然而，本發明者實際如專利文獻3般粉碎市售之反丁烯二酸，並依據JIS K 5101-13-2(2004)測定其吸油量，結果為36.6 mL/100 g(參照下文敍述之比較例4)。市售之C4二羧酸之吸油量為25～30 mL/100 g左右(參照下文敍述之比較例1～3)，粉碎之C4二羧酸之吸油量雖超過該吸油量，但未能藉由粉碎獲得充分之吸油量。因此，本發明係關於一種吸油量較高之C4二羧酸結晶之製造方法。本發明者發現：於陰離子性高分子之存在下，且陰離子性高分子之濃度相對於C4二羧酸之濃度為特定範圍的情形時，自包含C4二羧酸或其鹽之水溶液晶析而獲得之C4二羧酸結晶具有前所未有之高吸油量。根據本發明之方法，可提高C4二羧酸結晶之吸油量，獲得高吸油量之C4二羧酸結晶。[C4二羧酸結晶之製造方法]本發明之C4二羧酸結晶可藉由包含如下步驟之方法而製造：於陰離子性高分子之存在下，自包含C4二羧酸或其鹽之水溶液析出C4二羧酸之結晶。於本發明中，上述水溶液中之陰離子性高分子之含量相對於C4二羧酸之含量的質量比為5×10-4
以上且0.5以下。藉由設為該特定之質量比而獲得具有高吸油量之C4二羧酸結晶。陰離子性高分子只要於析出C4二羧酸結晶時存在即可，將該等添加至包含C4二羧酸或其鹽之水溶液之時點並無特別限制。(包含C4二羧酸或其鹽之水溶液)作為本發明之C4二羧酸之例，可列舉反丁烯二酸、琥珀酸、蘋果酸、酒石酸、順丁烯二酸、草醯乙酸等。較佳為反丁烯二酸或琥珀酸，更佳為反丁烯二酸。又，C4二羧酸或其鹽較佳為不具有胺基酸殘基之碳數為4之二羧酸或其鹽。C4二羧酸或其鹽並無特別限制，可藉由源自苯或丁烷等石化原料之化學合成、或微生物醱酵而獲得。作為生成C4二羧酸或其鹽之微生物，可列舉根黴菌屬等絲狀菌。於藉由微生物醱酵而獲得C4二羧酸或其鹽之情形時，亦可自包含C4二羧酸或其鹽之培養液析出C4二羧酸之結晶。(陰離子性高分子)本發明中使用之陰離子性高分子較佳為水溶性。作為陰離子性高分子，可列舉具有陰離子性基、例如羧基、硫酸基、磺酸基、磷酸基、硼酸基等之聚合物。具體而言，作為天然高分子，可列舉三仙膠或阿拉伯膠、海藻酸、聚麩胺酸或其等之鹽。作為合成高分子，可列舉包含(甲基)丙烯酸或順丁烯二酸、順丁烯二酸酐、反丁烯二酸、伊康酸、丁烯酸、或乙烯基磺酸等單體之聚合物或共聚物及其等之鹽。又，可列舉羧甲基纖維素或羧乙基纖維素等羧基烷基纖維素、羧基乙烯基聚合物。該等可單獨使用，又，亦可組合複數種而使用。作為鹽，可列舉鹼金屬鹽、鹼土金屬鹽、銨鹽、碳數1～22之烷基或烯基銨鹽、經碳數1～22之烷基或烯基取代之吡啶鎓鹽、碳數1～22之烷醇銨鹽、鹼性胺基酸鹽等。較佳為鹼金屬鹽，更佳為鈉鹽、鉀鹽。就使高吸油量之C4二羧酸結晶之觀點而言，陰離子性高分子較佳為聚(甲基)丙烯酸、聚麩胺酸、羧基烷基纖維素或其等之鹽，更佳為聚(甲基)丙烯酸或其鹽、或聚麩胺酸，更佳為聚丙烯酸或其鹽。(陰離子性高分子之分子量)就使高吸油量之C4二羧酸結晶之觀點而言，陰離子性高分子之重量平均分子量較佳為1,000以上、更佳為2,000以上、進而較佳為5,000以上，又，較佳為2,000,000以下、更佳為1,000,000以下、進而較佳為500,000以下。陰離子性高分子之重量平均分子量較佳為1,000以上且2,000,000以下、更佳為2,000以上且1,000,000以下、進而較佳為5,000以上且500,000以下。高分子之重量平均分子量可根據測定對象而藉由例如以聚丙烯酸鈉為換算標準之凝膠滲透層析法(GPC)法進行測定。(陰離子性高分子之含量相對於碳數為4為之二羧酸之含量的質量比)於本發明中，包含C4二羧酸或其鹽之水溶液中之陰離子性高分子之含量相對於C4二羧酸之含量的質量比為5×10-4
以上且0.5以下。就製造高吸油量之C4二羧酸結晶之觀點而言，較佳為0.001以上、更佳為0.002以上、進而較佳為0.004以上、進而較佳為0.01以上，又，就相同之觀點而言，較佳為0.3以下、更佳為0.2以下、更佳為0.1以下、更佳為0.05以下。包含C4二羧酸或其鹽之水溶液中之陰離子性高分子之含量相對於C4二羧酸之含量的質量比較佳為0.001以上且0.3以下、更佳為0.002以上且0.2以下、進而較佳為0.004以上且0.1以下、進而較佳為0.01以上且0.1以下。再者，於本說明書中，C4二羧酸之含量係C4二羧酸之含量與將C4二羧酸鹽換算成C4二羧酸時之含量的和。(陰離子性高分子之含量)就使高吸油量之C4二羧酸結晶之觀點而言，包含C4二羧酸或其鹽之水溶液中之陰離子性高分子之含量較佳為0.001質量%以上、更佳為0.005質量%以上、更佳為0.01質量%以上、更佳為0.03質量%以上、進而較佳為0.05質量%以上，又，就工業生產性、成本之觀點而言，較佳為5質量%以下、更佳為1.5質量%以下、進而較佳為1質量%以下、進而較佳為0.5質量%以下、進而較佳為0.2質量%以下、進而較佳為0.1質量%以下。包含C4二羧酸或其鹽之水溶液中之陰離子性高分子之含量較佳為0.001質量%以上且5質量%以下、更佳為0.005質量%以上且1.5質量%以下、進而較佳為0.01質量%以上且1質量%以下、進而較佳為0.05質量%以上且0.5質量%以下。(使C4二羧酸結晶析出之方法)使C4二羧酸結晶析出之方法並無特別限制，可利用藉由調整pH值進行之析出方法、藉由冷卻進行之析出方法、藉由濃縮進行之析出方法、藉由反應進行之析出方法等操作而進行。(晶析裝置)較佳為使用具有攪拌葉之反應槽，一面進行攪拌，一面進行C4二羧酸結晶之析出。攪拌葉可為任意形狀，特別是為了良好地混合結晶，較佳為槳葉、渦輪葉、螺旋槳葉、錨葉、大葉徑槳葉、大功率攪拌葉(Maxblend)。就使高吸油量之C4二羧酸均勻地晶析之觀點而言，攪拌之周速較佳為0.2 m/s以上、更佳為0.3 m/s以上、進而較佳為0.5 m/s以上，又，就使高吸油量之C4二羧酸結晶化之觀點而言，較佳為10 m/s以下、更佳為5 m/s以下、進而較佳為3 m/s以下。攪拌之周速較佳為0.2 m/s以上且10 m/s以下、更佳為0.3 m/s以上且5 m/s以下、進而較佳為0.5 m/s以上且3 m/s以下。(藉由調整pH值進行之析出方法)藉由調整pH值進行之析出方法可藉由如下方式晶析C4二羧酸：藉由添加酸而使C4二羧酸自C4二羧酸鹽游離，將C4二羧酸之濃度提高至溶解度以上。調整pH值所使用之酸只要為pKa小於C4二羧酸之酸，則可無特別限制地使用，特佳為無機酸。作為無機酸，例如可列舉鹽酸、硝酸、硫酸、磷酸等。較佳為硫酸、鹽酸。就C4二羧酸之回收率之觀點而言，關於進行析出時之pH值，較佳為將晶析開始時之pH值調整為9以下、較佳為6以下，藉由添加酸而調整成較佳為2.5以下。又，就反應槽等之腐蝕性之觀點而言，pH值較佳為0.5以上。進行結晶之析出時之pH值較佳為0.5以上且9以下、更佳為0.5以上且6以下、進而較佳為0.5以上且2.5以下。就使高吸油量之C4二羧酸結晶化之觀點而言，酸之添加速度較佳為0.1 mmol-酸/L/min以上、更佳為0.3 mmol-酸/L/min以上、進而較佳為1 mmol-酸/L/min以上，又，就C4二羧酸結晶懸浮液之過濾性、過濾後之C4二羧酸濾餅之含水率之觀點而言，較佳為10 mmol-酸/L/min以下、更佳為5 mmol-酸/L/min以下、進而較佳為3 mmol-酸/L/min以下、進而較佳為2 mmol-酸/L/min以下。酸之添加速度較佳為0.1 mmol-酸/L/min以上且10 mmol-酸/L/min以下、更佳為0.3 mmol-酸/L/min以上且5 mmol-酸/L/min以下、進而較佳為0.3 mmol-酸/L/min以上且3 mmol-酸/L/min以下、進而較佳為1 mmol-酸/L/min以上且2 mmol-酸/L/min以下。再者，mmol-酸/L/min係表示1分鐘於反應液中每1升所混合之酸量。藉由調整pH值進行析出時之溫度並無特別限定，就C4二羧酸之回收率之觀點而言，較佳為於較低之溫度下實施。就C4二羧酸之回收率之觀點而言，晶析溫度較佳為50℃以下、更佳為40℃以下、進而較佳為30℃以下，又，較佳為0℃以上、更佳為5℃以上。晶析溫度較佳為0℃以上且50℃以下、更佳為0℃以上且40℃以下、進而較佳為5℃以上且30℃以下。藉由調整pH值進行析出時之C4二羧酸或其鹽之濃度並無特別限定，就C4二羧酸之回收率之觀點而言，較佳為析出開始時之溫度下之溶解度量、或略微小於該溶解度量之量。具體而言，含有C4二羧酸或其鹽之水溶液中之C4二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上。藉由調整pH值進行析出時之含有C4二羧酸或其鹽之水溶液中的C4二羧酸或其鹽之含量較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。(藉由冷卻進行之析出方法)藉由冷卻進行之析出方法可藉由如下方式晶析C4二羧酸：藉由將含有C4二羧酸或其鹽之水溶液自高溫冷卻至低溫，而將C4二羧酸濃度提高至溶解度以上。C4二羧酸具有於溫度較高之情形時溶解度較高之性質，故而較佳為升溫而提高溶解之酸濃度後進行冷卻。升溫溫度較佳為60℃以上、更佳為70℃以上、進而較佳為80℃以上，又，較佳為120℃以下。升溫溫度較佳為60℃以上且120℃以下、更佳為70℃以上且120℃以下、進而較佳為80℃以上且120℃以下。就C4二羧酸之回收率之觀點而言，冷卻溫度較佳為50℃以下、更佳為40℃以下、進而較佳為30℃以下，又，較佳為0℃以上、更佳為5℃以上。冷卻溫度較佳為0℃以上且50℃以下、更佳為0℃以上且40℃以下、進而較佳為5℃以上且30℃以下。就C4二羧酸之回收率之觀點、使高吸油量之C4二羧酸結晶之觀點而言，根據自升溫溫度達到冷卻溫度所需之時間算出之平均冷卻速度較佳為0.05℃/min以上、更佳為0.1℃/min以上，又，就結晶向反應槽之附著性、C4二羧酸結晶懸浮液之過濾性、過濾後之C4二羧酸濾餅之含水率之觀點而言，較佳為20℃/min以下、更佳為10℃/min以下、進而較佳為5℃/min以下。根據自升溫溫度達到冷卻溫度所需之時間算出之平均冷卻速度較佳為0.05℃/min以上且20℃/min以下、更佳為0.1℃/min以上且10℃/min以下、進而較佳為0.1℃/min以上且5℃/min以下。就C4二羧酸之回收率之觀點而言，關於藉由冷卻進行之析出方法之pH值，較佳為將晶析開始時之pH值調整為4以下、更佳為2.5以下。又，就反應槽等之腐蝕性之觀點而言，pH值較佳為0.5以上。藉由冷卻進行析出時之C4二羧酸或其鹽之濃度並無特別限定，就C4二羧酸之回收率之觀點而言，較佳為析出開始時之溫度下之溶解度量、或略微小於該溶解度量之量。具體而言，含有C4二羧酸或其鹽之水溶液中之C4二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上。藉由冷卻進行析出時之含有C4二羧酸或其鹽之水溶液中的C4二羧酸或其鹽之含量較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。(藉由濃縮進行之析出方法)藉由濃縮進行之析出方法可藉由如下方式晶析C4二羧酸：藉由使含有C4二羧酸或其鹽之水溶液之溶劑(水)蒸發進行濃縮，而將C4二羧酸濃度提高至溶解度以上。濃縮之方法並無特別限制，可使用反應槽等，亦可藉由噴霧乾燥器(spray dryer)等噴霧乾燥機而使溶劑(水)瞬間蒸發。蒸發時之溫度並無特別限定，較佳為300℃以下、更佳為200℃以下，又，較佳為5℃以上。蒸發時之溫度較佳為5℃以上且300℃以下、更佳為5℃以上且200℃以下。再者，亦可於減壓下進行蒸發。藉由濃縮進行析出時之C4二羧酸或其鹽之濃度並無特別限定，較佳為設定成相當於C4二羧酸之各溫度之溶解度之濃度、或略微小於該溶解度之濃度。具體而言，就回收率之觀點而言，含有C4二羧酸或其鹽之水溶液中之C4二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上。藉由濃縮進行析出時之含有C4二羧酸或其鹽之水溶液中的C4二羧酸或其鹽之含量較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。又，就C4二羧酸之回收率之觀點而言，關於藉由濃縮進行之析出方法之pH值，較佳為將晶析開始時之pH值調整為4以下、更佳為2.5以下。又，就反應槽等之腐蝕性之觀點而言，pH值較佳為0.5以上。(藉由反應進行之析出方法)藉由反應進行之析出方法可根據C4二羧酸之種類而適當地設定。例如，於析出反丁烯二酸之情形時，可藉由如下方式晶析反丁烯二酸：藉由於含有順丁烯二酸或順丁烯二酸酐之水溶液中添加觸媒生成反丁烯二酸，而將反丁烯二酸濃度提高至溶解度以上。含有順丁烯二酸或順丁烯二酸酐之水溶液中之順丁烯二酸或順丁烯二酸酐之含量較佳為5質量%以上、更佳為10質量%以上、更佳為20質量%以上，又，較佳為70質量%以下、更佳為50質量%以下、更佳為40質量%以下、更佳為30質量%以下。反應時之溫度並無特別限定，較佳為60℃～100℃。作為觸媒，只要為推進C4二羧酸之生成反應之觸媒，則可無特別限制地使用，例如可列舉硫脲、溴酸鹽、過硼酸鹽等。又，於反應時，亦可添加硫酸或鹽酸等無機酸。該等析出方法可單獨實施，亦可組合複數種方法而實施。例如，於本發明中，作為C4二羧酸結晶之析出方法，考慮將包含C4二羧酸或其鹽之水溶液升溫至80℃以上，於確認溶解後，以0.05℃/min以上之平均冷卻速度藉由冷卻進行析出，於達到30℃後，添加無機酸而將pH值降至2.5以下等。(C4二羧酸結晶懸浮液之過濾)C4二羧酸之結晶可藉由離心分離、過濾、傾析等固液分離操作而分取。結晶之分離操作等較佳為於上述溫度範圍內進行。亦可視需要對以此方式獲得之C4二羧酸結晶進行清洗。視需要於清洗後進行乾燥，藉此可獲得C4二羧酸結晶。(C4二羧酸結晶之乾燥)乾燥可使用箱形乾燥機(shelf dryer)、錐形乾燥器、槳式乾燥器、諾塔混合器(nauta mixer)、流動層乾燥機、真空攪拌乾燥機、圓盤乾燥器、氣流式乾燥機等通常之乾燥機。為了維持吸油量較高之C4二羧酸結晶結構，較佳為不施加高剪切之乾燥方法。乾燥溫度較佳為70℃以上、更佳為80℃以上、進而較佳為100℃以上，又，較佳為300℃以下、更佳為250℃以下、進而較佳為200℃以下、進而較佳為150℃以下、進而較佳為130℃以下、進而較佳為120℃以下。再者，亦可進行減壓乾燥。乾燥後之C4二羧酸結晶亦可視需要而進行過篩等處理。[C4二羧酸結晶]以此方式獲得具有高吸油量之C4二羧酸結晶。本發明之C4二羧酸結晶之依據JIS K 5101-13-2(2004)而測定之吸油量較佳為40 mL/100 g～200 mL/100 g。JIS K 5101-13-2(2004)係藉由熟亞麻籽油法進行之吸油量之測定法。測定方法之詳細內容記載於實施例。於本說明書中，亦將「依據JIS K 5101-13-2(2004)而測定之吸油量」簡稱為「吸油量」。C4二羧酸結晶之吸油量較佳為50 mL/100 g～200 mL/100 g、更佳為60 mL/100 g～200 mL/100 g、進而較佳為70 mL/100 g～200 mL/100 g。高吸油性之C4二羧酸結晶並無特別限定，可用作樹脂原料或食品添加物等，特別是適於用作要求更多地擔載期待保濕效果之油劑之沐浴劑之原料。關於上述實施形態，本發明進而揭示以下之製造方法。＜1＞一種碳數為4之二羧酸結晶之製造方法，其包括於陰離子性高分子之存在下，自包含碳數為4之二羧酸或其鹽之水溶液析出碳數為4之二羧酸之結晶的步驟，且上述水溶液中之陰離子性高分子之含量相對於碳數為4之二羧酸之含量的質量比為5×10-4
以上且0.5以下。＜2＞如＜1＞所記載之製造方法，其中陰離子性高分子之重量平均分子量較佳為1,000以上、更佳為2,000以上、進而較佳為5,000以上，又，較佳為2,000,000以下、更佳為1,000,000以下、進而較佳為500,000以下，又，較佳為1,000以上且2,000,000以下、更佳為2,000以上且1,000,000以下、進而較佳為5,000以上且500,000以下。＜3＞如＜1＞或＜2＞所記載之製造方法，其中包含碳數為4之二羧酸或其鹽之水溶液中之陰離子性高分子之含量相對於碳數為4之二羧酸之含量的質量比較佳為0.001以上、更佳為0.002以上、進而較佳為0.004以上、進而較佳為0.01以上，又，較佳為0.3以下、更佳為0.2以下、進而較佳為0.1以下、進而較佳為0.05以下，又，較佳為0.001以上且0.3以下、更佳為0.002以上且0.2以下、進而較佳為0.004以上且0.1以下、進而較佳為0.01以上且0.1以下。＜4＞如＜1＞至＜3＞中任一項所記載之製造方法，其中包含碳數為4之二羧酸或其鹽之水溶液中之陰離子性高分子之含量較佳為0.001質量%以上、更佳為0.005質量%以上、進而較佳為0.01質量%以上、進而較佳為0.03質量%以上、進而較佳為0.05質量%以上，又，較佳為5質量%以下、更佳為1.5質量%以下、進而較佳為1質量%以下、進而較佳為0.5質量%以下、進而較佳為0.2質量%以下、進而較佳為0.1質量%以下，又，較佳為0.001質量%以上且5質量%以下、更佳為0.005質量%以上且1.5質量%以下、進而較佳為0.01質量%以上且1.5質量%以下、進而較佳為0.05質量%以上且0.5質量%以下。＜5＞如＜1＞至＜4＞中任一項所記載之製造方法，其中陰離子性高分子較佳為聚(甲基)丙烯酸或其鹽、聚麩胺酸或其鹽、羧基烷基纖維素或其鹽，更佳為聚丙烯酸或其鹽。＜6＞如＜1＞至＜5＞中任一項所記載之製造方法，其中析出結晶之方法為選自藉由調整pH值進行之析出、藉由冷卻進行之析出、藉由濃縮進行之析出、及藉由反應進行之析出中之1種以上之方法。＜7＞如＜6＞所記載之製造方法，其中藉由調整pH值進行結晶之析出時之pH值較佳為9以下、更佳為6以下、進而較佳為2.5以下，又，pH值較佳為0.5以上，又，較佳為0.5以上且9以下、更佳為0.5以上且6以下、進而較佳為0.5以上且2.5以下。＜8＞如＜6＞或＜7＞所記載之製造方法，其中藉由調整pH值進行結晶之析出時之酸之添加速度較佳為0.1 mmol-酸/L/min以上、更佳為0.3 mmol-酸/L/min以上、進而較佳為1 mmol-酸/L/min以上，又，較佳為10 mmol-酸/L/min以下、更佳為5 mmol-酸/L/min以下、進而較佳為3 mmol-酸/L/min以下、進而較佳為2 mmol-酸/L/min以下，又，較佳為0.1 mmol-酸/L/min以上且10 mmol-酸/L/min以下、更佳為0.3 mmol-酸/L/min以上且5 mmol-酸/L/min以下、進而較佳為0.3 mmol-酸/L/min以上且3 mmol-酸/L/min以下、進而較佳為1 mmol-酸/L/min以上且2 mmol-酸/L/min以下。