as Aspergillus niger and glucose oxidase, glucose is oxidized to gluconic acid under normal temperature and pressure (30-35°C, normal pressure). The reaction conditions are mild, no NOₓ waste gas is generated, and the enzyme catalytic specificity is strong. The glucose conversion rate can reach more than 95%, and only a small amount of biological residue is produced (which can be used as feed additives or organic fertilizer raw materials). For example, using an immobilized glucose oxidase reactor, glucose solution (concentration 20%-25%) and air are introduced into the reactor. Under enzyme catalysis, glucose is gradually oxidized to gluconic acid. After the reaction solution is ultrafiltered to remove the enzyme preparation, it can be directly used in subsequent complexation reactions, avoiding the waste gas treatment costs and equipment corrosion problems of nitric acid oxidation in traditional processes.

Electrochemical oxidation method: Using titanium-based lead dioxide (Ti/PbO₂) as the anode, in an acidic electrolyte (0.1mol/L sulfuric acid), glucose solution (concentration 15%-20%) is electrolyzed to generate gluconic acid. The electrolysis voltage is controlled at 2.0-2.5V, and the current density is 30-50mA/cm². The glucose oxidation rate can reach more than 90%. This process has no waste gas discharge, and the only by-product is hydrogen (generated at the cathode, which can be collected as clean energy). Moreover, the sulfuric acid electrolyte can be recycled (with a small amount of loss supplemented regularly), reducing the generation of salt-containing wastewater.

Recycling and Impurity Control of Iron Powder Raw Materials

The unreacted iron powder filter residue (containing Fe, FeO, Fe₂O₃) in traditional processes can be reused after "acid washing-reduction" treatment: the filter residue is added to dilute hydrochloric acid (concentration 5%-8) to dissolve and generate ferrous chloride solution. After filtration to remove insoluble impurities (such as dust and mechanical impurities), iron powder is added to reduce Fe³+ in the solution (to avoid Fe³+ affecting product purity), resulting in a pure ferrous chloride solution. This solution can replace part of ferrous sulfate for the displacement reaction in the "gluconate displacement method", or react with sodium carbonate to generate ferrous carbonate, which then reacts with gluconic acid to generate ferrous gluconate. This realizes the closed-loop recycling of iron resources, reducing iron powder procurement costs and slag discharge.

(2) Clean Reactions: Design of Waste-Free Side Reactions and Process Simplification

By optimizing the reaction path, reducing the generation of by-products, and shortening the process flow, the environmental load of intermediate links is reduced, which is the core of green processes.

"One-Step" Biocatalysis-Complexation Coupling Process

Breaking through the traditional "two-step reaction" mode, the "glucose oxidation" and "ferrous complexation" are integrated into one step, and the mildness of biocatalysis is used to achieve clean reactions:

Glucose (concentration 20%-25%), iron powder (5%-8% excess, lower than the excess ratio of traditional processes), and immobilized glucose oxidase are added to the reactor, along with a small amount of oxygen (air is introduced, oxygen concentration controlled at 5%-8%). Under the conditions of 35-40°C and pH 4.0-4.5 (no strong acid adjustment required, pH is self-regulated by the gluconic acid generated in the reaction), glucose oxidase first oxidizes glucose to gluconic acid, and the generated gluconic acid immediately reacts with iron powder to generate ferrous gluconate. The reaction process has no NOₓ waste gas, and the excess ratio of iron powder is reduced, reducing the generation of filter residue. After the reaction, the immobilized enzyme (which can be reused 5-8 times) and a small amount of unreacted iron powder (sent back to the raw material recycling system) are separated by filtration. The filtrate is concentrated and crystallized (temperature 60-70°C, vacuum degree -0.08~-0.09MPa) to obtain ferrous gluconate crystals. The crystallization mother liquor contains a small amount of ferrous gluconate (concentration 2%-3%), which can be returned to the reactor for re-reaction. The yield can reach 88%-90%, an increase of 3%-5% compared with traditional processes, and wastewater discharge is reduced by more than 40% (only a small amount of wastewater is generated from equipment cleaning).

