1015 results about "Carbonate" patented technology

The invention discloses a fluoride salt-free nitrate-free cast iron enamel semitransparent glaze as well as a preparation method and application thereof, and belongs to the technical field of enamel. The semitransparent glaze is prepared from quartz, anhydrous borax, sodium carbonate, potassium carbonate, calcium carbonate, potassium feldspar, zinc oxide, lithium carbonate, zirconium silicate, titanium dioxide, aluminum oxide, trisodium phosphate, antimony oxide, magnesium carbonate and barium carbonate according to a specific mass ratio. The preparation method comprises the steps of raw material mixing, high-temperature melting under a pure oxygen condition, wiredrawing detection, quenching and the like. The semitransparent glaze is completely free of fluoride salt and nitrate, the problems of fluoride emission and environmental protection equipment blockage are avoided from the source, the fluorine content of the product is not detected through detection, the product meets the requirements of European Union REACH regulations, meanwhile, the product has good porcelain surface, luster and low-temperature firing adaptability, and is suitable for cast iron matrixes, the firing temperature is 740-760 DEG C, and the firing time is 2-3 hours. The method can be used for producing green and environment-friendly enamel products.

A method of fixating carbon dioxide to a substituted cyclic carbonate includes a reaction of an epoxide and carbon dioxide in the presence of a zirconium-containing metal-organic framework to form the substituted cyclic carbonate. The zirconium-containing metal-organic framework is a UiO-66-based metal-organic framework including an aminomethylbenzoic acid. The zirconium-containing metal-organic framework catalyzes the formation of the substituted cyclic carbonate in a range of 40 to 97 percent yield.

The invention provides a preparation method of mannitol carbonic acid sulfate, mannitol and dimethyl carbonate are used as starting raw materials to prepare mannitol carbonic acid sulfate, a target compound is obtained through ester exchange, cyclization and oxidation reaction, the raw materials are low in price and easy to obtain, the reaction operation is simple and convenient, the yield of the obtained product is 81% or above, the purity is 99.6% or above, and the method is suitable for industrial production. The method is suitable for industrial production.

A system for capture of atmospheric CO2 includes a reaction reactor having a solid sorbent; and a mineralization reactor downstream from the reaction reactor, the system having three states including in the reaction reactor, a first state of sequestering CO2 from air into the solid sorbent; in the reaction reactor, a second state of releasing the sequestered CO2 from the sorbent by contacting the CO2-enriched sorbent with an exchange fluid having one or more amino acids and forming a CO2-enriched exchange fluid; and in the mineralization reactor, a third state of reacting the sequestered CO2 from the CO2-enriched exchange fluid with an alkaline feedstock to form a carbonate while regenerating the exchange fluid.

The invention belongs to the technical field of carbon materials, and relates to a coal pre-oxidation treatment-based high-layer spacing porous carbon material and a preparation method thereof.A large number of oxygen-containing functional groups are constructed in coal through low-temperature pre-oxidation, and a cross-linked structure is constructed through the oxygen-containing functional groups in the coal low-temperature pyrolysis process; free movement, stacking and growth of aromatic lamellas are inhibited, a graphitization process of a carbon material is hindered, a foundation is laid for improvement of interlayer spacing, carbon is activated and etched by adopting excessive carbonate with relatively weak corrosivity, a large number of graded pore structures are generated, activated carbon with high specific surface area is generated, an oxygen-containing functional group structure generated by coal pre-oxidation is generated, and the carbon material is prepared. And uniform generation of subsequent pore structures is also facilitated. The coal-based carbon material obtained by coupling pre-oxidation with medium-temperature carbonate activation has higher interlayer spacing and specific surface area, and has a hierarchical porous structure.

The invention discloses a synthesis method of a hydrazine dicarboxylate modified cerium dioxide abrasive, and belongs to the technical field of precise polishing materials. The method comprises the following steps: taking cerous nitrate as a raw material, and preparing a cerous carbonate precursor through an ammonium carbonate precipitation reaction; and grinding and calcining the obtained precursor and hydrazine dicarboxylate to obtain the CeO2 grinding material with the surface rich in oxygen vacancies. And Ce < 4 + > can be partially reduced into Ce < 3 + > by reducing fragments generated by decomposition of the hydrazine dicarboxylate in the calcining process, so that an oxygen vacancy structure is formed through induction, and the chemical activity and the reaction rate of the abrasive are improved. The obtained grinding material shows excellent performance in chemical mechanical polishing (CMP) of K9 glass, the material removal rate (MRR) is increased by about 3.6 times compared with that of unmodified CeO2, the surface roughness (Ra) is reduced to about 0.16 nm or below, and grinding material particles are good in dispersity and less in agglomeration. The used hydrazine dicarboxylate is stable, low in toxicity and mild in reaction, and the particle size can be prevented from excessively growing while the high oxygen vacancy concentration is kept.