＜9＞如＜8＞所記載之製造方法，其中酸較佳為無機酸，更佳為選自鹽酸、硝酸、硫酸、及磷酸中之1種以上，進而較佳為硫酸或鹽酸。＜10＞如＜6＞至＜9＞中任一項所記載之製造方法，其中藉由調整pH值進行結晶之析出時之溫度較佳為50℃以下、更佳為40℃以下、進而較佳為30℃以下，又，較佳為0℃以上、更佳為5℃以上，又，較佳為0℃以上且50℃以下、更佳為0℃以上且40℃以下、進而較佳為5℃以上且30℃以下。＜11＞如＜6＞至＜10＞中任一項所記載之製造方法，其中藉由調整pH值進行結晶之析出時之含有碳數為4之二羧酸或其鹽之水溶液中的碳數為4之二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上，又，較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。＜12＞如＜6＞所記載之製造方法，其中較佳為對包含碳數為4之二羧酸或其鹽之水溶液進行升溫後藉由冷卻進行結晶之析出。＜13＞如＜12＞所記載之製造方法，其中升溫溫度較佳為60℃以上、更佳為70℃以上、進而較佳為80℃以上，又，較佳為120℃以下，又，較佳為60℃以上且120℃以下、更佳為70℃以上且120℃以下、進而較佳為80℃以上且120℃以下。＜14＞如＜6＞、＜12＞或＜13＞所記載之製造方法，其中冷卻溫度較佳為50℃以下、更佳為40℃以下、進而較佳為30℃以下，又，較佳為0℃以上、更佳為5℃以上，又，較佳為0℃以上且50℃以下、更佳為0℃以上且40℃以下、進而較佳為5℃以上且30℃以下。＜15＞如＜12＞至＜14＞中任一項所記載之製造方法，其中根據自升溫溫度達到冷卻溫度所需之時間算出之平均冷卻速度較佳為0.05℃/min以上、更佳為0.1℃/min以上，又，較佳為20℃/min以下、更佳為10℃/min以下、進而較佳為5℃/min以下，又，較佳為0.05℃/min以上且20℃/min以下、更佳為0.1℃/min以上且10℃/min以下、進而較佳為0.1℃/min以上且5℃/min以下。＜16＞如＜6＞、＜12＞至＜15＞中任一項所記載之製造方法，其中藉由冷卻進行結晶之析出時之pH值較佳為4以下、更佳為2.5以下，又，pH值較佳為0.5以上，又，較佳為0.5以上且4以下，更佳為0.5以上且2.5以下。＜17＞如＜6＞、＜12＞至＜16＞中任一項所記載之製造方法，其中藉由冷卻進行結晶之析出時之含有碳數為4之二羧酸或其鹽之水溶液中的碳數為4之二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上，又，較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。＜18＞如＜6＞所記載之製造方法，其中藉由濃縮進行結晶之析出係藉由使含有碳數為4之二羧酸或其鹽之水溶液之溶劑蒸發，其次進行濃縮而進行，蒸發時之溫度較佳為300℃以下、更佳為200℃以下，又，較佳為5℃以上，又，較佳為5℃以上且300℃以下、更佳為5℃以上且200℃以下。＜19＞如＜6＞或＜18＞所記載之製造方法，其中藉由濃縮進行結晶之析出時之含有碳數為4之二羧酸或其鹽之水溶液中的碳數為4之二羧酸或其鹽之含量較佳為45質量%以下、更佳為40質量%以下、進而較佳為20質量%以下，又，較佳為1質量%以上、更佳為2質量%以上，又，較佳為1質量%以上且45質量%以下、更佳為1質量%以上且40質量%以下、進而較佳為2質量%以上且20質量%以下。＜20＞如＜6＞、＜18＞或＜19＞所記載之製造方法，其中藉由濃縮進行結晶之析出時之pH值較佳為4以下、更佳為2.5以下，又，pH值較佳為0.5以上，又，較佳為0.5以上且4以下、更佳為0.5以上且2.5以上。＜21＞如＜1＞至＜20＞中任一項所記載之製造方法，其中一面以較佳為周速0.2 m/s以上、更佳為周速0.3 m/s以上、進而較佳為周速0.5 m/s以上，又，較佳為周速10 m/s以下、更佳為周速5 m/s以下、進而較佳為周速3 m/s以下，又，較佳為周速0.2 m/s以上且10 m/s以下、更佳為周速0.3 m/s以上且5 m/s以下、進而較佳為周速0.5 m/s以上且3 m/s以下進行攪拌，一面進行結晶之析出。＜22＞如＜1＞至＜21＞中任一項所記載之製造方法，其中將包含碳數為4之二羧酸或其鹽之水溶液升溫至80℃以上，於確認溶解後，以0.05℃/min以上之平均冷卻速度藉由冷卻進行析出，於達到30℃後，添加無機酸而將pH值降至2.5以下，藉此進行結晶之析出。＜23＞如＜1＞至＜22＞中任一項所記載之製造方法，其中碳數為4之二羧酸結晶較佳為反丁烯二酸、琥珀酸、蘋果酸、酒石酸、順丁烯二酸、或草醯乙酸之結晶，更佳為反丁烯二酸或琥珀酸之結晶，進而較佳為反丁烯二酸結晶。＜24＞如＜6＞或＜21＞所記載之製造方法，其中碳數為4之二羧酸結晶為反丁烯二酸結晶，藉由在含有順丁烯二酸或順丁烯二酸酐之水溶液中添加觸媒而生成反丁烯二酸之反應進行結晶之析出。＜25＞如＜24＞所記載之製造方法，其中含有順丁烯二酸或順丁烯二酸酐之水溶液中之順丁烯二酸或順丁烯二酸酐之含量較佳為5質量%以上、更佳為10質量%以上、更佳為20質量%以上，又，較佳為70質量%以下、更佳為50質量%以下、更佳為40質量%以下、更佳為30質量%以下。＜26＞如＜24＞或＜25＞所記載之製造方法，其中反應時之溫度為60℃以上且100℃以下。＜27＞如＜24＞至＜26＞中任一項所記載之製造方法，其中觸媒較佳為硫脲、溴酸鹽、或過硼酸鹽。＜28＞如＜1＞至＜27＞中任一項所記載之製造方法，其中碳數為4之二羧酸結晶之依據JIS K 5101-13-2(2004)而測定之吸油量較佳為50 mL/100 g～200 mL/100 g、更佳為60 mL/100 g～200 mL/100 g、進而較佳為70 mL/100 g～200 mL/100 g。＜29＞一種碳數為4之二羧酸結晶，其係藉由如＜1＞至＜28＞中任一項所記載之製造方法而獲得。[實施例][C4二羧酸]・反丁烯二酸：川崎化成公司工業(股)製造・反丁烯二酸：日本觸媒(股)製造・琥珀酸：和光純藥工業(股)製造(陰離子性高分子)・聚丙烯酸(重量平均分子量為250000)(和光純藥工業(股)製造)・聚丙烯酸(重量平均分子量為5000)(和光純藥工業(股)製造)・聚丙烯酸(重量平均分子量為25000)(和光純藥工業(股)製造)・聚丙烯酸63%水溶液(重量平均分子量為1800)(Polysciences, Inc製造)・羧甲基纖維素鈉(東京化成工業(股)製造)[吸油量之測定]吸油量係依據JIS K 5101-13-2，將試樣1～5 g取至測定板(大於300×400 mm之平滑之玻璃板)上之中央部，按照1次4、5滴將熟亞麻籽油自滴定管緩緩地滴加至試樣之中央，每次利用平刮刀充分地對整體進行混練。反覆進行熟亞麻籽油之滴加及混練，於整體變為較硬之油灰狀之塊後以每次1滴進行混練，將藉由最後之1滴而成為可使用平刮刀捲成螺旋形之狀態時設為終點。但於無法捲成螺旋狀之情形時，將藉由1滴熟亞麻籽油而急遽地變柔軟之前設為終點。以達到終點前之操作時間成為7～15分鐘之間之方式調節操作。讀取達到終點時之滴定管內之熟亞麻籽油滴加量而設為吸油量(單位：相對於試樣每100 g之mL)。[比較例1]對川崎化成工業(股)製造之反丁烯二酸之吸油量進行測定，結果為29.5 mL/100 g。[比較例2]對日本觸媒(股)製造之反丁烯二酸之吸油量進行測定，結果為29.4 mL/100 g。[比較例3] 對和光純藥工業(股)製造之琥珀酸之吸油量進行測定，結果為27.3 mL/100 g。 [比較例4]藉由使用SUPERMIXER PICCOLO SMP2(KAWATAMFG(股)製造)，以葉徑140 mm、3000 r/min之速度進行20 min之攪拌而粉碎日本觸媒(股)製造之反丁烯二酸。對所粉碎之反丁烯二酸之吸油量進行測定，結果為36.6 mL/100 g。將比較例1～4之結果示於表1。[表1]
(藉由冷卻進行之析出)[實施例1]於在3 L之反應槽(直徑為130 mm)混合離子交換水2.14 kg、反丁烯二酸105 g後，升溫至85℃而進行溶解。繼而，於混合1.125 g之聚丙烯酸(分子量為5000 g/mol)後，以平均冷卻速度0.3℃/min自85℃冷卻至25℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。其次，使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以105℃進行乾燥。於乾燥後，通過網眼為500 μm之篩網，藉此獲得反丁烯二酸結晶。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為72.0 mL/100 g。[實施例2]於與實施例1相同之實驗中，將添加之聚丙烯酸之量變更為0.788 g而進行實驗。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為65.0 mL/100 g。[實施例3]於與實施例1相同之實驗中，將添加之聚丙烯酸之量變更為0.45 g而進行實驗。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為65.0 mL/100 g。[實施例4]於與實施例1相同之實驗中，將添加之聚丙烯酸之量變更為0.113 g而進行實驗。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為60.0 mL/100 g。[實施例5]於在100 L之反應槽(直徑為450 mm)混合離子交換水79.9 kg、反丁烯二酸3923 g後，升溫至85℃而進行溶解。繼而，於混合168 g之聚丙烯酸(分子量為5000 g/mol)後，以平均冷卻速度0.3℃/min自85℃冷卻至25℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為405 mm之攪拌葉，以50 r/min之條件進行。 其次，使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以60℃進行乾燥。於乾燥後，通過網眼為500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為60.0 mL/100 g。[比較例5]於在3 L之反應槽(直徑為130 mm)混合離子交換水2.09 kg、反丁烯二酸105 g、聚丙烯酸(分子量為250000 g/mol)0.034 g後，升溫至80℃而進行溶解。繼而，以平均冷卻速度0.23℃/min自80℃冷卻至24℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以150 r/min之條件進行。其次，於使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾後，添加500 g之離子交換水而進行過濾清洗。濾液之pH值為2.1。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT以105℃進行乾燥。於乾燥後，通過網眼為500 μm之篩網，藉此獲得反丁烯二酸結晶。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為26.8 mL/100 g。[實施例6] 於在3 L之反應槽(直徑為450 mm)混合離子交換水2.14 kg、反丁烯二酸105.8 g後，升溫至85℃而進行溶解。繼而，於混合1.13 g之聚丙烯酸(分子量為250000 g/mol)後，以平均冷卻速度0.3℃/min自80℃冷卻至25℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以105℃進行乾燥。於乾燥後，通過網眼為500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為162.0 mL/100 g。 [實施例7] 於在3 L之反應槽(直徑為450 mm)混合離子交換水2.14 kg、反丁烯二酸105.8 g後，升溫至85℃而進行溶解。繼而，於混合1.13 g之聚丙烯酸(分子量為25000 g/mol)後，以平均冷卻速度0.3℃/min自80℃冷卻至25℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以105℃進行乾燥。於乾燥後，通過網眼為500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為110.0 mL/100 g。 [實施例8]於與實施例1相同之實驗中，將添加之聚丙烯酸之種類變更為聚丙烯酸(分子量為1800 g/mol)而進行實驗。但由於所使用之試劑為聚丙烯酸之63%水溶液，故而水溶液之添加量係設為實施例1之添加量之1.59倍。對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為59.0 mL/100 g。 [實施例9] 於在3 L之反應槽(直徑為450 mm)混合離子交換水2.14 kg、反丁烯二酸105.8 g後，升溫至85℃而進行溶解。繼而，於混合1.13 g之羧甲基纖維素鈉(東京化成工業(股))後，以平均冷卻速度0.3℃/min自80℃冷卻至25℃，藉此使反丁烯二酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以105℃進行乾燥。於乾燥後，通過網眼500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為100.0 mL/100 g。 將實施例1～9及比較例5之結果示於表2及表3。 [表2]
[表3]
如表2及表3，確認到於陰離子性高分子之存在下，且陰離子性高分子之濃度相對於反丁烯二酸之濃度為特定比之情形時，自包含反丁烯二酸或其鹽之水溶液晶析而獲得之反丁烯二酸結晶表現出較高之吸油量。 (藉由反應進行之晶析) [實施例10] 於在3 L之反應槽(直徑為130 mm)混合離子交換水1.55 kg、順丁烯二酸酐450 g、聚丙烯酸(分子量為5000g/mol)9.58 g、98%硫酸24.2 g後，升溫至70℃而進行溶解。繼而，以1.8 ml/min之速度花費120分鐘連續添加6.19%硫脲水溶液，藉此進行自順丁烯二酸向反丁烯二酸之異構化反應而使反丁烯二酸析出。於經過120分鐘後，為了提高反丁烯二酸之產率而以70℃保持30分鐘。此後，以平均冷卻速度0.6℃/min自70℃冷卻至25℃。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，於使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾後，添加1000 g之離子交換水而進行過濾清洗。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT以60℃進行乾燥。於乾燥後，通過網眼500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為80 mL/100 g。 [比較例6] 於在3 L之反應槽(直徑為130 mm)混合離子交換水1.56 kg、順丁烯二酸酐450 g、98%硫酸24.2 g後，升溫至70℃而進行溶解。繼而，以1.8 ml/min之速度花費120分鐘而連續添加6.19%硫脲水溶液，藉此進行自順丁烯二酸向反丁烯二酸之異構化反應而使反丁烯二酸析出。於經過120分鐘後，為了提高反丁烯二酸之產率而以70℃保持30分鐘。此後，以平均冷卻速度0.6℃/min自70℃冷卻至25℃。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，於使用No.2之濾紙對所析出之反丁烯二酸懸浮液進行抽吸過濾後，添加1000 g之離子交換水而進行過濾清洗。過濾後之反丁烯二酸濾餅係利用熱風循環乾燥器FS-60WT以60℃進行乾燥。於乾燥後，通過網眼500 μm之篩網，藉此獲得反丁烯二酸結晶。 對所獲得之反丁烯二酸結晶之吸油量進行測定，結果為20 mL/100 g。 將實施例10及比較例6之結果示於表4。 [表4]
如上述表4所示，確認到利用藉由反應進行之析出方法亦獲得高吸油量之反丁烯二酸結晶。 (藉由冷卻進行之析出方法) [實施例11] 於在3 L之反應槽(直徑為450 mm)混合離子交換水1.80 kg、琥珀酸450.0 g後，升溫至80℃而進行溶解。繼而，於混合4.77 g之聚丙烯酸(重量平均分子量為5000)(東京化成工業(股))後，以平均冷卻速度0.3℃/min自80℃冷卻至25℃，藉此使琥珀酸析出。攪拌係使用葉徑為121 mm之攪拌葉，以250 r/min之條件進行。 其次，使用No.2之濾紙對所析出之琥珀酸懸浮液進行抽吸過濾。過濾後之琥珀酸濾餅係利用熱風循環乾燥器FS-60WT(東京硝子器械公司製造)以105℃進行乾燥。於乾燥後，通過網眼500 μm之篩網，藉此獲得琥珀酸結晶。 對所獲得之琥珀酸結晶之吸油量進行測定，結果為50.0 mL/100 g。將實施例11之結果示於表5。 [表5]
