«○●—[Preface]—●○»
Residues are part of the natural cycle and economy, and the amount of residues has increased significantly over the past few decades due to the overdevelopment of industrial processes.
Industrial manufacturing of alumina uses the Bayer process to produce large quantities of solid residues in powder form, which are mainly composed of alumina, aluminum metal hydroxide and iron ore.
The production stage with the highest waste generation rate is primary production with 136.5 kg/ton, followed by secondary production with 122.0 kg/ton, the material does not meet industrial specifications, so it is stacked in landfills and causes environmental pollution, while another process to produce aluminum-containing waste is aluminum anodizing, which consumes a lot of water and may lead to the generation of a large amount of sludge.
«○●—[Characteristics of aluminum hydroxide]—●○»
Aluminium hydroxide is the most commonly used flame retardant in the polymer conversion industry because of its low cost, easy availability and good processability thermoplastics.
Among the various polymers currently modified as flame retardants, polypropylene (PP) and polyethylene (PE) are most commonly studied to produce charged materials.
In this context, this study employs aluminum nanocomposites to improve the thermal and mechanical properties of polymeric materials used as flame retardant additives in the future, and the use of aluminum anodized scrap as a precursor for aluminum nanoparticles to bind into polymers is considered advantageous.
The advantages include reduced environmental impact associated with disposing of these residues in landfills, the use of low temperatures and atmospheric pressure in precipitation, which ensures process safety and low energy consumption, and reduced consumption of analytical reagents used to obtain aluminum nanoparticles.
Low concentrations of extractants and precipitants and consequent reduced environmental concerns reduce or inhibit the toxicity of combustion by-products, as well as the versatility of nanoparticles in different processes.
«○●—[Preparation of aluminum hydroxide nanocomposites]—●○»
Prior to the synthesis of aluminum hydroxide nanoparticles, this step involves obtaining nanoparticles (NP-Al) from AAW using a wet chemical precipitation method that follows the method described in the literature.
First, AAW needs to be dried in an oven at 105 degrees, after which it is impregnated, weighed, and characterized, and for each assay, 10 g of treated residue is dissolved in 200 ml of deionized water.
One containing 100 ml of hydrochloric acid at a concentration of 0.96 ml was then added to the mixture, the extraction process was carried out in a glass reactor, using a digital thermostatic bath as the heating source, and the stirring was carried out in a 300-degree digital overhead agitator after rpm filtered with UNIFIL filter paper.
The resulting materials were used to evaluate the effects of temperature, sodium hydroxide concentration and precipitant (HCI) on the efficiency of the aluminum nanoparticle precipitation process, and the parameters were determined as 50 and 80 temperatures, NaOH concentrations of 3.3 and 6.1 mol/L (both 50 mL) for the production of sodium aluminate, and 4.8 and 8.4 mol/L (both 50 mL) for the precipitant HCl.
Once the acid leaching is complete, 10 ml of deionized water is added to the solid material at 200 and the mixture is reconditioned in a reactor for the production of sodium aluminate by dropwise addition of NaOH according to the conditions established in the experimental design.
Nanoaluminum and aluminum anodized scrap (AAW) are then added to linear low-density polyethylene (LLDPE) polymers to produce LLDPE/nanoaluminum and LLDPE/AAW composites in different proportions.
Different recipes containing NP-Al and AAW are prepared at 145C in a blow molded film extruder that produces a masterbatch, which consists of an industrial single screw extruder with seven heating zones, the heating zone is kept at 165c for the first two zones, 170c for zones 3-6 and zone 180c for the last zone at 90 rpm.
This introduction parameter is based on LLDPE specifications and thermogravimetric analysis, and different concentrations of LLDPE/NP-Al nanocomposites are formed in a blow molding cavity through a feed hopper and polymer film to a thickness of 0.25.
The samples were processed in a furnace at a temperature of 1200 to obtain alumina, with a heating slope of 10C/min and a combustion standard of 2h, XRF analysis allowed the quantification of residues and oxides present in eight synthetic samples, in the EDX7000 device, the parameters used in the test were 15 volts kV and 10uA.
