Slag Deposits Research Articles

Wear mechanism of oxide-bonded SiC refractory tiles in the combustion zone of waste-to-energy facilities

Refractory tiles used in waste-to-energy facilities are exposed to high temperatures (Tflue-gas ≈ 1200 °C) and corrosive environment with gases and fly ashes. Post-mortem examination of refractory tiles from the combustion zone of waste-to-energy plants was conducted to understand the damage mechanism of the refractory material. Corroded refractory tiles from this area showed an important loss of matter and large cavities on the hot surface. Investigations highlighted the weakness of the refractory matrix of the material. In contact with slag deposit and corrosive gases, the bonding phase of the refractory material is dissolved and SiC grains are oxidized which causes the spalling of the hot face of refractory tiles.

Recycling of Local Qatar’s Steel Slag and Gravel Deposits in Road Construction

Recycling of Local Qatar’s Steel Slag and Gravel Deposits in Road Construction

Tracing Soil Contamination from Pre-Roman Slags at the Monte Romero Archaeological Site, Southwest Spain

Soil serves as a repository of human history, preserving artifacts within its horizons. However, the presence of chemically reactive remnants, such as ancient slags, can significantly impact the surrounding soil environment. This paper addresses this scarcely explored issue by focusing on soil contamination arising from pre-Roman slag deposits at the Monte Romero archaeological site in southwest Spain, dating back to the Tartessian period (c. 7th century BC). Through the high-resolution microscopy examination of slag wastes and the trace element analysis of soil samples by ICP-OES, this study evaluated current contamination status using a multi-index approach. The results revealed markedly high levels of Pb (>5000 mg kg−1), Cu (up to 2730 mg kg−1), and As (up to 445 mg kg−1) in the soil compared to a control sample. The identification of secondary complex compounds like Cu arsenates and Pb arsenates/antimonates within slag cavities suggests post-depositional weathering processes, leading to the dispersion of potentially toxic elements into the surrounding soil. Assessments through indices of contamination and potential ecological risk highlighted severe contamination, particularly concerning Ag, Pb, Sb, Cu, and As. This study underscores the importance of addressing potential environmental hazards associated with archaeological sites hosting remnants of metal production.

Environmental behaviour of iron and steel slags in coastal settings

Iron and steel slags have a long history of both disposal and beneficial use in the coastal zone. Despite the large volumes of slag deposited, comprehensive assessments of potential risks associated with metal(loid) leaching from iron and steel by-products are rare for coastal systems. This study provides a national-scale overview of the 14 known slag deposits in the coastal environment of Great Britain (those within 100 m of the mean high-water mark), comprising geochemical characterisation and leaching test data (using both low and high ionic strength waters) to assess potential leaching risks. The seaward facing length of slag deposits totalled at least 76 km, and are predominantly composed of blast furnace (iron-making) slags from the early to mid-20th Century. Some of these form tidal barriers and formal coastal defence structures, but larger deposits are associated with historical coastal disposal in many former areas of iron and steel production, notably the Cumbrian coast of England. Slag deposits are dominated by melilite phases (e.g. gehlenite), with evidence of secondary mineral formation (e.g. gypsum, calcite) indicative of weathering. Leaching tests typically show lower element (e.g. Ba, V, Cr, Fe) release under seawater leaching scenarios compared to deionised water, largely ascribable to the pH buffering provided by the former. Only Mn and Mo showed elevated leaching concentrations in seawater treatments, though at modest levels (<3 mg/L and 0.01 mg/L, respectively). No significant leaching of potentially ecotoxic elements such as Cr and V (mean leachate concentrations <0.006 mg/L for both) were apparent in seawater, which micro-X-Ray Absorption Near Edge Structure (μXANES) analysis show are both present in slags in low valence (and low toxicity) forms. Although there may be physical hazards posed by extensive erosion of deposits in high-energy coastlines, the data suggest seawater leaching of coastal iron and steel slags in the UK is likely to pose minimal environmental risk.