如上述表5所示，確認到藉由本發明之方法而晶析之琥珀酸結晶表現出較高之吸油量。However, the inventors actually pulverized commercially available fumaric acid as in Patent Document 3, and measured the oil absorption amount thereof according to JIS K 5101-13-2 (2004), and the result was 36.6 mL/100 g (refer to the following description). Comparative Example 4). The oil absorption of commercially available C4 dicarboxylic acid is about 25 to 30 mL/100 g (refer to Comparative Examples 1 to 3 described below), and the oil absorption of the pulverized C4 dicarboxylic acid exceeds the oil absorption amount, but it cannot be borrowed. A sufficient oil absorption is obtained by pulverization. Accordingly, the present invention relates to a process for producing a C4 dicarboxylic acid crystal having a relatively high oil absorption. The present inventors have found that in the presence of an anionic polymer, when the concentration of the anionic polymer is within a specific range with respect to the concentration of the C4 dicarboxylic acid, it is crystallized from an aqueous solution containing a C4 dicarboxylic acid or a salt thereof. The obtained C4 dicarboxylic acid crystal has an unprecedented high oil absorption. According to the method of the present invention, the oil absorption of the C4 dicarboxylic acid crystal can be increased, and a C4 dicarboxylic acid crystal having a high oil absorption can be obtained. [Method for Producing C4 Dicarboxylic Acid Crystal] The C4 dicarboxylic acid crystal of the present invention can be produced by a method comprising the steps of: precipitating from an aqueous solution containing a C4 dicarboxylic acid or a salt thereof in the presence of an anionic polymer; Crystallization of C4 dicarboxylic acid. In the present invention, the mass ratio of the content of the anionic polymer in the aqueous solution to the content of the C4 dicarboxylic acid is 5 × 10 -4 Above and below 0.5. A C4 dicarboxylic acid crystal having a high oil absorption amount is obtained by setting this specific mass ratio. The anionic polymer may be present in the case where a C4 dicarboxylic acid crystal is precipitated, and the addition of the anionic polymer to the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is not particularly limited. (An aqueous solution containing a C4 dicarboxylic acid or a salt thereof) Examples of the C4 dicarboxylic acid of the present invention include fumaric acid, succinic acid, malic acid, tartaric acid, maleic acid, and oxalic acid. It is preferably fumaric acid or succinic acid, more preferably fumaric acid. Further, the C4 dicarboxylic acid or a salt thereof is preferably a dicarboxylic acid having 4 or less carbon atoms which does not have an amino acid residue. The C4 dicarboxylic acid or a salt thereof is not particularly limited and can be obtained by chemical synthesis derived from a petrochemical raw material such as benzene or butane or by microbial fermentation. Examples of the microorganism which produces the C4 dicarboxylic acid or a salt thereof include filamentous fungi such as Rhizopus. When a C4 dicarboxylic acid or a salt thereof is obtained by microbial fermentation, a crystal of a C4 dicarboxylic acid may be precipitated from a culture solution containing a C4 dicarboxylic acid or a salt thereof. (Anionic Polymer) The anionic polymer used in the present invention is preferably water-soluble. Examples of the anionic polymer include polymers having an anionic group such as a carboxyl group, a sulfate group, a sulfonic acid group, a phosphoric acid group, or a boric acid group. Specifically, examples of the natural polymer include a salt of tricyon gum or gum arabic, alginic acid, polyglutamic acid, or the like. Examples of the synthetic polymer include polymerization of monomers including (meth)acrylic acid or maleic acid, maleic anhydride, fumaric acid, itaconic acid, crotonic acid, or vinylsulfonic acid. a salt of a substance or a copolymer and the like. Further, examples thereof include a carboxyalkyl cellulose such as carboxymethyl cellulose or carboxyethyl cellulose, and a carboxyvinyl polymer. These may be used singly or in combination of plural kinds. Examples of the salt include an alkali metal salt, an alkaline earth metal salt, an ammonium salt, an alkyl group or an alkenyl ammonium salt having 1 to 22 carbon atoms, a pyridinium salt substituted with an alkyl group having 1 to 22 carbon atoms or an alkenyl group, and a carbon number. An alkanolammonium salt of 1 to 22, a basic amino acid salt or the like. It is preferably an alkali metal salt, more preferably a sodium salt or a potassium salt. From the viewpoint of crystallization of a high oil absorption C4 dicarboxylic acid, the anionic polymer is preferably a poly(meth)acrylic acid, a polyglutamic acid, a carboxyalkyl cellulose or the like, more preferably a poly (Meth)acrylic acid or a salt thereof, or polyglutamic acid, more preferably polyacrylic acid or a salt thereof. (Molecular weight of the anionic polymer) The weight average molecular weight of the anionic polymer is preferably 1,000 or more, more preferably 2,000 or more, and still more preferably 5,000 or more from the viewpoint of the high oil absorption C4 dicarboxylic acid crystal. Further, it is preferably 2,000,000 or less, more preferably 1,000,000 or less, still more preferably 500,000 or less. The weight average molecular weight of the anionic polymer is preferably 1,000 or more and 2,000,000 or less, more preferably 2,000 or more and 1,000,000 or less, still more preferably 5,000 or more and 500,000 or less. The weight average molecular weight of the polymer can be measured by, for example, a gel permeation chromatography (GPC) method using sodium polyacrylate as a standard. (The mass ratio of the content of the anionic polymer to the content of the dicarboxylic acid having 4 carbon atoms) In the present invention, the content of the anionic polymer in the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is relative to C4 The mass ratio of the dicarboxylic acid content is 5 × 10 -4 Above and below 0.5. From the viewpoint of producing a C4 dicarboxylic acid crystal having a high oil absorption, it is preferably 0.001 or more, more preferably 0.002 or more, still more preferably 0.004 or more, still more preferably 0.01 or more, and from the same viewpoint It is preferably 0.3 or less, more preferably 0.2 or less, still more preferably 0.1 or less, still more preferably 0.05 or less. The content of the content of the anionic polymer in the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is preferably 0.001 or more and 0.3 or less, more preferably 0.002 or more and 0.2 or less, more preferably 0.002 or more, more preferably 0.2 or less, based on the mass of the C4 dicarboxylic acid. It is 0.004 or more and 0.1 or less, More preferably, it is 0.01 or more and 0.1 or less. Further, in the present specification, the content of the C4 dicarboxylic acid is the sum of the content of the C4 dicarboxylic acid and the content of the C4 dicarboxylic acid salt in terms of C4 dicarboxylic acid. (the content of the anionic polymer) The content of the anionic polymer in the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is preferably 0.001% by mass or more from the viewpoint of the C4 dicarboxylic acid crystal having a high oil absorption amount. More preferably, it is 0.005 mass% or more, more preferably 0.01 mass% or more, more preferably 0.03 mass% or more, further preferably 0.05 mass% or more, and further preferably 5 in terms of industrial productivity and cost. The mass% or less is more preferably 1.5% by mass or less, further preferably 1% by mass or less, further preferably 0.5% by mass or less, further preferably 0.2% by mass or less, and further preferably 0.1% by mass or less. The content of the anionic polymer in the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is preferably 0.001% by mass or more and 5% by mass or less, more preferably 0.005% by mass or more and 1.5% by mass or less, and further preferably 0.01% by mass. It is 100% or more and 1% by mass or less, and more preferably 0.05% by mass or more and 0.5% by mass or less. (Method for Crystallizing C4 Dicarboxylic Acid) The method for crystallizing the C4 dicarboxylic acid is not particularly limited, and it can be carried out by a precipitation method by adjusting the pH value, a precipitation method by cooling, and concentration. The precipitation method, the