The invention relates to the technical field of mineralogy, in particular to a complex iron mineral distinguishing and quantifying method and an iron mineral content analysis method, which innovatively combines chemical dissolution, automatic mineralogy analysis and a micro-area magnetic verification technology to realize distinguishing and quantifying of complex iron minerals. According to the analysis method, a closed and mutual verification analysis system is formed, acid treatment of carbonate, automatic mineralogy analysis of residual minerals, micro-area magnetic verification correction of misjudgment of maghemite, element surface scanning verification of mineralogy total quantification accuracy and a final calculation result, and closed verification of the total iron content of chemical analysis are combined, and therefore the comprehensive performance of the magnetic separation system is improved. The reliability of a final result is ensured and standardized popularization and application are facilitated.

The invention discloses fluoride salt-free and nitrate-free low-temperature matte glaze for steel plate enamel as well as a preparation method and application of the low-temperature matte glaze, and belongs to the technical field of enamel. The matte glaze is prepared from quartz, borax pentahydrate, sodium carbonate, titanium dioxide, magnesium oxide, monopotassium phosphate, calcium carbonate, zinc oxide, magnesium carbonate, lithium carbonate, aluminum oxide and zirconium silicate according to a specific mass ratio. The preparation method comprises the following steps: mixing the raw materials, melting at 1300 + / -10 DEG C in a pure oxygen environment, and quenching after wiredrawing to detect a molten state. The matte glaze completely avoids the use of fluoride salt and nitrate, the problems of fluoride emission and environmental protection equipment blockage are eliminated from the source, the fluorine content of the product is not detected through detection, and the limit value requirements of European Union REACH regulations are met. Zinc oxide and ZnO / TiO2 are added, so that the liquid phase viscosity is reduced, ion diffusion is accelerated, and the opacity (opacification performance) and gloss of the porcelain glaze are enhanced.

The invention discloses a fluoride-salt-free and nitrate-free cast iron enamel zirconium white glaze as well as a preparation method and application thereof, and belongs to the technical field of enamel. The zirconium white glaze is prepared from quartz, borax pentahydrate, magnesium carbonate, potassium carbonate, potassium feldspar, zinc oxide, aluminum hydroxide, calcium carbonate, zirconium silicate, sodium tripolyphosphate and lithium carbonate according to a specific mass ratio. The preparation method comprises the steps of raw material mixing, high-temperature melting under a pure oxygen condition, wiredrawing detection, quenching and the like. The zirconium white glaze is completely free of fluoride salt and nitrate, the problems of fluoride emission and environmental protection equipment blockage are avoided from the source, the fluorine content of the product is not detected through detection, the product meets the requirements of European Union REACH regulations, and meanwhile, the zirconium white glaze has good porcelain surface, luster and firing adaptability, is suitable for cast iron matrixes, has the firing temperature of 740-760 DEG C and the firing time of 87-87 hours, and can be widely applied to cast iron substrates. The method can be used for producing green and environment-friendly enamel products.

The invention discloses a method for synthesizing alkyl carbonate through gas-phase carbonylation. Comprising a dechlorination method and a chlorine circulation process in the process of preparing alkyl carbonate through a gas-phase oxidative carbonylation reaction of carbon monoxide and alkyl nitrite. The content of water and alkyl alcohol in circulating gas is controlled through solvent absorption, alkyl chloride is synthesized with a chlorine supplementing agent HCl and alkyl chloroformate under the action of a dechlorinating agent, and the alkyl chloride is separated from a product through gas-liquid separation. When alkyl carbonate is produced by adopting the dechlorination method and the dechlorination process, the problem of equipment corrosion caused by chlorides can be avoided, the dechlorination process is a low-cost and environment-friendly chlorine circulation process, and stable operation of an industrial device is realized.