Dittrichia graveolens (Asteraceae), rapidly spreading along European highways, recorded in Lithuania

For a long time, railways were the main pathway for the accidental introduction of alien plant species, but now highways and other roads are becoming increasingly important in this process. Considering that Dittrichia graveolens is spreading rapidly along highways in Central Europe and neighbouring Poland, a targeted search was conducted in Lithuania in 2023. Dittrichia graveolens was recorded at two sites along the Kalvarija–Kaunas highway (Kazlų Rūda district, in the environs of Pentupiai, and Kaunas city, in the environs of Jonučiai). At both sites, the species occupied relatively compact stands consisting of several hundred individuals, suggesting that the stands were formed from locally matured seeds. The mean number of flowers per capitulum in Lithuania in 2023 was 20.5 ± 4.9, and the realised fecundity was 89.7% of the potential fecundity. It is presumed that the seeds of Dittrichia graveolens were accidentally introduced into Lithuania by trucks, probably attached to tyres. Currently, this species is classified as a casual alien, but its naturalisation is expected. The study revealed that the first record of Dittrichia graveolens in Poland was not in 2013, as previously reported, but more than 20 years earlier, in 1990, near Kraków, at the slag deposit site of the Nowa Huta Steelworks.

Dittrichia graveolens (Asteraceae), rapidly spreading along European highways, recorded in Lithuania

For a long time, railways were the main pathway for the accidental introduction of alien plant species, but now highways and other roads are becoming increasingly important in this process. Considering that Dittrichia graveolens is spreading rapidly along highways in Central Europe and neighbouring Poland, a targeted search was conducted in Lithuania in 2023. Dittrichia graveolens was recorded at two sites along the Kalvarija–Kaunas highway (Kazlų Rūda district, in the environs of Pentupiai, and Kaunas city, in the environs of Jonučiai). At both sites, the species occupied relatively compact stands consisting of several hundred individuals, suggesting that the stands were formed from locally matured seeds. The mean number of flowers per capitulum in Lithuania in 2023 was 20.5 ± 4.9, and the realised fecundity was 89.7% of the potential fecundity. It is presumed that the seeds of Dittrichia graveolens were accidentally introduced into Lithuania by trucks, probably attached to tyres. Currently, this species is classified as a casual alien, but its naturalisation is expected. The study revealed that the first record of Dittrichia graveolens in Poland was not in 2013, as previously reported, but more than 20 years earlier, in 1990, near Kraków, at the slag deposit site of the Nowa Huta Steelworks.

FEATURES OF THE FORMATION, INTAKE AND DISTRIBUTION OF THE IRON-CONTAINING COMPONENT IN THE WATER SUSPENSION OF THE DNIEPER RIVER WITHIN ZAPORIZHZHIA CITY

The article presents the results of research devoted to one of the aspects of environmental issues, namely: the determination of the distribution features of the solid component of the river suspension containing iron oxide; investigation of the morphological characteristics of iron-containing particles; and the discovering of their genetic affiliation to various technological processes of metallurgical production. The results of many years of continuous comprehensive research on the distribution of iron in the suspended matter of the Dnieper water within Zaporizhzhia city are presented. Field observations were carried out by scientists of the Institute of Geological Sciences of the National Academy of Sciences of Ukraine (IGS) in cooperation with specialists from the State Institution “Scientific Hydrophysical Center of the National Academy of Sciences of Ukraine” (SHC). Analytical studies of the solid phase of the suspended matter were done in the Center for Collective Use of Scientific Equipment of the National Academy of Sciences of Ukraine and the Educational and Scientific Institute “Institute of Geology” of Taras Shevchenko Kyiv National University. The publication considers the seasonal features of the distribution of the total iron content in suspension and the relationship of its monthly changes with the distribution of concentrations of such heavy metals as copper, chromium, nickel, zinc. Considerable attention is paid to the examination of iron-containing particles present in suspension, their chemical and trace element composition and the allocation of their individual categories in accordance with the technogenic processes during which they were formed. Comprehensive analysis of suspension samples made it possible to identify and classify a group of typical iron-bearing formations that are inherent in the territory of Zaporizhzhia city. Based on a set of characteristics, three main genetic categories have been identified: spherical mineral aggregates represented by iron oxide; clastic fragments of Fe2O3, and aluminosilicate spherules (formation, the main or essential component of which is aluminum oxide with an admixture of iron). The obtained data showed a significant influence of the metallurgical industry on the chemical composition of the Dnieper suspension within Zaporizhzhia. Comparison of iron-containing particles observed in the Dnieper suspended matter with the substance filtered out by gas purification systems of various parts of metallurgical production and slag deposits showed that the most active emission to the environment occurs during the preparation and smelting of iron ore.