precipitation method by the reaction, and the like are carried out. (Crystalizer) It is preferred to carry out precipitation of C4 dicarboxylic acid crystals while stirring using a reaction vessel having a stirring blade. The agitating blades may be of any shape, particularly for good mixing of the crystals, preferably blades, turbine blades, propeller blades, anchor blades, large blade blades, high power mixing blades (Maxblend). From the viewpoint of uniformly crystallizing the C4 dicarboxylic acid having a high oil absorption amount, the peripheral speed of the stirring is preferably 0.2 m/s or more, more preferably 0.3 m/s or more, and still more preferably 0.5 m/s or more. Further, from the viewpoint of crystallizing the C4 dicarboxylic acid having a high oil absorption amount, it is preferably 10 m/s or less, more preferably 5 m/s or less, still more preferably 3 m/s or less. The peripheral speed of the stirring is preferably 0.2 m/s or more and 10 m/s or less, more preferably 0.3 m/s or more and 5 m/s or less, further preferably 0.5 m/s or more and 3 m/s or less. (Precipitation method by adjusting pH) The precipitation method by adjusting the pH value can crystallize the C4 dicarboxylic acid by dissociating the C4 dicarboxylic acid from the C4 dicarboxylate by adding an acid, The concentration of the C4 dicarboxylic acid is increased above the solubility. The acid to be used for adjusting the pH value is not particularly limited as long as it is an acid having a pKa of less than C4 dicarboxylic acid, and particularly preferably an inorganic acid. Examples of the inorganic acid include hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and the like. Preferred is sulfuric acid or hydrochloric acid. From the viewpoint of the recovery ratio of the C4 dicarboxylic acid, the pH at the time of precipitation is preferably adjusted to a pH of 9 or less, preferably 6 or less, at the start of crystallization, and is adjusted by adding an acid. It is preferably 2.5 or less. Further, the pH is preferably 0.5 or more from the viewpoint of corrosion of the reaction vessel or the like. The pH at the time of precipitation of the crystal is preferably 0.5 or more and 9 or less, more preferably 0.5 or more and 6 or less, still more preferably 0.5 or more and 2.5 or less. From the viewpoint of crystallizing the C4 dicarboxylic acid having a high oil absorption amount, the acid addition rate is preferably 0.1 mmol-acid/L/min or more, more preferably 0.3 mmol-acid/L/min or more, and further preferably. It is preferably 1 mmol-acid/L/min or more, and in view of the filterability of the C4 dicarboxylic acid crystal suspension and the water content of the filtered C4 dicarboxylic acid filter cake, it is preferably 10 mmol-acid/ L/min or less is more preferably 5 mmol-acid/L/min or less, further preferably 3 mmol-acid/L/min or less, further preferably 2 mmol-acid/L/min or less. The acid addition rate is preferably 0.1 mmol-acid/L/min or more and 10 mmol-acid/L/min or less, more preferably 0.3 mmol-acid/L/min or more and 5 mmol-acid/L/min or less. Further, it is preferably 0.3 mmol-acid/L/min or more and 3 mmol-acid/L/min or less, more preferably 1 mmol-acid/L/min or more and 2 mmol-acid/L/min or less. Further, mmol-acid/L/min represents the amount of acid mixed per 1 liter in the reaction liquid for 1 minute. The temperature at the time of precipitation by adjusting the pH value is not particularly limited, and from the viewpoint of the recovery ratio of the C4 dicarboxylic acid, it is preferably carried out at a relatively low temperature. The crystallization temperature is preferably 50 ° C or lower, more preferably 40 ° C or lower, further preferably 30 ° C or lower, and more preferably 0 ° C or higher, more preferably from the viewpoint of the recovery ratio of the C4 dicarboxylic acid. Above 5 °C. The crystallization temperature is preferably 0° C. or higher and 50° C. or lower, more preferably 0° C. or higher and 40° C. or lower, and still more preferably 5° C. or higher and 30° C. or lower. The concentration of the C4 dicarboxylic acid or a salt thereof at the time of precipitation by adjusting the pH value is not particularly limited, and from the viewpoint of the recovery ratio of the C4 dicarboxylic acid, it is preferably a dissolution metric at a temperature at the start of precipitation, or Slightly smaller than the amount of the dissolution metric. Specifically, the content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, still more preferably 20% by mass or less. Further, it is preferably 1% by mass or more, and more preferably 2% by mass or more. The content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof by precipitation at a pH value is preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more. 40% by mass or less, more preferably 2% by mass or more and 20% by mass or less. (Precipitation method by cooling) The precipitation method by cooling can crystallize the C4 dicarboxylic acid by cooling the aqueous solution containing the C4 dicarboxylic acid or its salt from a high temperature to a low temperature, and C4 The dicarboxylic acid concentration is increased above the solubility. The C4 dicarboxylic acid has a high solubility when the temperature is high. Therefore, it is preferred to increase the temperature and increase the dissolved acid concentration, followed by cooling. The temperature rise temperature is preferably 60 ° C or higher, more preferably 70 ° C or higher, further preferably 80 ° C or higher, and further preferably 120 ° C or lower. The temperature rise temperature is preferably 60° C. or higher and 120° C. or lower, more preferably 70° C. or higher and 120° C. or lower, and still more preferably 80° C. or higher and 120° C. or lower. From the viewpoint of the recovery ratio of the C4 dicarboxylic acid, the cooling temperature is preferably 50 ° C or lower, more preferably 40 ° C or lower, further preferably 30 ° C or lower, and further preferably 0 ° C or higher, more preferably 5 Above °C. The cooling temperature is preferably 0° C. or higher and 50° C. or lower, more preferably 0° C. or higher and 40° C. or lower, and still more preferably 5° C. or higher and 30° C. or lower. From the viewpoint of the recovery ratio of the C4 dicarboxylic acid and the viewpoint of the high oil absorption C4 dicarboxylic acid crystal, the average cooling rate calculated from the time required for the temperature from the temperature rise to reach the cooling temperature is preferably 0.05 ° C / min or more. More preferably, it is 0.1 ° C / min or more, and in view of the adhesion of the crystal to the reaction tank, the filterability of the C4 dicarboxylic acid crystal suspension, and the water content of the filtered C4 dicarboxylic acid filter cake, It is preferably 20 ° C / min or less, more preferably 10 ° C / min or less, further preferably 5 ° C / min or less. The average cooling rate calculated from the time required for the temperature rise to reach the cooling temperature is preferably 0.05 ° C / min or more and 20 ° C / min or less, more preferably 0.1 ° C / min or more and 10 ° C / min or less, and further preferably 0.1 ° C / min or more and 5 ° C / min or less. From the viewpoint of the recovery ratio of the C4 dicarboxylic acid, the pH of the precipitation method by cooling is preferably adjusted to a pH of 4 or less, more preferably 2.5 or less at the start of crystallization. Further, the pH is preferably 0.5 or more from the viewpoint of corrosion of the reaction vessel or the like. The concentration of the C4 dicarboxylic acid or a salt thereof during precipitation by cooling is not particularly limited, and from the viewpoint of the recovery ratio of the C4 dicarboxylic acid, it is preferably a dissolution metric at a temperature at the start of precipitation, or slightly smaller than The amount of the dissolution metric. Specifically, the content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, still