Using the LabXXRD-6100 diffractometer, with copper radiation at power of 40, the crystal phase was evaluated by XRD technology, measured using a step size of 0.02 and a step duration of 0.6, and the database available in the software Match3 was used to interpret the results and identify the phase.
The percentage of crystalline material in the sample is specified by the diffraction pattern, peak integration is performed using the Origin software, and the crystallinity is calculated by dividing the value corresponding to the peak area by the estimate of the total area.
The morphology of the processed material was evaluated using electron microscopy techniques in SEM and TEM microscopy, and for SEM analysis, samples were coated with a gold layer prior to analysis.
The transition induced in materials over different temperature ranges is determined by thermogravimetric analysis using a synchronous analyzer, which is made of synthetic air (21% O2/79% Nz) heated at a rate of 10C/min until a temperature of 1200 is reached.
High purity oxygen in the device (model TGA50) is evaluated for determining the chemical composition of the nanocomposites and the temperature range of dehydration, oxidation, combustion and decomposition reactions corresponding to the nanocomposites, for analysis in the temperature range from 25 to 800c, heating rate of 10 and flow rate of 50 ° C / min ml/min.
The properties of flame-retardant materials are determined by the following method of analysis adapted according to UL94VB and UL94HB standards, and the specimen used in the analysis consists of a sample of size 12.5.
The sample was secured with grippers to allow 12 inches of distance between the claw and the bottom end of the material, and a distance of 20 cm from the floor where the flame was placed.
The material is labeled 10 cm to indicate the end point of combustion, the sample is exposed to a flame with a height of about 3 cm, a total of 5, once the combustion is complete, the flame is removed and the linear combustion rate of the sample is calculated.
«○●—[Chemical Analysis]—●○»
The main component of AAW is aluminum (Al2O3) at a level of 60.42%, and the concentration of aluminum in the residue is the result of the decoating step during the surface treatment, and the results also indicate the presence of other compounds (oxides) or impurities in the material, such as silica (SiO2), magnesium oxide (MgO) and sulfur trioxide (SO3).
These results are expected, especially since the electrolysis process using sulfuric acid solutions results in the formation of an anode layer on the surface of the aluminum, and the high level of loss on ignition is related to the presence of hydroxyl groups in the anodic oxidation residue.
AAW exhibits a high water content, which is due to the presence of strongly hydrated minerals and minor elements such as calcium oxide (CaO), iron ore (iron 2O3) and nickel oxide (NiO), which are formed due to contamination and chemical reagents used in washing water treatment, and AAW's chemical analysis data highlights its potential as a raw material for the production of aluminum hydroxide nanoparticles.
The chemical composition of the AAW used in this study is similar to that studied in the literature, and it is important to emphasize that these authors used AAW as a component in several processes, none of which synthesized aluminum hydroxide nanoparticles from this residue.
Differences in the chemical composition of AAW and synthetic NP-Al(1-8), except for samples NP-Al1 and NP-Al3, a high percentage of aluminum was observed after the precipitation process in almost all nanoparticles obtained in this study.
Although the percentages of other compounds occasionally vary, aluminum is the dominant component in all synthetic samples with a percentage of more than 50%, and the reduction in the percentage of oxide after NP-Al(1-8) synthesis is consistent with the synthesis process, where the residue undergoes several purification steps until the final product is formed.
Following the experimental design, we need to characterize NP-Al in pairs, using the lowest concentrations of NaOH and HCl for NP-Al1 and NP-A15 synthesized at 50 and 80.
According to the XRF results, the percentage of aluminum (Al) in NP-Al5 is better than the percentage in NP-Al1, indicating the positive effect of temperature in the process.
For NP-Al2 and NP-Al6, using the highest concentration of NaOH and the lowest amount of HC1 at temperatures of 50C (NP-A12) and 80C (NP-Al6), the results of Al composition in the sample showed a higher percentage of Al in NP-Al6, indicating that both temperature and NaOH concentration had a positive effect on the results.