Laser modification of heating surfaces: A new approach to reduce boiler slagging

Slagging and fouling of boilers are typical technological problems in the combustion of solid fuels (especially coal and biomass). This study proposes a novel approach based on laser texturing of the near-surface layer of steel used to make heating surfaces of coal-fired boilers. This approach is aimed at improving the resistance of metal surfaces to slagging. The formation of slag deposits on the surfaces of AISI 310 S steel modified by laser radiation has been experimentally studied. The formation of slag melt was analyzed on the surface of modified and polished steel plates during heating to 1400 °C. High-temperature heating and interaction with slag led to a change in the chemical composition of the steel. A significant increase in the chromium content (up to 47%) of the steel surface and the transformation of its texture with the formation of grains up to 2.5 µm in size were established. Laser surface treatment significantly increased the resistance of the steel to such changes and also reduced the grain size to 1.5 µm. As a result of laser modification of surfaces, the temperature of slag formation shifts to higher values (higher by 60–75 °C compared to the unmodified polished surface). The anisotropic texture (cauliflower type) contributed to a decrease in the size of the slag spreading front, and also had by self-cleaning properties from the mineral melt. The latter allows to increase the reliability and energy efficiency of boiler equipment.

Research on Water Quality Indicators at the Rovinari Power Plant

Abstract Indicators of: pH, temperature, conductivity, filtered residue at 105°C, suspended matter, ammonium, chemical oxygen consumption-CCOCr, chlorides, biochemical oxygen consumption - CBO5, sulfates, extractable substances with organic solvents, petroleum products, total iron, calcium, magnesium, sulfides and hydrogen sulfide. The excesses recorded in the sulfate and ammonium indicators in the phreatic waters within the unit premises and in the area of the slag and ash deposits are mainly due to natural causes and to a small extent to anthropogenic action. In accordance with the specialized literature, but also with the studies and research carried out, it is found that the exceedances recorded in the Oltenia area of the specified indicators are due to:- for ammonium (NH4+) - the primary conditions in the Pliocene basin for the formation of aquifer sand deposits; - for sulfates (SO42-) - washing by underground waters of lignite layers in which sulfides frequently appear, especially in the form of pyrite.

Quantifying CO2 Removal at Enhanced Weathering Sites: a Multiproxy Approach.

Enhanced weathering is a carbon dioxide (CO2) mitigation strategy that promises large scale atmospheric CO2 removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO2 mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ13C, 87Sr/86Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO3 deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans.

Unreported Fe and Pb-Cu slag heaps in Populonia hinterland (Tuscany, Italy): quick geochemical discrimination and mapping

This paper describes a topographic and geochemical quick field survey approach for the investigation of Fe and Pb-Cu slag heaps deriving from the ancient smelting of Fe-oxides and Pb(-Ag)-Cu sulfides. The quick field survey method, joined together with geochemical analyses (HH-XRF) of slags and ore samples provided: (i) the extension of the slag heaps, (ii) the environmental contextualization, (iii) the type and chemistry of the slags and ore minerals, (iv) the connection between mining and smelting sites. The several Pb-Cu slag heaps in Schiumai site (nearby Campiglia Marittima town) were described in detail, since this site is still undescribed despite its importance as one of the most extended slag deposits in the area. The other smaller slag heaps detected in the Populonia hinterland were also mapped and a general description was provided. The HH-XRF geochemical analyses, coupled with macroscopic observations, have proven to be sufficient to discriminate between Fe and Pb-Cu slags: the Fe-slags have FeO >60 wt% and Pb+Cu+Zn

Analytical study on heavy metal output fluxes and source apportionment of a non-ferrous smelter in southwest China