more preferably 20% by mass or less. Further, it is preferably 1% by mass or more, and more preferably 2% by mass or more. The content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof during precipitation by cooling is preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more and 40%. The mass% or less is more preferably 2% by mass or more and 20% by mass or less. (Precipitation method by concentration) The precipitation method by concentration can be carried out by crystallizing a C4 dicarboxylic acid by concentrating a solvent (water) containing an aqueous solution of a C4 dicarboxylic acid or a salt thereof, The C4 dicarboxylic acid concentration is increased above the solubility. The method of concentration is not particularly limited, and a reaction tank or the like may be used, and the solvent (water) may be instantaneously evaporated by a spray dryer such as a spray dryer. The temperature at the time of evaporation is not particularly limited, but is preferably 300 ° C or lower, more preferably 200 ° C or lower, and further preferably 5 ° C or higher. The temperature at the time of evaporation is preferably 5 ° C or more and 300 ° C or less, more preferably 5 ° C or more and 200 ° C or less. Further, evaporation can also be carried out under reduced pressure. The concentration of the C4 dicarboxylic acid or a salt thereof during precipitation by concentration is not particularly limited, and is preferably set to a concentration corresponding to the solubility of each temperature of the C4 dicarboxylic acid or a concentration slightly smaller than the solubility. Specifically, the content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, and further, from the viewpoint of the recovery ratio. It is preferably 20% by mass or less, more preferably 1% by mass or more, and still more preferably 2% by mass or more. The content of the C4 dicarboxylic acid or a salt thereof in the aqueous solution containing a C4 dicarboxylic acid or a salt thereof by concentration is preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more and 40%. The mass% or less is more preferably 2% by mass or more and 20% by mass or less. Further, from the viewpoint of the recovery ratio of the C4 dicarboxylic acid, the pH of the precipitation method by concentration is preferably adjusted to a pH of 4 or less, more preferably 2.5 or less at the start of crystallization. Further, the pH is preferably 0.5 or more from the viewpoint of corrosion of the reaction vessel or the like. (Precipitation method by reaction) The precipitation method by a reaction can be suitably set according to the kind of C4 dicarboxylic acid. For example, in the case of the precipitation of fumaric acid, the fumaric acid can be crystallized by adding a catalyst to an aqueous solution containing maleic acid or maleic anhydride to form a fubutene. The diacid increases the concentration of fumaric acid above the solubility. The content of maleic acid or maleic anhydride in the aqueous solution containing maleic acid or maleic anhydride is preferably 5% by mass or more, more preferably 10% by mass or more, and still more preferably 20% by mass. More preferably, it is 70% by mass or less, more preferably 50% by mass or less, still more preferably 40% by mass or less, and still more preferably 30% by mass or less. The temperature at the time of the reaction is not particularly limited, but is preferably 60 ° C to 100 ° C. The catalyst is not particularly limited as long as it is a catalyst for promoting the formation reaction of the C4 dicarboxylic acid, and examples thereof include thiourea, bromate, and perborate. Further, an inorganic acid such as sulfuric acid or hydrochloric acid may be added during the reaction. These precipitation methods can be carried out separately or in combination with a plurality of methods. For example, in the present invention, as a method for precipitating a C4 dicarboxylic acid crystal, it is considered that the aqueous solution containing the C4 dicarboxylic acid or a salt thereof is heated to 80 ° C or higher, and after confirming dissolution, the average cooling rate is 0.05 ° C / min or more. The precipitation was carried out by cooling, and after reaching 30 ° C, a mineral acid was added to lower the pH to 2.5 or less. (Filtering of C4 Dicarboxylic Acid Crystal Suspension) The crystal of C4 dicarboxylic acid can be obtained by solid-liquid separation operation such as centrifugation, filtration, decantation or the like. The separation operation of the crystals or the like is preferably carried out in the above temperature range. The C4 dicarboxylic acid crystals obtained in this manner can also be washed as needed. Drying is carried out after washing as needed, whereby C4 dicarboxylic acid crystals can be obtained. (drying of C4 dicarboxylic acid crystals) drying can be carried out using a shelf dryer, a conical dryer, a paddle dryer, a nauta mixer, a fluidized bed dryer, a vacuum agitation dryer, A general dryer such as a disk dryer or an airflow dryer. In order to maintain the crystal structure of the C4 dicarboxylic acid having a high oil absorption amount, a drying method in which high shear is not applied is preferred. The drying temperature is preferably 70 ° C or higher, more preferably 80 ° C or higher, further preferably 100 ° C or higher, and more preferably 300 ° C or lower, more preferably 250 ° C or lower, further preferably 200 ° C or lower, and further It is preferably 150 ° C or lower, more preferably 130 ° C or lower, and still more preferably 120 ° C or lower. Further, it can also be dried under reduced pressure. The dried C4 dicarboxylic acid crystals may also be subjected to sieving or the like as needed. [C4 Dicarboxylic Acid Crystal] In this way, a C4 dicarboxylic acid crystal having a high oil absorption amount was obtained. The oil absorption amount of the C4 dicarboxylic acid crystal of the present invention measured according to JIS K 5101-13-2 (2004) is preferably 40 mL/100 g to 200 mL/100 g. JIS K 5101-13-2 (2004) is a method for measuring the oil absorption by the cooked linseed oil method. The details of the measurement method are described in the examples. In the present specification, the "oil absorption amount measured in accordance with JIS K 5101-13-2 (2004)" is also simply referred to as "oil absorption amount". The oil absorption of the C4 dicarboxylic acid crystal is preferably from 50 mL/100 g to 200 mL/100 g, more preferably from 60 mL/100 g to 200 mL/100 g, and further preferably from 70 mL/100 g to 200 mL. /100 g. The C4 dicarboxylic acid crystal having high oil absorption property is not particularly limited, and it can be used as a resin raw material, a food additive, or the like, and is particularly suitable as a raw material for a bathing agent which is required to further support an oil agent which is expected to have a moisturizing effect. In the above embodiment, the present invention further discloses the following manufacturing method. <1> A method for producing a dicarboxylic acid crystal having a carbon number of 4, which comprises, in the presence of an anionic polymer, a carbon number of 4 in an aqueous solution containing a dicarboxylic acid having a carbon number of 4 or a salt thereof a step of crystallizing the carboxylic acid, and the mass ratio of the content of the anionic polymer in the aqueous solution to the content of the dicarboxylic acid having a carbon number of 4 is 5 × 10 -4 Above and below 0.5. <2> The production method according to <1>, wherein the weight average molecular weight of the anionic polymer is preferably 1,000 or more, more preferably 2,000 or more, still more preferably 5,000 or more, further preferably 2,000,000 or less, more preferably It is preferably 1,000,000 or less, more preferably 500,000 or less, further preferably 1,000 or more and 2,000,000 or less, more preferably 2,000 or more and 1,000,000 or less, still more