The embodiment of the invention discloses a fluorine removal agent and a fluorine removal process for rare earth feed liquid. The fluorine removal agent comprises a first fluorine removal agent and a second fluorine removal agent, the first fluorine removal agent is polycrystal system composite calcium, the polycrystal system composite calcium is composed of calcium oxide / calcium hydroxide and calcium carbonate according to the mass ratio of (0.1-1): (0.1-1), and the second fluorine removal agent is composed of polycrystal system composite calcium and an inorganic flocculant according to the mass ratio of (0.01-1): (0.01-1). According to the method, fluorine is removed under the condition that the pH value is 4.6-5.8, fluoride composite salt precipitation is formed, fluorine in the rare earth solution can be efficiently removed, rare earth carbonate precipitated from subsequent oxalic acid or carbonate (sodium carbonate, sodium bicarbonate, ammonium bicarbonate and the like) or high-concentration rare earth feed liquid can be recycled, and the quality of the rare earth is improved. The requirements (the F content in the mixed rare earth oxide is less than 0.3%, and the F content in the high-concentration rare earth feed liquid (the concentration is greater than 100g / L) is less than 100mg / L) are met. The method is simple in process and high in operability, and has a relatively good application prospect.

The invention discloses a method for reducing the content of rare earth carbonate oil, which comprises the following steps: adding rare earth carbonate seed crystals and pure water into a precipitation tank, feeding a rare earth chloride solution after extraction separation into the precipitation tank, adding an ammonium bicarbonate solution, and carrying out precipitation reaction to obtain rare earth carbonate; the method comprises the following steps: treating rare earth carbonate with hydrochloric acid, converting into a rare earth chloride solution, and filtering the rare earth chloride solution to remove oil to obtain a refined rare earth chloride solution; adding rare earth carbonate into a precipitation tank as a seed crystal, adding the refined rare earth chloride solution and the prepared ammonium bicarbonate solution into the precipitation tank, and carrying out precipitation reaction to produce oil-free rare earth carbonate. According to the invention, oil entrained in the extraction feed liquid and oil dissolved in the feed liquid can be removed, and the quality of the rare earth carbonate product is improved.

The invention relates to the technical field of purification of fine chemical intermediates, in particular to a purification method of a mannitol carbonate sulfate preparation process intermediate, which comprises the following steps: S1, synthesizing diacetone-D-mannitol; s2, preparing an organic solution containing solids; s3, performing deprotection reaction to prepare a solid crude product; s4, removing unreacted solids; s5, extracting and separating by-product solids; and S6, purifying the target product solid and recovering mannitol. Through system separation and multi-component recovery, preparation of a high-purity product and cyclic utilization of unreacted substances, byproducts and raw materials are realized, the yield is remarkably improved, the cost is reduced, and the quality of a final product is guaranteed.

The invention provides coal gangue-based artificial soil as well as a preparation method and application thereof. The preparation method of the coal gangue-based artificial soil comprises the following steps: S1, modifying coal gangue by adopting a solution containing citric acid and ethylenediamine tetraacetic acid to obtain modified coal gangue; s2, preparing the modified coal gangue into artificial soil. According to the invention, the citric acid can dissolve carbonate in the coal gangue, the released carbonate can be combined with heavy metals, and citrate ions and ferric ions can form a complex to destroy the outer layer of ferric oxide of the heavy metals; ethylenediamine tetraacetic acid can permeate into mineral lattices of the coal gangue, lattice bound state heavy metal is released through octahedral Al-O bond breakage, the ethylenediamine tetraacetic acid can capture heavy metal combined with citric acid, a more stable heavy metal complex is formed, the heavy metal complex carries strong negative charges, and the heavy metal complex can be separated from the coal gangue. And electrostatic repulsion can be generated on the surface of the coal gangue with negative electricity, so that the heavy metal removal effect is improved.