Abandoned Pb/Zn smelters are often accompanied by a large amount of smelting slag, which is a serious environmental problem. Previous studies have demonstrated that slag deposits pose an environmental threat even if the smelters are shut down. Herein, a Pb/Zn smelter and its impacted zone in GeJiu, Yunnan, China were selected as the study area. The risk and source apportionment of heavy metals (HMs) in the soil of the impacted zone were systematically studied. Based on the hydrogeological features, the migration path and output fluxes of the HMs released from smelting slag to the impacted zone were investigated. The HM contents (Cd, As, Zn, Pb, and Cu) in the soil substantially exceeded the screening values of the Chinese soil standard (GB15618-2018). Based on the results of the Pb isotopic and statistical analyses for source apportionment, the contaminated sites and agricultural irrigation water had a large impact on the HMs of soil. The hydrological analysis results showed that runoff, as an HM migration path under rainfall, continued to affect the environment. The water balance calculations using the Hydrologic Evaluation of Landfill Performance model showed that the rainfall was distributed on site as follows: evaporation (57.35%), runoff (32.63%), and infiltration (10.02%). Finally, the output fluxes were calculated in combination with the leaching experiment. As, Zn, Cd, Pb, and Cu runoff had the output fluxes of 6.1 × 10−3, 4.2 × 10−3, 4.1, 1.4 × 10−2, and 7.2 × 10−4 mg/kg/y, and infiltration of 1.9 × 10−3, 1.3 × 10−3, 1.3, 4.0 × 10−4, and 2.2 × 10−4 mg/kg/y, respectively. Therefore, this study offers theoretical and scientific recommendations for effective environmental management and engineering remediation.

Lead isotope fingerprinting techniques help identify and quantify 3000 years of atmospheric lead pollution from Laguna Roya, northwestern Iberia

Legacy pollution research has established that over the past 3000 years, mining and metallurgical activities have resulted in widespread deposition of lead (Pb) pollution. However, there is still a limited understanding of how humans have impacted the long-term cycling of Pb in the environment. We present a 4,000-year lake sediment Pb isotope record from Laguna Roya, northwestern Iberia, that identifies and quantifies the predominant sources of atmospheric Pb pollution. For the first time, Pb isotopic compositions of ancient slag samples dated (∼600 BCE–200 CE) from a mining district in the southwest of the Iberian Peninsula are compared to Pb isotope ratios of Pb pollution deposited contemporaneously in lake sediments. In addition, literature Pb isotope ratios of ores from mining regions throughout Iberia are compared with those of leaded gasoline and coal to identify additional sources of anthropogenic Pb. Deposition of atmospheric Pb pollution begins after 950 BCE, and until 1750 CE, the Pb isotopic composition most resembles the southwestern slag deposits, containing a mixture of Pb ores from southeast Iberia (up to 36%) and southwest Iberia (∼74%). Between 1750 and 1960 CE, Pb pollution is attributed to Pb mining in southcentral Iberia. After 1960 CE, the dominant Pb pollution source (∼85%) is again metal refining in southwestern Iberia, and only ∼15% is from leaded gasoline. Provenance and reconstruction of the temporal and spatial distribution of legacy Pb pollution further our understanding of how humans have affected the biogeochemical cycle of this toxic element in the environment over time.

Recycling of steel slag powder in green ultra-high strength concrete (UHSC) mortar at various curing conditions

The large quantity of steel slag deposit has caused great environmental pressure. This study aims to recycle steel slag as binder in the production of green ultra-high strength concrete (UHSC) subjected to different curing regimes (i.e. standard and steam curing). The fresh behaviors, mechanical properties, microstructure and ecological value of the UHSC are explicitly assessed with the help of the Funk and Dinger particle packing model. The results show that the inclusion of steel slag powder (SSP) increases the workability (up to about 13%) and wet packing density of UHSC. The use of SSP results in a decrease of early compressive strength, because the SSP inhibits cement hydration at early stage. The steam curing can minimize this negative impact of SSP on the early compressive strength (>120 MPa). Besides, the increase of SSP dosage results in a reduction in the flexural strength for both early and later ages, but steam curing reduces the decline ratio. The high temperature curing not only promotes the hydration process of both cement and SSP, but also accelerates the pozzolanic reaction of silica fume, resulting in a lower porosity (4.78%) of UHSC with SSP. Finally, the ecological assessment shows that the recycling SSP as a replacement of cement in the UHSC system can reduce energy consumption and carbon emissions. Therefore, the results in this study indicate the feasibility of utilizing SSP in the fabrication of sustainable UHSC products with excellent performance.