Supercritical CO₂-Assisted Reaction: Reducing Solvent Pollution

Traditional processes require water as a solvent (solvent dosage is 3-5 times the mass of raw materials), and subsequent concentration consumes a large amount of steam (energy consumption accounts for 40%-50% of total energy consumption). Wastewater mainly comes from crystallization mother liquor and cleaning water. Supercritical CO₂ (temperature 31.1°C, pressure 7.38MPa) can be introduced as a reaction medium and extractant:

Glucose, iron powder, and a small amount of glucose oxidase are added to a supercritical CO₂ reactor, and a small amount of water (only 1/10 of that in traditional processes) is introduced as the reaction medium. At 35-40°C and 8-10MPa, glucose is oxidized to gluconic acid and reacts with iron powder to generate ferrous gluconate. Supercritical CO₂ has both solvent and extraction functions. After the reaction, the pressure is reduced to normal pressure, CO₂ is vaporized and separated (recyclable with a recovery rate ≥95%), and ferrous gluconate precipitates as a solid. No concentration and crystallization steps are required, significantly reducing steam energy consumption (energy consumption is reduced by more than 60%). At the same time, the trace amount of wastewater only comes from the water generated in the reaction, which can reach the discharge standard after simple treatment (such as activated carbon adsorption), solving the problem of "high water consumption and high energy consumption" in traditional processes.

(3) Resource Utilization of Wastewater and Slag: Building a Circular Economy System

If wastewater and slag generated in the production process can be recycled, environmental discharge can be completely eliminated, forming a circular chain of "raw materials-products-waste-regenerated raw materials".

Hierarchical Treatment and Reuse of Wastewater

Even with clean reaction processes, a small amount of wastewater (such as equipment cleaning water and crystallization mother liquor) is still generated, which needs to be treated hierarchically before reuse:

Primary treatment (pretreatment): For wastewater containing gluconic acid and ferrous gluconate (COD ≥500mg/L), a combined "anaerobic fermentation-aerobic aeration" treatment is adopted. First, in an anaerobic reactor (temperature 35°C, hydraulic retention time 24h), microorganisms decompose organic matter such as gluconic acid into methane (which can be collected as boiler fuel to replace natural gas) and CO₂. Then, it enters an aerobic aeration tank (dissolved oxygen 2-3mg/L, hydraulic retention time 12h) to further degrade the remaining organic matter, reducing the COD to ≤100mg/L.

Secondary treatment (advanced purification): The pretreated wastewater adopts "ultrafiltration-reverse osmosis" membrane separation technology. Ultrafiltration (pore size 0.01-0.1μm) removes suspended solids and colloids in the water, and reverse osmosis (operating pressure 1.5-2.0MPa) removes salts and trace organic matter. The produced water quality meets industrial water standards (conductivity ≤100μS/cm, COD ≤20mg/L) and can be reused for reactor water replenishment, equipment cleaning, or cooling water, with a reuse rate ≥80%. The reverse osmosis concentrated water (containing a small amount of gluconate and salt) can be used to prepare sodium gluconate or as boiler desulfurization water (using the weak reducibility of gluconate to assist desulfurization), realizing "zero discharge" of wastewater.

High-Value Utilization of Slag

The slag generated in the production process mainly includes "unreacted iron powder filter residue" and "enzyme residue from the bio-oxidation method":

Iron powder filter residue: As mentioned earlier, after acid washing-reduction treatment, it generates ferrous chloride solution, which can be returned to the process as a raw material. If the iron content of the filter residue is low (≤50%), it can be mixed with clay and coal powder to make iron-based desulfurizers (for industrial waste gas desulfurization) or sent to steel plants as ironmaking raw materials, avoiding land occupation by slag landfill.