The invention discloses a high-aluminum wear-resistant ceramic grinding ball and a preparation method thereof. The grinding ball comprises the following raw materials in percentage by weight: 85%-95% of aluminum oxide, 2%-10% of silicon dioxide, 1%-3% of calcium carbonate, 0.2%-0.5% of magnesium carbonate, 0.03%-0.05% of titanium dioxide, 0.01%-0.03% of sodium oxide and 1%-5% of modified zirconium dioxide. The preparation method of the modified zirconium dioxide comprises the following steps: adding hydrogen peroxide into a mixed solution of soluble cerium salt and soluble zirconium salt to obtain a reaction system; adding the reaction system into an alkaline solution for reaction, and collecting an insoluble substance after the reaction is finished; and calcining the insoluble substance to obtain the modified zirconium dioxide. The ceramic ball is simple in formula, rich in raw materials, low in price and low in production cost, the operation stability of the ceramic ball is improved, and the service life is prolonged.

The present application belongs to the technical field of nanometer material, and discloses a kind of sheet porous nanometer zinc oxide and its preparation method and application.The present application first prepares basic zinc carbonate precursor, then nitrogen is passed into, the pressure of system is >0.4MPa and <0.6MPa, hydrothermal reaction is carried out, and the reaction product is calcined, and sheet porous nanometer zinc oxide is prepared.The sheet porous nanometer zinc oxide prepared by the method of the present application has uniform and regular sheet porous structure, the holes on the sheet are uniform and regular, dense, small in particle size, large in specific surface area, and can be widely applied in the degradation of organic pollutants in photocatalytic system and the field of heavy metal adsorption.The preparation method of the present application can obtain raw materials easily and can be mass-produced.

The invention belongs to the field of hydrometallurgy, and particularly provides an ionic type rare earth ore leachate composite precipitation treatment method which comprises the steps that magnesium oxide is adopted for one-step precipitation of rare earth leachate, and rare earth and impurities are co-precipitated to form an enriched product; carrying out acid dissolution treatment on the rare earth enrichment, carrying out solid-liquid separation to obtain filter residues, using the filter residues for mining and covering soil, neutralizing and removing impurities from the filtrate by adopting magnesium oxide or the rare earth enrichment, adjusting the pH value of one part of the filtrate after impurity removal by adopting magnesium oxide, and sufficiently aging to serve as a precipitator for later use; the pH of the other part is adjusted by the precipitator, and then sodium bicarbonate or sodium hydroxide composite precipitation is adopted to recover the rare earth. The treatment method provided by the invention effectively solves the problems of high content of silicon, aluminum, sulfur and magnesium impurities and low total amount of rare earth in a rare earth enriched product when rare earth is recovered by a single precipitator, and poor crystallization, slow sedimentation, difficult filtration, high rare earth loss and the like of a rare earth carbonate product during traditional sodium bicarbonate precipitation, and realizes mining and smelting cyclic utilization of magnesium resources; and the overall development and utilization efficiency of the ionic rare earth ore is improved.

The invention relates to a method for preparing basic magnesium carbonate from heavy magnesium water under the assistance of nano-micro bubbles, and belongs to the technical field of preparation of basic magnesium carbonate. According to the method, nano-microbubbles are generated and are stirred and mixed with heavy magnesium water, and energy released when the nano-microbubbles are broken is utilized to promote the decomposition reaction of the heavy magnesium water, so that efficient crystallization of basic magnesium carbonate is realized under a low-temperature condition. According to the technical scheme, the dependence of a traditional heavy magnesium water preparation process on high-temperature and vacuum conditions is broken through, the energy consumption and the production cost for preparing the basic magnesium carbonate are remarkably reduced, and a new path is opened up for preparing the basic magnesium carbonate from the heavy magnesium water.

An electrocoagulation (EC) reactor and method for simultaneous CO2 capture and brine desalination. The EC reactor includes a reaction chamber that holds brine solution with contaminants, an inlet tube for CO2 gas mixture supply, and a plurality of electrodes (130), submerged in the brine. The electrodes are connected to a power source, applying a voltage to induce electrochemical reactions that precipitate dissolved salts and heavy metals, reducing brine salinity and contaminants, while capturing CO2 in the form of carbonates to form solid coagulants. The method involves introducing brine into the EC reactor's reaction chamber, adding calcium oxide (CaO) and ammonium bicarbonate (NH4HCO3) to create a homogeneous solution, and applying electric current to dissolve the electrodes, producing solid coagulants. A CO2 gas mixture is introduced to facilitate carbonation reactions, forming precipitates like calcium carbonate (CaCO3) and magnesium carbonate (MgCO3), which are filtered to obtain desalinated brine.