preferably 5,000 or more and 500,000 or less. <3> The production method according to <1> or <2>, wherein the content of the anionic polymer in the aqueous solution containing a carbon number of 4 dicarboxylic acid or a salt thereof is 4 dicarboxylic acid with respect to the carbon number The content of the content is preferably 0.001 or more, more preferably 0.002 or more, still more preferably 0.004 or more, still more preferably 0.01 or more, further preferably 0.3 or less, more preferably 0.2 or less, still more preferably 0.1 or less. Further, it is preferably 0.05 or less, more preferably 0.001 or more and 0.3 or less, more preferably 0.002 or more and 0.2 or less, still more preferably 0.004 or more and 0.1 or less, still more preferably 0.01 or more and 0.1 or less. The production method according to any one of <1> to <3> wherein the content of the anionic polymer in the aqueous solution containing a dicarboxylic acid having a carbon number of 4 or a salt thereof is preferably 0.001% by mass. The above is more preferably 0.005 mass% or more, further preferably 0.01 mass% or more, further preferably 0.03 mass% or more, further preferably 0.05 mass% or more, further preferably 5% by mass or less, more preferably 1.5% by mass or less, more preferably 1% by mass or less, further preferably 0.5% by mass or less, further preferably 0.2% by mass or less, further preferably 0.1% by mass or less, and more preferably 0.001% by mass or more. It is 5% by mass or less, more preferably 0.005% by mass or more and 1.5% by mass or less, further preferably 0.01% by mass or more and 1.5% by mass or less, and further preferably 0.05% by mass or more and 0.5% by mass or less. The production method according to any one of <1> to <4> wherein the anionic polymer is preferably poly(meth)acrylic acid or a salt thereof, polyglutamic acid or a salt thereof, or a carboxyalkyl group. Cellulose or a salt thereof is more preferably polyacrylic acid or a salt thereof. The production method according to any one of <1> to <5> wherein the method of depositing crystals is selected from the group consisting of precipitation by pH adjustment, precipitation by cooling, and concentration by precipitation. One or more methods of precipitation and precipitation by reaction. <7> The production method according to <6>, wherein the pH at the time of precipitation by crystallization by adjusting the pH is preferably 9 or less, more preferably 6 or less, still more preferably 2.5 or less, and further, pH It is preferably 0.5 or more, and more preferably 0.5 or more and 9 or less, more preferably 0.5 or more and 6 or less, still more preferably 0.5 or more and 2.5 or less. <8> The production method according to <6> or <7>, wherein the acid addition rate at the time of precipitation by crystallization by adjusting the pH is preferably 0.1 mmol-acid/L/min or more, more preferably 0.3. Methyl-acid/L/min or more, further preferably 1 mmol-acid/L/min or more, further preferably 10 mmol-acid/L/min or less, more preferably 5 mmol-acid/L/min or less Further, it is preferably 3 mmol-acid/L/min or less, further preferably 2 mmol-acid/L/min or less, and further preferably 0.1 mmol-acid/L/min or more and 10 mmol-acid/L. /min or less, more preferably 0.3 mmol-acid/L/min or more and 5 mmol-acid/L/min or less, further preferably 0.3 mmol-acid/L/min or more and 3 mmol-acid/L/min or less Further, it is preferably 1 mmol-acid/L/min or more and 2 mmol-acid/L/min or less. <9> The production method according to <8>, wherein the acid is preferably an inorganic acid, more preferably one or more selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid, and more preferably sulfuric acid or hydrochloric acid. The production method according to any one of <6> to <9> wherein the temperature at which the precipitation of the crystal is adjusted by adjusting the pH is preferably 50 ° C or lower, more preferably 40 ° C or lower, and further It is preferably 30 ° C or lower, more preferably 0 ° C or higher, more preferably 5 ° C or higher, and further preferably 0 ° C or higher and 50 ° C or lower, more preferably 0 ° C or higher and 40 ° C or lower, and further preferably 5 ° C or more and 30 ° C or less. The production method according to any one of <6> to <10> wherein the carbon in the aqueous solution containing a carbon number of 4 dicarboxylic acid or a salt thereof when the crystal is precipitated by adjusting the pH value The content of the dicarboxylic acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, still more preferably 20% by mass or less, still more preferably 1% by mass or more, and still more preferably 2% by mass or more, more preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and still more preferably 2% by mass or more and 20% by mass or less. <12> The production method according to <6>, wherein it is preferred that the aqueous solution containing a dicarboxylic acid having a carbon number of 4 or a salt thereof is heated and then precipitated by cooling. <13> The production method according to <12>, wherein the temperature increase temperature is preferably 60 ° C or higher, more preferably 70 ° C or higher, further preferably 80 ° C or higher, and more preferably 120 ° C or lower. It is preferably 60° C. or higher and 120° C. or lower, more preferably 70° C. or higher and 120° C. or lower, and still more preferably 80° C. or higher and 120° C. or lower. <14> The production method according to <6>, <12> or <13>, wherein the cooling temperature is preferably 50 ° C or lower, more preferably 40 ° C or lower, further preferably 30 ° C or lower, and further preferably It is 0 ° C or more, more preferably 5 ° C or more, and further preferably 0 ° C or more and 50 ° C or less, more preferably 0 ° C or more and 40 ° C or less, further preferably 5 ° C or more and 30 ° C or less. The production method according to any one of <12> to <14>, wherein the average cooling rate calculated from the time required for the temperature from the temperature rise to reach the cooling temperature is preferably 0.05 ° C / min or more, more preferably 0.1 ° C / min or more, more preferably 20 ° C / min or less, more preferably 10 ° C / min or less, further preferably 5 ° C / min or less, further preferably 0.05 ° C / min or more and 20 ° C / More preferably, it is 0.1 ° C / min or more and 10 ° C / min or less, more preferably 0.1 ° C / min or more and 5 ° C / min or less. The production method according to any one of <6>, wherein the pH at the time of precipitation by crystallization by cooling is preferably 4 or less, more preferably 2.5 or less, and further The pH is preferably 0.5 or more, more preferably 0.5 or more and 4 or less, still more preferably 0.5 or more and 2.5 or less. The production method according to any one of <6>, wherein the precipitation of the crystal by cooling is carried out in an aqueous solution containing a dicarboxylic acid having a carbon number of 4 or a salt thereof. The content of the carbon tetracarboxylic acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, still more preferably 20% by mass or less, and further preferably 1% by mass or more. It is preferably 2% by mass or more, more preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and still more preferably 2% by mass or more and 20% by mass or less. <18> The production method according to <6>, wherein the precipitation by crystallization is carried out by evaporating a solvent containing an aqueous solution of a dicarboxylic acid having a carbon number of 4 or a salt thereof, followed by concentration, and evaporation. The temperature is preferably 300 ° C or lower, more preferably 200 ° C or lower, further preferably 5 ° C or higher, and more preferably 5 ° C or higher and 300 ° C or lower, more preferably 5 ° C or higher and 200 ° C or lower. <19> The production method according to <6> or <18>, wherein the carbon number of the dicarboxylic acid having a carbon number of 4 or a salt