Slagging Characteristics of a Steam Boiler Furnace with Flare Combustion of Solid Fuel When Switching to Composite Slurry Fuel

Two interconnected mathematical models have been developed to describe slagging of a steam boiler furnace at the macro and micro levels. The macro-level model is implemented in Ansys Fluent. Using the fuel characteristics and temperature in the furnace, this model can predict the characteristics of ash formation on heat exchanger tubes when the melting temperature of the mineral part of solid fossil fuel is exceeded. The obtained values of slagging rates are used as initial data in the software implementation of the original Matlab microlevel model. Under conditions of dynamic change in the thickness of the slag layer, this model can evaluate the heat transfer characteristics in the hot gas/slag layer/tube wall/water coolant system. The results showed that switching a coal-fired boiler from a solid fossil fuel to a fuel slurry will improve stability and uninterrupted boiler operation due to a lower slagging rate. The combustion of coal water slurries with petrochemicals compared with coal–water fuel is characterized by higher maximum temperatures in the furnace (13–38% higher) and a lower average growth rate of slag deposits (5% lower), which reduces losses during heat transfer from flue gases to water coolant by 2%.

Assessment of potential exposure to As, Cd, Pb and Zn in vegetable garden soils and vegetables in a mining region

Mining and smelting activities can contaminate soils and affect farming due to high emissions and input of potentially toxic elements (PTE) into the environment. Soils (sampled from two depths) and market vegetables from vegetable gardens located within the vicinity of unconfined slag deposits from decades of mining and smelting activities in Kutná Hora, Czechia were assessed to determine to what extent they pose a health hazard to communities that use these gardens. Pseudo-total As concentrations in the soils exceeded background levels (4.5 mg kg−1) 1.9–93 times, with higher concentrations in the deeper layer. The pseudo-total concentrations of PTE in soils ranked in the order As > Zn > Cd > Pb. Phyto-available concentrations of PTE in soils were relatively low, compared to pseudo-total concentrations. Concentration of As, Cd, Pb and Zn in the vegetables exceeded guideline values, with the highest concentrations found in the fruits of cucumber, peppers, and zucchini. Despite low phyto-available PTE concentrations in soils, all the PTE concentrations in the vegetables surpassed the guidelines set by the Czech Ministry of Health and EU directive, indicating a health hazard to consumers.

Penetration of a Pulsed Gas Jet through a Package of Heat Exchange Tubes

The slagging of heating surfaces by fly ash significantly complicates the long term operation of solid-fuel boilers. Gas pulse technologies offer a potentially convenient, inexpensive, yet efficient way for online slag deposits removal. The creation and application of real boiler cleaning systems using gas pulse technologies is a relatively new direction in the energy sector and requires scientific research on various aspects. In this work, the numerical investigation of the gas-dynamic flow arising when the package of heat-exchange tubes is blown with single air jet created by a pneumopulse cleaning system has been carried out. The forces acting on the heat-exchange tubes located at distances 1.0 m and 2.0 m from the exhaust nozzle with various configurations of tubes in the package have been determined. It is shown that an increase in the pressure of the generated air jet makes it possible to significantly increase the acting forces at large distances from the exhaust nozzle. The results of this study and similar calculations for real geometry will increase the validity of engineering solutions used in the development of pneumopulse cleaning systems for large power boilers.