In contrast, the lowest concentration of NaOH and the highest amounts of HC1C (NP-Al3) and 80C (NP-Al7) were used in the synthesis of NP-Al3 and NP-A17.
The percentage of Al observed in NP-A17 is higher than that of NP-Al3, which has the lowest percentage of Al of all synthetic samples, and as for NP-A14 and NP-A18, which are synthesized at 50C (NP-A14) and 80C (NP-A18), a higher percentage of Al is observed in NP-A14, which presents the highest percentage of Al among all synthesized samples.
The concentration of NaOH in these samples appears to be a decisive factor in the formation of sodium aluminate from residues, suggesting that higher NaOH concentrations combined with higher temperatures favor the formation of aluminate, a precursor to sodium hydroxide nanoparticles.
XRF results show that the concentration of HCl has no effect on the process, and the leaching rate at the beginning of the reaction is strongly dependent on the proton concentration in the oxide (CI) anion.
Higher HCI concentrations result in less acid ionization occurring, thus negatively affecting leaching rates and process efficiency, as an increase in acid concentration increases the amount of metal cations in the solution that compete with aluminum to attract chloride ions.
The addition of NP-Al appears to alter the maximum tension polymer film; The values of pure LLDPE membranes are significantly different from all membranes to which NP-Al charges are added, and as the concentration of NP-Al in the LLDPE matrix increases, the maximum tension of the sample decreases.
Significant reduction in deformation in membranes containing 1/1% and 5% NP-Al and 2/3% and 5% NP-Al, although no significant differences were observed between pure LLDPE and other samples, but the addition of higher percentages of NP-Al to LLDPE resulted in a reduction in their deformation.
This behavior is to be expected, as the addition of nanoparticles to the polymer chain prevents them from forming a more rigid structure during analysis, i.e. at high concentrations, discontinuous effects between the two phases result in lower elongation at break.
The modulus of elasticity of the polymer film, which did not show significant variation between pure LLDPE and most samples doped with AAW or NP-Al, observed the opposite behavior for sample NP-Al6/3%; The modulus of elasticity is statistically higher than pure LLDPE.
This may be related to boehmite, the main component of NP-Al, which is able to improve the performance of polymers by improving their mechanical properties.
The polymer nanocomposites, which exhibit good flame retardant properties while maintaining the mechanical properties of the base polymer, are quite promising for this field.
Studies have shown that flame retardancy increases with the reduction in the size of aluminum hydroxide particles, which is often used in nanoscale sizes or in combination with other halogen-free flame retardants to enhance its flame retardant effect.
The effect of flammability properties can be attributed to the rate of decomposition of filler materials and/or the formation of more stable ash, and nanoparticles as flame retardants show higher performance than AAW, confirming XRD results for calcined materials.
An important difference between the two compounds is that calcination of AAW produces alumina (corundum), while NP-Al forms corundum and hydrated alumina, the formation of hydrated alumina can be correlated with improvements in flammability tests.
«○●—[Conclusion]—●○»
The anodizing residue of aluminum is successfully converted into aluminum hydroxide nanoparticles in the form of trihydroxyaluminite and boehmite with filamentous morphology, and temperature is one of the factors affecting morphology and phase formation.
The resulting nanoparticles exhibit a crystallinity of about 44%, which is nearly 50% higher than that of aluminum anodizing residues, as observed in the analysis of material characterization.
The addition of aluminum hydroxide nanoparticles (1%, 3%, and 5%) to LLDPE did not significantly alter the mechanical properties of the material, which was demonstrated by the following analysis.
Nanocomposites (NP-Al6) containing 3% aluminum hydroxide obtained in condition 6 exhibit a higher average modulus of elasticity than pure polymers and other nanocomposites.
Aluminium hydroxide nanoparticles used as flame retardant additives in polymeric materials and their traction mechanical properties exhibit satisfactory properties sufficient for more in-depth study of potential future applications.
Other results of this study suggest that it is possible to convert aluminum hydroxide nanoparticles into alumina nanoparticles (smaller than 50 nm in size) by a heat treatment process.