thereof when the crystallization is precipitated by concentration is 4 carboxylic acid The content of the acid or a salt thereof is preferably 45% by mass or less, more preferably 40% by mass or less, still more preferably 20% by mass or less, and further preferably 1% by mass or more, more preferably 2% by mass or more. It is preferably 1% by mass or more and 45% by mass or less, more preferably 1% by mass or more and 40% by mass or less, and still more preferably 2% by mass or more and 20% by mass or less. <20> The production method according to <6>, <18> or <19>, wherein the pH at the time of precipitation by crystallization by concentration is preferably 4 or less, more preferably 2.5 or less, and further, pH is higher than It is preferably 0.5 or more, and more preferably 0.5 or more and 4 or less, more preferably 0.5 or more and 2.5 or more. The manufacturing method as described in any one of <1> to <20>, wherein one side is preferably 0.2 m/s or more of the peripheral speed, more preferably 0.3 m/s or more of the peripheral speed, and further preferably The peripheral speed is 0.5 m/s or more, and more preferably, the peripheral speed is 10 m/s or less, more preferably the peripheral speed is 5 m/s or less, further preferably the peripheral speed is 3 m/s or less, and more preferably, the circumference is preferably 5 m/s or less. The speed is 0.2 m/s or more and 10 m/s or less, more preferably 0.3 m/s or more and 5 m/s or less, and more preferably 0.5 m/s or more and 3 m/s or less. The crystallization is precipitated on one side. The production method according to any one of <1> to <21> wherein the aqueous solution containing a dicarboxylic acid having a carbon number of 4 or a salt thereof is heated to 80 ° C or higher, and after confirming dissolution, 0.05 is obtained. The average cooling rate of ° C/min or more is precipitated by cooling, and after reaching 30 ° C, the inorganic acid is added to lower the pH to 2.5 or less, thereby precipitating the crystal. <23> The production method according to any one of <1> to <22> wherein the dicarboxylic acid crystal having a carbon number of 4 is preferably fumaric acid, succinic acid, malic acid, tartaric acid or cistern. The crystal of enedic acid or oxalic acid is more preferably a crystal of fumaric acid or succinic acid, and further preferably a crystal of fumaric acid. <24> The production method according to <6> or <21> wherein the crystal of the dicarboxylic acid having a carbon number of 4 is a crystal of fumaric acid by containing maleic acid or maleic anhydride. The catalyst is added to the aqueous solution to form a reaction of fumaric acid to precipitate crystals. <25> The production method according to <24>, wherein the content of maleic acid or maleic anhydride in the aqueous solution containing maleic acid or maleic anhydride is preferably 5% by mass or more More preferably, it is 10% by mass or more, more preferably 20% by mass or more, and further preferably 70% by mass or less, more preferably 50% by mass or less, still more preferably 40% by mass or less, and still more preferably 30% by mass or less. . <26> The production method according to <24> or <25>, wherein the temperature at the time of the reaction is 60 ° C or more and 100 ° C or less. <27> The production method according to any one of <24> to <26> wherein the catalyst is preferably thiourea, bromate or perborate. <28> The production method according to any one of <1> to <27> wherein the oil absorption amount of the dicarboxylic acid crystal having a carbon number of 4 is preferably measured in accordance with JIS K 5101-13-2 (2004). It is 50 mL/100 g to 200 mL/100 g, more preferably 60 mL/100 g to 200 mL/100 g, and further preferably 70 mL/100 g to 200 mL/100 g. <29> A carboxylic acid crystal having a carbon number of 4, which is obtained by the production method according to any one of <1> to <28>. [Examples] [C4 dicarboxylic acid], fumaric acid: Kawasaki Kasei Co., Ltd. Industrial (stock) manufacturing, fumaric acid: Japanese catalyst (stock) manufacturing, succinic acid: Wako Pure Chemical Industries Co., Ltd. Production (anionic polymer), polyacrylic acid (weight average molecular weight: 250,000) (manufactured by Wako Pure Chemical Industries, Ltd.), polyacrylic acid (weight average molecular weight: 5,000) (manufactured by Wako Pure Chemical Industries, Ltd.), polyacrylic acid (weight average molecular weight: 25,000) (manufactured by Wako Pure Chemical Industries, Ltd.), polyacrylic acid 63% aqueous solution (weight average molecular weight: 1800) (manufactured by Polysciences, Inc.), sodium carboxymethyl cellulose (Tokyo Chemical Industry Co., Ltd.) Manufactured] [Measurement of oil absorption] Oil absorption is based on JIS K 5101-13-2, and samples 1 to 5 g are taken to the center of the measuring plate (smooth glass plate larger than 300 × 400 mm). The next 4 and 5 drops of the cooked linseed oil were slowly dropped from the titration tube to the center of the sample, and the whole was thoroughly kneaded each time by a flat blade. Repeatedly adding and mixing the cooked linseed oil, and after mixing the whole into a hard putty-like block, mixing one drop at a time, and using the last one drop, it can be rolled into a spiral shape by using a flat blade. Set to the end point in the state. However, when it is impossible to roll into a spiral shape, it will be set as an end point before it is softened by one drop of cooked linseed oil. The operation is adjusted in such a manner that the operation time before reaching the end point becomes between 7 and 15 minutes. The amount of cooked linseed oil in the burette at the end of the end is read and set as the oil absorption (unit: 100 g per 100 g of the sample). [Comparative Example 1] The oil absorption of fumaric acid produced by Kawasaki Chemical Industry Co., Ltd. was measured and found to be 29.5 mL/100 g. [Comparative Example 2] The oil absorption of fumaric acid produced by a Japanese catalyst (stock) was measured and found to be 29.4 mL/100 g. [Comparative Example 3] The oil absorption of succinic acid produced by Wako Pure Chemical Industries Co., Ltd. was measured and found to be 27.3 mL/100 g. [Comparative Example 4] The use of SUPERMIXER PICCOLO SMP2 (manufactured by KAWATAMFG Co., Ltd.) was carried out by stirring at a leaf diameter of 140 mm and 3000 r/min for 20 minutes to pulverize the antibutene produced by Nippon Shokubai Co., Ltd. acid. The oil absorption of the pulverized fumaric acid was measured and found to be 36.6 mL/100 g. The results of Comparative Examples 1 to 4 are shown in Table 1. [Table 1] (Precipitation by cooling) [Example 1] After mixing 2.14 kg of ion-exchanged water and 105 g of fumaric acid in a reaction tank of 3 L (diameter: 130 mm), the mixture was heated to 85 ° C to be dissolved. Then, after 1.125 g of polyacrylic acid (molecular weight: 5000 g/mol) was mixed, it was cooled from 85 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper. The filtered fumaric acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, a mesh of 500 μm was passed through the mesh, whereby crystals of fumaric acid were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 72.0 mL/100 g. [Example 2] In the same experiment as in Example 1, the experiment was carried out by changing the amount of the added polyacrylic acid to 0.788 g. The oil absorption of the obtained fumaric acid crystal was measured and found to be 65.0 mL/100 g. [Example 3] In the same experiment as in Example 1, the experiment was carried out by changing the amount of the added polyacrylic acid to 0.45 g. The oil absorption of the obtained fumaric acid crystal was measured and found to be 65.0 mL/100 g. [Example 4] In the same experiment as in Example 1, the experiment was carried out by changing the amount of the added polyacrylic acid to 0.113 g. The oil absorption of the obtained fumaric acid crystal was measured and found to be 60.0 mL/100 g. [Example 