The invention discloses a preparation method of polyether-ether-ketone. The preparation method of the polyether-ether-ketone comprises the step of preparing the polyether-ether-ketone in the presence of carbonate and / or bicarbonate of alkali metal, wherein the D90 of the carbonate and / or bicarbonate of the alkali metal is less than 45 [mu] m. Under the condition of reduced pressure operation, when carbonate and / or bicarbonate of alkali metal with D90 smaller than 45 microns are / is used for preparing polyether-ether-ketone, polyether-ether-ketone with properties (such as color, mechanical property and the like) equivalent to those of a conventional process can be obtained, and the gel content is lower.

The invention provides spherical cobaltosic oxide as well as a preparation method and application thereof, the preparation method comprises the following steps: (1) injecting a cobalt salt-containing solution and a carbonate solution into a base solution in a parallel flow manner for a coprecipitation reaction to obtain a mixed-phase precursor; and (2) calcining the mixed phase precursor to obtain the spherical cobaltosic oxide, wherein the solute of the base solution in the step (1) is alkali metal hydroxide. According to the method disclosed by the invention, the precursor of the hydroxyl cobalt oxide and cobalt carbonate mixed phase can be prepared without separating kettles, the content of foreign ions in the precursor is low, and small-particle and high-purity cobaltosic oxide can be prepared by directly sintering.

The invention relates to the technical field of water pollution treatment, and discloses a sulfur autotrophic denitrification filler for nitrogen wastewater treatment and a preparation method thereof.The sulfur autotrophic denitrification filler comprises composite particles capable of being degraded step by step, and the composite particles are synergistically limited by chemical compositions and microscopic physical structures; the main alkali source calcium-based carbonate and the sacrificial alkali source magnesium-based oxide establish an interface self-updating mechanism by utilizing the reaction rate difference; physically, a pore structure composed of a reaction chamber and a throat is built in the filler, self-cleaning of an endogenous mass transfer channel is achieved through denitrification product nitrogen, and through internal cooperation of chemical and physical mechanisms, the problems of interface passivation and pore silting self-locking of traditional sulfur-based filler caused by reaction products are solved; the active self-updating and long-term stable operation capabilities of the filler are realized, so that the application potential of a cheap alkali source is released.

The invention discloses a method for monitoring atmospheric carbon dioxide removal efficiency of seawater alkalization, and belongs to the technical field of ocean negative emission and geochemical monitoring. The method comprises the following steps: 1) collecting a seawater sample; 2) determining the alkalinity of carbonate; (3) calculating the CO2 removal efficiency; based on the difference value of carbonate alkalinity before and after alkalization, the removal effectiveness CDR of atmospheric carbon dioxide is calculated, and the calculation formula is as follows: CDR = CAT-CA0; wherein CDR is atmospheric CO2 removal efficiency after seawater alkalization engineering is implemented, and the unit is mol CO2 kg <-1 >; cA0 and CAt are respectively carbonate alkalinity before and after seawater alkalization, and the unit is mol kg <-1 >.

The invention provides a method for producing monochloroethylene carbonate with low ethylene carbonate content, and the monochloroethylene carbonate prepared by the method provided by the invention can improve the production efficiency and yield, and can significantly reduce the EC content in a CEC product at the same time. The method comprises the following steps: (1) in a main reactor, chlorine is introduced into a material at least containing an ethylene carbonate liquid to carry out a chlorination reaction in a boiling state, and the reaction pressure of the chlorination reaction is lower than normal pressure; and (2) feeding a gas-phase material flow obtained in the main reactor into a rectifying tower for separation, and obtaining a gas-phase product from the top of the rectifying tower.

The invention discloses a novel method for solidifying a foundation through cooperation of microorganisms and active magnesium oxide, and belongs to the technical field of foundation treatment. In a traditional foundation treatment method, a large amount of ore energy needs to be consumed in the production process of a reinforcing material, harm can be brought to the surrounding environment, and serious noise pollution, air pollution and the like are generated. According to the novel method for solidifying the foundation through cooperation of microorganisms and active magnesium oxide, the carbonation reaction between HCO3 <-> in a NaHCO3 solution and active MgO is accelerated through the catalytic acceleration effect of carbonic anhydrase on the CO2 hydration reaction, so that a large number of carbonates such as magnesium carbonate and basic magnesium carbonate with the good gelling effect are generated, solidified soil particles are cemented, and the effect of solidifying the foundation through cooperation of microorganisms and active magnesium oxide is achieved. And the effect of reinforcing the foundation is achieved.