Enzyme residue: The ultrafiltration residue of enzyme preparations generated by the bio-oxidation method (containing protein and microbial cells) has a protein content of 40%-50% after drying (temperature 60-70°C). It can be used as a feed additive (for livestock feed to supplement protein) or made into organic fertilizer through composting (adding straw and manure, composting temperature 55-65°C, maturity time 15-20 days) for agricultural planting, realizing the transformation of "slag to resource".

(4) Low-Carbon Energy Consumption: Clean Energy Replacement and Process Energy Saving

Energy consumption is an important environmental impact factor in the production process. Carbon emissions can be reduced through clean energy replacement and process energy saving.

Clean Energy-Driven Production

The heating in traditional processes (such as reactor heating and concentration crystallization) mainly relies on coal-fired or natural gas boilers, which have high carbon emissions. It can be replaced by "solar energy + biomass energy" combined energy supply:

Solar heating: A solar heat collection system (using flat-panel solar collectors with a heat collection efficiency ≥45%) is built in the factory area to heat water to 60-80°C, and then provide preheating energy for the reactor through a heat exchanger to meet the initial temperature requirements of the reaction (which can cover 30%-40% of the heating load).

Biomass energy supplementary energy: Agricultural wastes (such as straw and rice husks) are made into biomass pellet fuel, which is burned to provide steam for the concentration and crystallization process (the carbon emission of biomass combustion is much lower than that of coal combustion, and carbon cycle can be realized). Alternatively, a biomass gasification device is built to convert biomass into biomass gas (mainly composed of CO, H₂, and CH₄) to drive gas turbines for power generation, providing electricity for workshop equipment (such as pumps, fans, and electrolysis devices) to achieve energy self-sufficiency.

Process Energy Saving Optimization

Energy consumption is reduced by optimizing reaction parameters and equipment design:

Reaction temperature optimization: The bio-oxidation method reduces the reaction temperature from the traditional 80-90°C to 30-35°C, reducing heating energy consumption. At the same time, using the slight exothermicity of the reaction (the heat of reaction for glucose oxidation to gluconic acid is about 120kJ/mol), the reaction heat is transferred to the raw material liquid through a heat recovery device (such as a spiral plate heat exchanger) to preheat the raw material, reducing heating demand.

Equipment energy-saving transformation: The traditional atmospheric pressure concentration and crystallization are changed to "vacuum low-temperature concentration" (vacuum degree -0.09~-0.095MPa, concentration temperature 45-50°C) to reduce the temperature required for concentration and reduce steam consumption. The reactor uses a high-efficiency stirring device (such as a propeller stirrer, with stirring efficiency increased by 20%-30%) to shorten the reaction time (from the traditional 4-6h to 2-3h) and reduce equipment operation energy consumption.

III. Challenges and Prospects of Green Processes

Currently, environmentally friendly processes still face some challenges: the high cost of enzyme preparations in the bio-oxidation method (although the service life of immobilized enzymes can reach 5-8 times, the initial investment is large), the higher equipment investment of the supercritical CO₂ process (high-pressure reactors and CO₂ recovery systems) compared with traditional processes, and the easy contamination of membrane modules for wastewater reuse (regular cleaning is required, increasing maintenance costs). However, with technological iteration, these problems can be gradually solved. For example, genetic engineering modification of enzyme preparations (improving enzyme activity and stability, reducing costs), localization of supercritical equipment (reducing equipment prices), and anti-pollution coating technology for membrane modules (extending membrane life) all provide support for the industrial application of green processes.

In the future, the green process of ferrous gluconate will develop towards "full-chain greening": from "bio-based glucose" at the raw material end (preparing glucose from biomass such as straw to replace grain-based glucose), to "solvent-free/less-solvent reaction" at the reaction end, and then to "full resource utilization" at the waste end. Finally, a production model of "zero pollution, zero discharge, and low carbonization" will be realized. This will not only improve the environmental competitiveness of enterprises but also provide a replicable process model for the green development of the food additive and pharmaceutical industries.