The Impact of Medieval Mining upon the Environment of the Central Balkans

The paper examines the impact of medieval mining upon the environment, based upon the present state of the historiographical, archaeological, and geological research. By the middle of the 13th century a massive increase in the exploitation of precious metals is noted, inducing the appearance of settlements in the vicinity of the locations of extraction and production of ores. The problems of water and air pollution occurred, raised by the operation of smelteries inside these settlements or in their immediate vicinity. Although the presence of slag indicates certain metallurgical activity, it is hard to discern the precise purpose and the period of their activity without archaeological excavations. The increase in demand for fuel may have induced deforestation, so the rights to exploit forests were legally regulated. The scope of wood exploitation in the region of Šumadija has been judged by the research estimating the presence of potassium in the previously registered old slag deposits. It is hard to discern the general health status of the inhabitants of the mining regions, since a small portion of skeletal material is examined, not including the one from the most important mining center of Novo Brdo. The anthropological analysis indicates the problems caused by hard physical activity and poor hygiene. The present knowledge on the influence of heavy metals indicates the impact of past mining practices upon the environment and human health. Decomposition of sulphides in the discard deposits and old slag, as well as the leak of acid waters from old shafts, still affect the present pollution of water and soil in the vicinity of mining centres such as Rudnik and Srebrenica. These same reasons may have presented dangers for the population living there during the period of 15th to 17th century. The future archaeological research, with the inclusion of other disciplines, will bring more detailed understanding of the relationship between people and their environment in the mining regions of the Medieval Serbia.

ИССЛЕДОВАНИЕ МЕХАНИЗМА ШЛАКОНАКОПЛЕНИЯ В КОНТУРАХ СО СВИНЕЦСОДЕРЖАЩИМИ ТЕПЛОНОСИТЕЛЯМИ И ЕГО ПРЕДОТВРАЩЕНИЕ ВЫСОКОТЕМПЕРАТУРНОЙ ОБРАБОТКОЙ СМЕСЯМИ ВОДОРОДА И ВОДЯНОГО ПАРА

The results of computational and experimental studies of the properties of lead and lead-bismuth coolants and the mechanism of physicochemical processes occurring in their circulation circuits explain the reasons for the formation of slag deposits based on lead oxide. First of all, this is the uncontrolled interaction of lead and lead-bismuth coolants with gaseous oxygen, which, along with nitrogen, is one of the main components of air. Slag deposits are based on lead oxides, but they also contain a significant amount of unoxidized lead (and bismuth in the case of using a lead-bismuth coolant) bound by a lead oxide framework into a single structure, as well as a small amount of components of construction materials (iron , chrome, nickel). The formation of slags in the circulation loop with a lead-containing coolant has a negative effect on its operation: the flow cross-sections are narrowed; heat transfer surfaces are blocked; the normal operation of pipeline fittings is disrupted. To minimize slag accumulation, it is necessary to limit the contact of the coolant with air oxygen, avoid draining the coolant from the circulation circuit, control whether the circulation circuit is in a depressurized state both during research and during non-working periods, when heating and filling the circuits with a coolant, create and maintain a vacuum. The best way to prevent slag accumulation is periodic hydrogen cleaning of the circuit with gas mixtures "hydrogen - water vapor - inert gas". The greatest cleaning efficiency is achieved when gas mixtures are introduced directly into the coolant flow using gas injection devices. Mechanical devices with moving parts, ejectors, nozzle nozzles can act as gas injection devices.

Experimental and numerical modal analysis of wall tubes in the coal‐fired boiler or radiant syngas cooler

AbstractThe slag deposited on the wall tubes in the coal‐fired boiler or radiant syngas cooler (RSC) of the entrained flow gasifier could reduce the heat transfer efficiency and degrade the tubes by corrosion. During the operation of the boiler or RSC, the rapping‐off‐ash method is an effective method to remove slag deposits, especially in a high‐pressure environment. The operating parameters of the rapping‐off‐ash method are related to the modal parameters of wall tubes, including the natural frequency, damping ratio, and mode shapes. In this study, modal parameters of wall tubes composed of three single tubes and fins were experimentally determined through the modal test analysis under both empty and water‐filled conditions. The dynamic characteristics of wall tubes were extracted by using the roving hammer‐impact test and the frequency response function (FRF). A numerical simulation based on the finite element method in ABAQUS was conducted to compare with the experimental modal analysis. The first five natural modes ranged from 14 to 154 Hz, and the maximum vibration or torsion locations of wall tubes under the corresponding mode frequencies were investigated. The water inside the wall tubes could reduce the stiffness of the structure and decrease the values of natural frequency.