5] After mixing 79.9 kg of ion-exchanged water and 3923 g of fumaric acid in a reaction tank of 100 L (diameter: 450 mm), the temperature was raised to 85 ° C to dissolve. Then, after mixing 168 g of polyacrylic acid (molecular weight: 5000 g/mol), it was cooled from 85 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 405 mm at 50 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper. The filtered fumaric acid filter cake was dried at 60 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, a mesh of 500 μm was passed through the mesh, whereby crystals of fumaric acid were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 60.0 mL/100 g. [Comparative Example 5] After mixing ion exchange water (2.09 kg, fumaric acid 105 g, polyacrylic acid (molecular weight: 250,000 g/mol) 0.034 g in a 3 L reaction tank (diameter: 130 mm), the temperature was raised to 80. Dissolved at °C. Then, it was cooled from 80 ° C to 24 ° C at an average cooling rate of 0.23 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 150 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper, and then 500 g of ion-exchanged water was added thereto to carry out filtration washing. The pH of the filtrate was 2.1. The filtered fumaric acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT. After drying, a mesh of 500 μm was passed through the mesh, whereby crystals of fumaric acid were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 26.8 mL/100 g. [Example 6] After mixing 2.14 kg of ion-exchanged water and 105.8 g of fumaric acid in a reaction tank of 3 L (diameter: 450 mm), the temperature was raised to 85 ° C to dissolve. Then, after mixing 1.13 g of polyacrylic acid (having a molecular weight of 250,000 g/mol), it was cooled from 80 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper. The filtered fumaric acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, a mesh of 500 μm was passed through the mesh, whereby crystals of fumaric acid were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 162.0 mL/100 g. [Example 7] After mixing 2.14 kg of ion-exchanged water and 105.8 g of fumaric acid in a reaction tank of 3 L (diameter: 450 mm), the temperature was raised to 85 ° C to dissolve. Then, after mixing 1.13 g of polyacrylic acid (having a molecular weight of 25,000 g/mol), it was cooled from 80 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper. The filtered fumaric acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, a mesh of 500 μm was passed through the mesh, whereby crystals of fumaric acid were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 110.0 mL/100 g. [Example 8] In the same experiment as in Example 1, an experiment was carried out by changing the type of polyacrylic acid to be added to polyacrylic acid (having a molecular weight of 1,800 g/mol). However, since the reagent used was a 63% aqueous solution of polyacrylic acid, the amount of the aqueous solution added was set to 1.59 times the amount of addition of Example 1. The oil absorption of the obtained fumaric acid crystal was measured and found to be 59.0 mL/100 g. [Example 9] After mixing 2.14 kg of ion-exchanged water and 105.8 g of fumaric acid in a reaction tank of 3 L (diameter: 450 mm), the mixture was heated to 85 ° C to be dissolved. Then, after mixing 1.13 g of sodium carboxymethylcellulose (Tokyo Chemical Industry Co., Ltd.), it was cooled from 80 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby the fumaric acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper. The filtered fumaric acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, a mesh of 500 μm was passed through the mesh, whereby the fumaric acid crystals were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 100.0 mL/100 g. The results of Examples 1 to 9 and Comparative Example 5 are shown in Tables 2 and 3. [Table 2] [table 3] As shown in Tables 2 and 3, when it is confirmed that the concentration of the anionic polymer is a specific ratio with respect to the concentration of fumaric acid in the presence of an anionic polymer, self-contained fumaric acid or The fumarate crystal obtained by crystallization of the salt aqueous solution exhibits a higher oil absorption. (Crystalization by reaction) [Example 10] 1.55 kg of ion-exchanged water, 450 g of maleic anhydride, and polyacrylic acid (molecular weight: 5000 g/mol) in a 3 L reaction vessel (diameter 130 mm) After 9.58 g and 98% sulfuric acid 24.2 g, the temperature was raised to 70 ° C to dissolve. Then, a 6.19% aqueous thiourea solution was continuously added at a rate of 1.8 ml/min for 120 minutes to carry out an isomerization reaction from maleic acid to fumaric acid to precipitate fumaric acid. After 120 minutes, it was kept at 70 ° C for 30 minutes in order to increase the yield of fumaric acid. Thereafter, it was cooled from 70 ° C to 25 ° C at an average cooling rate of 0.6 ° C / min. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper, and then 1000 g of ion-exchanged water was added thereto to carry out filtration washing. The filtered fumaric acid filter cake was dried at 60 ° C using a hot air circulation drier FS-60WT. After drying, a mesh of 500 μm was passed through the mesh, whereby the fumaric acid crystals were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 80 mL/100 g. [Comparative Example 6] After mixing 1.56 kg of ion-exchanged water, 450 g of maleic anhydride, and 24.2 g of 98% sulfuric acid in a reaction tank of 3 L (diameter: 130 mm), the mixture was heated to 70 ° C to be dissolved. Then, a 0.19% thiourea aqueous solution was continuously added at a rate of 1.8 ml/min for 120 minutes to carry out an isomerization reaction from maleic acid to fumaric acid to precipitate fumaric acid. After 120 minutes, it was kept at 70 ° C for 30 minutes in order to increase the yield of fumaric acid. Thereafter, it was cooled from 70 ° C to 25 ° C at an average cooling rate of 0.6 ° C / min. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated fumaric acid suspension was suction-filtered using No. 2 filter paper, and then 1000 g of ion-exchanged water was added thereto to carry out filtration washing. The filtered fumaric acid filter cake was dried at 60 ° C using a hot air circulation drier FS-60WT. After drying, a mesh of 500 μm was passed through the mesh, whereby the fumaric acid crystals were obtained. The oil absorption of the obtained fumaric acid crystal was measured and found to be 20 mL/100 g. The results of Example 10 and Comparative Example 6 are shown in Table 4. [Table 4] As shown in the above Table 4, it was confirmed that a high oil absorption amount of fumaric acid crystals was obtained by a precipitation method by a reaction. (Precipitation method by cooling) [Example 11] After mixing 1.80 kg of ion-exchanged water and 450.0 g of succinic acid in a reaction tank of 3 L (diameter: 450 mm), the temperature was raised to 80 ° C to be dissolved. Then, after mixing 4.77 g of polyacrylic acid (weight average molecular weight: 5000) (Tokyo Chemical Industry Co., Ltd.), it was cooled from 80 ° C to 25 ° C at an average cooling rate of 0.3 ° C / min, whereby succinic acid was precipitated. The stirring system was carried out using a stirring blade having a leaf diameter of 121 mm at 250 r/min. Next, the precipitated succinic acid suspension was suction-filtered using No. 2 filter paper. The filtered succinic acid filter cake was dried at 105 ° C using a hot air circulation drier FS-60WT (manufactured by Tokyo Glass Instruments Co., Ltd.). After drying, it passed through a mesh of 500 μm mesh, whereby succinic acid crystals were obtained. The oil absorption of the obtained succinic acid crystal was measured and found to be 50.0 mL/100 g. The results of Example 11 are shown in Table 5. [table 5] As shown in the above Table 5, it was confirmed that the succinic acid crystal which was crystallized by the method of the present invention exhibited a higher oil absorption amount.