The invention relates to the technical field of oily sewage treatment, and discloses a flocculant as well as a preparation method and application thereof. The flocculant contains a component A, a component B, a component C and a component D, the component A contains etherified starch and nano magnetic iron oxide, the component B contains sodium bentonite and nano magnetic iron oxide, the component C contains tannin modified chitosan and nano magnetic iron oxide, and the component D contains carbonate, citric acid and auxiliary materials. The flocculant is easy to store, does not need to be compounded on site, flocculates quickly in water, only needs 60-120 seconds in flocculation time, and has an excellent flocculation effect; the emergency requirement of an accident site can be met, and the good popularization value is achieved; the method is suitable for an oil-containing sewage system with coexistence of anions and cations, can also adapt to a wide pH range, is good in salt resistance and has a good dehydration and filtration aiding effect.

The invention belongs to the technical field of synthesis, and particularly relates to a synthesis method of urea [14C]. The synthesis method of the urea [14C] comprises the following steps: (1) reacting carbonate [14C] with ammonia gas to obtain cyanamide salt [14C]; (2) hydrolyzing the cyanamide salt [14C] obtained in the step (1) with concentrated sulfuric acid at the hydrolysis temperature of 35-45 DEG C; and (3) adjusting the pH value of the hydrolysate obtained in the step (2) with ammonia water, concentrating and purifying to obtain urea [14C]. The synthesis method provided by the invention has the advantages of mild conditions, efficient reaction, high product yield (up to 95% or above), high purity (the chemical purity is greater than or equal to 99%) and the like, and is suitable for large-scale preparation of key raw materials of helicobacter pylori diagnostic reagents.

The invention provides high-aluminum doped cobalt carbonate and a preparation method and application thereof, and the preparation method comprises the following steps: (1) injecting a cobalt-aluminum mixed salt solution, a carbonate solution and an organic weak acid solution into a base solution in a parallel flow manner, and carrying out a co-precipitation reaction to obtain a precursor; and (2) stopping injection of the organic weak acid solution, continuing to inject the cobalt-aluminum mixed salt solution and the carbonate solution, and carrying out a co-precipitation reaction to obtain the high-aluminum doped cobalt carbonate. According to the method, through segmented coprecipitation and control of the types of precipitants used for coprecipitation of all the segments, the doping amount and uniformity of aluminum in cobalt carbonate can be improved, aluminum segregation is avoided, and meanwhile high-aluminum-doped cobalt carbonate higher in sphericity degree and better in compactness can be obtained.

Disclosed in the present invention is a preparation method for vinylene carbonate. Specifically, the preparation method of the present invention comprises the following steps: introducing a material containing chloroethylene carbonate into a reaction bed layer containing a catalyst to perform a contact reaction, so as to obtain vinylene carbonate, wherein the catalyst is prepared by means of the following method: performing forming by using a catalyst raw material, then optionally calcining same, and then cooling same, so as to obtain catalyst particles. The preparation method of the present invention has a low cost, is easy to operate, has a high conversion rate and a high yield and is beneficial to industrial production.

The present invention discloses a method for preparing molten salt-modified magnesium oxide adsorbent particles for carbon dioxide capture. The method involves reacting magnesium chloride as a magnesium source with sodium bicarbonate to produce light basic magnesium carbonate, which is then calcined in stages to produce a microporous magnesium oxide precursor. The microporous magnesium oxide precursor is then impregnated with a mixed salt consisting of nitrate and carbonate, and dried to produce an alkali metal molten salt-modified magnesium oxide powder. A binder is added to the powder, and the powder is extruded to form a granular adsorbent. Applications of the adsorbent are also disclosed. The prepared adsorbent particles have good structural stability and are recyclable. When adsorbing and capturing CO2 at approximately 300°C, the loaded mixed salt forms a molten salt layer on the microporous surface of the magnesium oxide, enhancing CO2 adsorption. The CO2 adsorption capacity reaches over 10 mmol / g (adsorbent), which is more than 10 times higher than that of unmodified magnesium oxide adsorbent. The CO2 desorption rate is also significantly improved, reaching 100% at 400°C.